Broken QSC RMX 2450 Amp

[Peter Jones]

Hi all,
I'm in possesion of a QSC RMX 2450 amplifier that has had a little
accident. The amp was on monitors when a digi desk recall resulted in
excessive feedback at about 1 - 2kHz. The HF diaphragms on channel B blew,
whilst the 15" on channel A remained intact. Channel A clip light remains
constantly lit, and no sound is heard when connected normally (I do not know
if this is true on channel B or not). I have tested the PSU and all rails
are correct. When powered up with no load and no input, the "A" heatsink
warms moderately quickly (about 10 - 20 C / minute). I measured the
temperature of the power transistors, and found the 4 to the rear of the
amplifier were responsible for the heating. I have now removed these.

What I am after is circuit diagrams for the amplifier (or links to them) if
these are available. And I'm wondering if anybody has dealt with this
problem before and can give me any advice on the issue? Also I cannot find a
supplier of the power transistors, which I am currently assumng to be at
fault.

Thanks for taking the time to help me.

Pete Jones
Freelance Live-Event Technology Engineer
BSC Live Performance Technology

[George]

Post by Peter Jones
Hi all,
I'm in possesion of a QSC RMX 2450 amplifier that has had a little
accident. The amp was on monitors when a digi desk recall resulted in
excessive feedback at about 1 - 2kHz. The HF diaphragms on channel B blew,
whilst the 15" on channel A remained intact. Channel A clip light remains
constantly lit, and no sound is heard when connected normally (I do not know
if this is true on channel B or not). I have tested the PSU and all rails
are correct. When powered up with no load and no input, the "A" heatsink
warms moderately quickly (about 10 - 20 C / minute). I measured the
temperature of the power transistors, and found the 4 to the rear of the
amplifier were responsible for the heating. I have now removed these.
What I am after is circuit diagrams for the amplifier (or links to them) if
these are available. And I'm wondering if anybody has dealt with this
problem before and can give me any advice on the issue? Also I cannot find a
supplier of the power transistors, which I am currently assumng to be at
fault.
Thanks for taking the time to help me.
Pete Jones
Freelance Live-Event Technology Engineer
BSC Live Performance Technology

Have you contacted QSC
they are very service friendly
George

[BOB URZ]

Post by Peter Jones
Hi all,
I'm in possesion of a QSC RMX 2450 amplifier that has had a little
accident. The amp was on monitors when a digi desk recall resulted in
excessive feedback at about 1 - 2kHz. The HF diaphragms on channel B blew,
whilst the 15" on channel A remained intact. Channel A clip light remains
constantly lit, and no sound is heard when connected normally (I do not know
if this is true on channel B or not). I have tested the PSU and all rails
are correct. When powered up with no load and no input, the "A" heatsink
warms moderately quickly (about 10 - 20 C / minute). I measured the
temperature of the power transistors, and found the 4 to the rear of the
amplifier were responsible for the heating. I have now removed these.
What I am after is circuit diagrams for the amplifier (or links to them) if
these are available. And I'm wondering if anybody has dealt with this
problem before and can give me any advice on the issue? Also I cannot find a
supplier of the power transistors, which I am currently assumng to be at
fault.
Thanks for taking the time to help me.
Pete Jones
Freelance Live-Event Technology Engineer
BSC Live Performance Technology

While you might get lucky and replace the bad output transistors
and bring it back to life, high power amplifier repair is best left
to the pros. One slip and there goes a Hundred dollars of parts.

If your up to the task, post the transistor numbers.
I do agree with George, sending back to QSC might not
be a bad idea. Maybe its still under warranty?
Did your check?

Bob

[Pooh Bear]

Post by BOB URZ

Post by Peter Jones
Hi all,
I'm in possesion of a QSC RMX 2450 amplifier that has had a little
accident. The amp was on monitors when a digi desk recall resulted in
excessive feedback at about 1 - 2kHz. The HF diaphragms on channel B blew,
whilst the 15" on channel A remained intact. Channel A clip light remains
constantly lit, and no sound is heard when connected normally (I do not know
if this is true on channel B or not). I have tested the PSU and all rails
are correct. When powered up with no load and no input, the "A" heatsink
warms moderately quickly (about 10 - 20 C / minute). I measured the
temperature of the power transistors, and found the 4 to the rear of the
amplifier were responsible for the heating. I have now removed these.
What I am after is circuit diagrams for the amplifier (or links to them) if
these are available. And I'm wondering if anybody has dealt with this
problem before and can give me any advice on the issue? Also I cannot find a
supplier of the power transistors, which I am currently assumng to be at
fault.
Thanks for taking the time to help me.
Pete Jones
Freelance Live-Event Technology Engineer
BSC Live Performance Technology

While you might get lucky and replace the bad output transistors
and bring it back to life, high power amplifier repair is best left
to the pros. One slip and there goes a Hundred dollars of parts.
If your up to the task, post the transistor numbers.
I do agree with George, sending back to QSC might not
be a bad idea. Maybe its still under warranty?
Did your check?

Good point - QSC have generous warranties. Good thing too after I managed to make
a PLX go bang !

Graham

[mike diack]

On Mon, 5 Apr 2004 21:28:22 +0100, "Peter Jones"

Post by Peter Jones
Hi all,
I'm in possesion of a QSC RMX 2450 amplifier that has had a little
accident. The amp was on monitors when a digi desk recall resulted in
excessive feedback at about 1 - 2kHz. The HF diaphragms on channel B blew,
whilst the 15" on channel A remained intact. Channel A clip light remains
constantly lit, and no sound is heard when connected normally (I do not know
if this is true on channel B or not). I have tested the PSU and all rails
are correct. When powered up with no load and no input, the "A" heatsink
warms moderately quickly (about 10 - 20 C / minute). I measured the
temperature of the power transistors, and found the 4 to the rear of the
amplifier were responsible for the heating. I have now removed these.
What I am after is circuit diagrams for the amplifier (or links to them) if
these are available. And I'm wondering if anybody has dealt with this
problem before and can give me any advice on the issue? Also I cannot find a
supplier of the power transistors, which I am currently assumng to be at
fault.

Pacific Semiconductors (Google it for the URL) have what you need
(2SA1943, 2SC5200 etc) Remember that if the outputs are stuffed, check
the emitter resistors and drivers, if the drivers are stuffed, check
the predrivers etc etc ad infinitum. Be careful where you buy semis,
there are counterfeits out there which can bite you in the bum really
badly.
M
(BTW with 2450s , make sure you check the rail commutating fets
(IRFZ44N) and disconnect diodes as well)

[Pooh Bear]

Post by Peter Jones
Hi all,
I'm in possesion of a QSC RMX 2450 amplifier that has had a little
accident. The amp was on monitors when a digi desk recall resulted in
excessive feedback at about 1 - 2kHz. The HF diaphragms on channel B blew,
whilst the 15" on channel A remained intact. Channel A clip light remains
constantly lit, and no sound is heard when connected normally (I do not know
if this is true on channel B or not). I have tested the PSU and all rails
are correct. When powered up with no load and no input, the "A" heatsink
warms moderately quickly (about 10 - 20 C / minute). I measured the
temperature of the power transistors, and found the 4 to the rear of the
amplifier were responsible for the heating. I have now removed these.
What I am after is circuit diagrams for the amplifier (or links to them) if
these are available. And I'm wondering if anybody has dealt with this
problem before and can give me any advice on the issue? Also I cannot find a
supplier of the power transistors, which I am currently assumng to be at
fault.

IIRC a fault on Ch A can mute both ch's or vice-versa. Certainly the overtemp
does this.

The power devices are made by Toshiba btw - and any can be supplied by any
competent distrubutor with that franchise.

Clip light constantly lit means that the op-amp stage output is greater than
'several' volts. This indicates a serious fault in the absence of signal.

Note that the 5532 op-amp that drives both ? channels is normally 'blown' when
there's an output fault. That's why they socket it and use a DIL package rather
than SMT ( for ease of replacement ).

QSC used to have the schematics on their website but I don't think they're
freely downloadable anymore. In any case it's not the simplest amp to work on
unless you understand the design and repair philosophy. Took me a while.

Do you know how to test for a good device with a DVM ? If not - get an expert to
fix it.

Not a good call for an RMX that a bit of feedback killed it. Did you have the
limiters engaged btw ?


Graham

[Phil Allison]

Post by Peter Jones
I measured the
temperature of the power transistors, and found the 4 to the rear of the
amplifier were responsible for the heating. I have now removed these.

** Power transistors fail short circuit after which they cannot generate
heat - so the ones you have removed are most likely OK.

If you did not know that already then keep out of that amplfier since its
repair is way beyond YOU.

Find a competent amp tech and make the job theirs.




......... Phil

[Peter Jones]

Cheers for the ideas,
we've actually had two of these go in the last
couple of months. I'm an electronic / electrical engineer, and have been
trained to degree level in power amp design. I felt the other amplifier was
beyond me and that has been sent for proffesional service. I have worked in
R & D departments for High power industrial motor controll companies,
helping to design and develop their products,so I have a lot of electronics
experience. I generally start with a circuit diagram and fault find from
there - does anyone know where I can trace these? The amplifier will be
retruned to a service engineer if it is beyond me, but we like to develop
in-house expertise.

The reasons we are attempting this ourselves is that the amplifier is out of
warranty :-( And we have been quoted £70 / hour for labour (or so I'm
told by the amps owner). I know you say that the amp is complicated, but
I've repaired, designed and built more compicated electronics in the past,
and am keen to develop the necessary skills.

If heat is generated by output devices, there must be current flow through
them (and a voltage drop across them) which should not happen under
quiescent conditions. I'm not sure untill I see the circuit diagrams how
this current is flowing - It can't be through the 'normal' route - the
transducer - as this is not connected (open circuit). Therefore current must
be flowwing abnormally through the power output sections - this may be due
to a fauly output device or drive circuitry. Once again the schematics would
enable me to see this more clearly. I would like to test the drive circuitry
in situ - using the usual tools.

Is it possible that a short circuit output device is causing one of the
other devices to drive current through the fulty device? I expect that is
highly possible. So phil - explain to me how a healthy output drive stage
generates heat with no load connected? I know it could be more than possible
by blown drive circuitry, but why would the drive circuitry fail under high
output conditions whilst the output stage remains intact? once again I
believe you have offended someone within the first sentence of talking to
them - your people skills could do with a service!

I have found the QSC website - the UK service centre is listed as :-
H.W. International
167-171 Willoughby Lane
Brantwood Ind. Area
London N17 OSB
Tel #[phone]
Fax #[phone]

I can't find the address in any phone listing and the phone number is
rejected as number not in use!!

does anyone know the UK QSC contacts?

Also I've found the source of the schematics - QSC are selling them for $15
+ P$P. Does anyone know if this is a hard or soft copy? I could do with a
link to download them from really.

Thanks for everyone's input, and don't worry If I feel out of my depth I
will send the amplifier to hospital. I don't believe in electronic
euthanasia.

Pete

Post by Phil Allison

Post by Peter Jones
I measured the
temperature of the power transistors, and found the 4 to the rear of the
amplifier were responsible for the heating. I have now removed these.

** Power transistors fail short circuit after which they cannot generate
heat - so the ones you have removed are most likely OK.
If you did not know that already then keep out of that amplfier since its
repair is way beyond YOU.
Find a competent amp tech and make the job theirs.
......... Phil

[Phil Allison]

This post might be inappropriate. Click to display it.

[VMI]

Hey Phil, you again?
I saw the message and said to myself "Hey that must be Phil!"
Only you are able to write such what ever it might be called.

Re-read what Peter wrote...
"once again I believe you have offended someone within the first sentence of
talking to
them - your people skills could do with a service!"

In case you're not able to trace that message.

Cheers and until the next vague of offending messages

[Ian McKeown]

HW International is now known as Shure Distribution UK and they still
distribute QSC. Address is still the same but the contact numbers you have
listed have the old area code.
Numbers are phone [phone] and fax [phone]

Post by Peter Jones
Cheers for the ideas,
we've actually had two of these go in the last
couple of months. I'm an electronic / electrical engineer, and have been
trained to degree level in power amp design. I felt the other amplifier was
beyond me and that has been sent for proffesional service. I have worked in
R & D departments for High power industrial motor controll companies,
helping to design and develop their products,so I have a lot of electronics
experience. I generally start with a circuit diagram and fault find from
there - does anyone know where I can trace these? The amplifier will be
retruned to a service engineer if it is beyond me, but we like to develop
in-house expertise.
The reasons we are attempting this ourselves is that the amplifier is out of
warranty :-( And we have been quoted £70 / hour for labour (or so I'm
told by the amps owner). I know you say that the amp is complicated, but
I've repaired, designed and built more compicated electronics in the past,
and am keen to develop the necessary skills.
If heat is generated by output devices, there must be current flow through
them (and a voltage drop across them) which should not happen under
quiescent conditions. I'm not sure untill I see the circuit diagrams how
this current is flowing - It can't be through the 'normal' route - the
transducer - as this is not connected (open circuit). Therefore current must
be flowwing abnormally through the power output sections - this may be due
to a fauly output device or drive circuitry. Once again the schematics would
enable me to see this more clearly. I would like to test the drive circuitry
in situ - using the usual tools.
Is it possible that a short circuit output device is causing one of the
other devices to drive current through the fulty device? I expect that is
highly possible. So phil - explain to me how a healthy output drive stage
generates heat with no load connected? I know it could be more than possible
by blown drive circuitry, but why would the drive circuitry fail under high
output conditions whilst the output stage remains intact? once again I
believe you have offended someone within the first sentence of talking to
them - your people skills could do with a service!
I have found the QSC website - the UK service centre is listed as :-
H.W. International
167-171 Willoughby Lane
Brantwood Ind. Area
London N17 OSB
Tel #[phone]
Fax #[phone]
I can't find the address in any phone listing and the phone number is
rejected as number not in use!!
does anyone know the UK QSC contacts?
Also I've found the source of the schematics - QSC are selling them for $15
+ P$P. Does anyone know if this is a hard or soft copy? I could do with a
link to download them from really.
Thanks for everyone's input, and don't worry If I feel out of my depth I
will send the amplifier to hospital. I don't believe in electronic
euthanasia.
Pete

Post by Phil Allison

Post by Peter Jones
I measured the
temperature of the power transistors, and found the 4 to the rear of the
amplifier were responsible for the heating. I have now removed these.

** Power transistors fail short circuit after which they cannot generate
heat - so the ones you have removed are most likely OK.
If you did not know that already then keep out of that amplfier since its
repair is way beyond YOU.
Find a competent amp tech and make the job theirs.
......... Phil

[Peter Jones]

Thanks. they are only a few years out of date then!!

Post by Ian McKeown
HW International is now known as Shure Distribution UK and they still
distribute QSC. Address is still the same but the contact numbers you have
listed have the old area code.
Numbers are phone [phone] and fax [phone]

Post by Peter Jones
Cheers for the ideas,
we've actually had two of these go in the last
couple of months. I'm an electronic / electrical engineer, and have been
trained to degree level in power amp design. I felt the other amplifier

was

Post by Peter Jones
beyond me and that has been sent for proffesional service. I have worked

in

Post by Peter Jones
R & D departments for High power industrial motor controll companies,
helping to design and develop their products,so I have a lot of

electronics

Post by Peter Jones
experience. I generally start with a circuit diagram and fault find from
there - does anyone know where I can trace these? The amplifier will be
retruned to a service engineer if it is beyond me, but we like to develop
in-house expertise.
The reasons we are attempting this ourselves is that the amplifier is

out

Post by Ian McKeown
of

Post by Peter Jones
warranty :-( And we have been quoted £70 / hour for labour (or so I'm
told by the amps owner). I know you say that the amp is complicated, but
I've repaired, designed and built more compicated electronics in the past,
and am keen to develop the necessary skills.
If heat is generated by output devices, there must be current flow through
them (and a voltage drop across them) which should not happen under
quiescent conditions. I'm not sure untill I see the circuit diagrams how
this current is flowing - It can't be through the 'normal' route - the
transducer - as this is not connected (open circuit). Therefore current

must

Post by Peter Jones
be flowwing abnormally through the power output sections - this may be due
to a fauly output device or drive circuitry. Once again the schematics

would

Post by Peter Jones
enable me to see this more clearly. I would like to test the drive

circuitry

Post by Peter Jones
in situ - using the usual tools.
Is it possible that a short circuit output device is causing one of the
other devices to drive current through the fulty device? I expect that is
highly possible. So phil - explain to me how a healthy output drive stage
generates heat with no load connected? I know it could be more than

possible

Post by Peter Jones
by blown drive circuitry, but why would the drive circuitry fail under

high

Post by Peter Jones
output conditions whilst the output stage remains intact? once again I
believe you have offended someone within the first sentence of talking to
them - your people skills could do with a service!
I have found the QSC website - the UK service centre is listed as :-
H.W. International
167-171 Willoughby Lane
Brantwood Ind. Area
London N17 OSB
Tel #[phone]
Fax #[phone]
I can't find the address in any phone listing and the phone number is
rejected as number not in use!!
does anyone know the UK QSC contacts?
Also I've found the source of the schematics - QSC are selling them for

$15

Post by Peter Jones
+ P$P. Does anyone know if this is a hard or soft copy? I could do with a
link to download them from really.
Thanks for everyone's input, and don't worry If I feel out of my depth I
will send the amplifier to hospital. I don't believe in electronic
euthanasia.
Pete

Post by Phil Allison

Post by Peter Jones
I measured the
temperature of the power transistors, and found the 4 to the rear of

the

Post by Peter Jones

Post by Phil Allison

Post by Peter Jones
amplifier were responsible for the heating. I have now removed these.

** Power transistors fail short circuit after which they cannot

generate

Post by Peter Jones

Post by Phil Allison
heat - so the ones you have removed are most likely OK.
If you did not know that already then keep out of that amplfier since

its

Post by Peter Jones

Post by Phil Allison
repair is way beyond YOU.
Find a competent amp tech and make the job theirs.
......... Phil

[bob urz]

Post by Peter Jones
Thanks. they are only a few years out of date then!!

Post by Ian McKeown
HW International is now known as Shure Distribution UK and they still
distribute QSC. Address is still the same but the contact numbers you have
listed have the old area code.
Numbers are phone [phone] and fax [phone]

Post by Peter Jones
Cheers for the ideas,
we've actually had two of these go in the

last

Post by Ian McKeown

Post by Peter Jones
couple of months. I'm an electronic / electrical engineer, and have been
trained to degree level in power amp design. I felt the other amplifier

was

Post by Peter Jones
beyond me and that has been sent for proffesional service. I have worked

in

Post by Peter Jones
R & D departments for High power industrial motor controll companies,
helping to design and develop their products,so I have a lot of

electronics

Post by Peter Jones
experience. I generally start with a circuit diagram and fault find from
there - does anyone know where I can trace these? The amplifier will be
retruned to a service engineer if it is beyond me, but we like to

develop

Post by Ian McKeown

Post by Peter Jones
in-house expertise.
The reasons we are attempting this ourselves is that the amplifier is

out

Post by Ian McKeown
of

Post by Peter Jones
warranty :-( And we have been quoted £70 / hour for labour (or so

I'm

Post by Ian McKeown

Post by Peter Jones
told by the amps owner). I know you say that the amp is complicated, but
I've repaired, designed and built more compicated electronics in the

past,

Post by Ian McKeown

Post by Peter Jones
and am keen to develop the necessary skills.
If heat is generated by output devices, there must be current flow

through

Post by Ian McKeown

Post by Peter Jones
them (and a voltage drop across them) which should not happen under
quiescent conditions. I'm not sure untill I see the circuit diagrams how
this current is flowing - It can't be through the 'normal' route - the
transducer - as this is not connected (open circuit). Therefore current

must

Post by Peter Jones
be flowwing abnormally through the power output sections - this may be

due

Post by Ian McKeown

Post by Peter Jones
to a fauly output device or drive circuitry. Once again the schematics

would

Post by Peter Jones
enable me to see this more clearly. I would like to test the drive

circuitry

Post by Peter Jones
in situ - using the usual tools.
Is it possible that a short circuit output device is causing one of the
other devices to drive current through the fulty device? I expect that

is

Post by Ian McKeown

Post by Peter Jones
highly possible. So phil - explain to me how a healthy output drive

stage

Post by Ian McKeown

Post by Peter Jones
generates heat with no load connected? I know it could be more than

possible

Post by Peter Jones
by blown drive circuitry, but why would the drive circuitry fail under

high

Post by Peter Jones
output conditions whilst the output stage remains intact? once again I
believe you have offended someone within the first sentence of talking

to

Post by Ian McKeown

Post by Peter Jones
them - your people skills could do with a service!
I have found the QSC website - the UK service centre is listed as :-
H.W. International
167-171 Willoughby Lane
Brantwood Ind. Area
London N17 OSB
Tel #[phone]
Fax #[phone]
I can't find the address in any phone listing and the phone number is
rejected as number not in use!!
does anyone know the UK QSC contacts?
Also I've found the source of the schematics - QSC are selling them for

$15

Post by Peter Jones
+ P$P. Does anyone know if this is a hard or soft copy? I could do

with

Post by Peter Jones
a

Post by Ian McKeown

Post by Peter Jones
link to download them from really.
Thanks for everyone's input, and don't worry If I feel out of my depth I
will send the amplifier to hospital. I don't believe in electronic
euthanasia.
Pete

Post by Phil Allison

Post by Peter Jones
I measured the
temperature of the power transistors, and found the 4 to the rear of

the

Post by Peter Jones

Post by Phil Allison

Post by Peter Jones
amplifier were responsible for the heating. I have now removed

these.

Post by Ian McKeown

Post by Peter Jones

Post by Phil Allison
** Power transistors fail short circuit after which they cannot

generate

Post by Peter Jones

Post by Phil Allison
heat - so the ones you have removed are most likely OK.
If you did not know that already then keep out of that amplfier since

its

Post by Peter Jones

Post by Phil Allison
repair is way beyond YOU.
Find a competent amp tech and make the job theirs.
......... Phil

Well, if your a competent design engineer, you should know that
transistors have PN junctions. With a simple test with a DVM, you can
measure the PN junction voltage on the OHMs scale of most DVM's. BC will
measure slightly lower than BC. (around .6 volt)A simple sweep with the
meter would ferret out most shorted devices for removal and further testing.
A junction test is not a 100% test (will not test for leakage or HFE) but i
would say its useful 95%+ of the time . Once all the shorted outputs are
noted, move on to the drivers. Then to emitter resistors and any other
components that may have gotten stressed.

As a matter of procedure, i usually replace all the output devices in the
bad channel. Why? because some of the other ones could have been stressed
and fail soon after. It may not be cheap insurance if only a couple devices
are bad, buts its insurance.

This is all standard tech stuff for those of us who do it every day.
Most of us would always like a schematic or service manual, but sometimes
you just don't have one when you need it. You can usually get the amp fixed
or get pretty far on one without one if you know what your doing. But you
must have the chops.

Bob

[ceedub]

Peter:

Here is QSC's USA site (just down the road from me here in Southern
California)...

http://www.qscstore.com/rmxserman.html

The link is for a download ($15 USD) for a PDF link to their schematics. I
think you should be able to download the information you need.

Good luck.

Craig
--
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There is a solution!"

Protected by GIANT Company's Spam Inspector
The most powerful anti-spam software available.
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Post by Peter Jones
Cheers for the ideas,
we've actually had two of these go in the last
couple of months. I'm an electronic / electrical engineer, and have been
trained to degree level in power amp design. I felt the other amplifier was
beyond me and that has been sent for proffesional service. I have worked in
R & D departments for High power industrial motor controll companies,
helping to design and develop their products,so I have a lot of electronics
experience. I generally start with a circuit diagram and fault find from
there - does anyone know where I can trace these? The amplifier will be
retruned to a service engineer if it is beyond me, but we like to develop
in-house expertise.
The reasons we are attempting this ourselves is that the amplifier is out of
warranty :-( And we have been quoted £70 / hour for labour (or so I'm
told by the amps owner). I know you say that the amp is complicated, but
I've repaired, designed and built more compicated electronics in the past,
and am keen to develop the necessary skills.
If heat is generated by output devices, there must be current flow through
them (and a voltage drop across them) which should not happen under
quiescent conditions. I'm not sure untill I see the circuit diagrams how
this current is flowing - It can't be through the 'normal' route - the
transducer - as this is not connected (open circuit). Therefore current must
be flowwing abnormally through the power output sections - this may be due
to a fauly output device or drive circuitry. Once again the schematics would
enable me to see this more clearly. I would like to test the drive circuitry
in situ - using the usual tools.
Is it possible that a short circuit output device is causing one of the
other devices to drive current through the fulty device? I expect that is
highly possible. So phil - explain to me how a healthy output drive stage
generates heat with no load connected? I know it could be more than possible
by blown drive circuitry, but why would the drive circuitry fail under high
output conditions whilst the output stage remains intact? once again I
believe you have offended someone within the first sentence of talking to
them - your people skills could do with a service!
I have found the QSC website - the UK service centre is listed as :-
H.W. International
167-171 Willoughby Lane
Brantwood Ind. Area
London N17 OSB
Tel #[phone]
Fax #[phone]
I can't find the address in any phone listing and the phone number is
rejected as number not in use!!
does anyone know the UK QSC contacts?
Also I've found the source of the schematics - QSC are selling them for $15
+ P$P. Does anyone know if this is a hard or soft copy? I could do with a
link to download them from really.
Thanks for everyone's input, and don't worry If I feel out of my depth I
will send the amplifier to hospital. I don't believe in electronic
euthanasia.
Pete

Post by Phil Allison

Post by Peter Jones
I measured the
temperature of the power transistors, and found the 4 to the rear of the
amplifier were responsible for the heating. I have now removed these.

** Power transistors fail short circuit after which they cannot generate
heat - so the ones you have removed are most likely OK.
If you did not know that already then keep out of that amplfier since its
repair is way beyond YOU.
Find a competent amp tech and make the job theirs.
......... Phil

[Hawker]

On 4/7/2004 1:11 AM The digits of ceedub's hands composed the
following:<br>

Post by ceedub
The link is for a download ($15 USD) for a PDF link to their schematics. I
think you should be able to download the information you need.

Interesting. I wonder when that changed?
Last time I needed a schematic for a QSC amp (an old - MX1500 - pre
MX1500a) I was able to download it for free. I since have gotten access
to the Tech CD so I haven't used it for a while.
Are all the Schematics pay now or just the newer ones?

Hawker

[Pooh Bear]

Post by Peter Jones
Cheers for the ideas,
we've actually had two of these go in the last
couple of months.

Any idea why ?

Post by Peter Jones
I'm an electronic / electrical engineer, and have been
trained to degree level in power amp design.

I spent 4 yrs in higher education including 1 at UCL and 3 on a specialist audio
cousre.

Not one moment ( maybe a few ? ) was spent regarding *practical* power amp
designs and their pitfalls.

There's more to power amps than 'dry theory'.

Your approach to fault finding is very flawed sadly. A 'tech' will approach it
very differently.


Graham

[Ernie Garner]

Look up "secondary breakdown" and "safe operating area" and you'll
understand the most likely reason for the shorts.

Ernie

[unitron]

Post by Phil Allison

Post by Peter Jones
I measured the
temperature of the power transistors, and found the 4 to the rear of the
amplifier were responsible for the heating. I have now removed these.

** Power transistors fail short circuit after which they cannot generate
heat - so the ones you have removed are most likely OK.
If you did not know that already then keep out of that amplfier since its
repair is way beyond YOU.
Find a competent amp tech and make the job theirs.
......... Phil

Time for me to reveal my ignorance, I guess. Wouldn't transistors
that failed open, i.e., unable to pass current, stay cold, and shorted
ones, i.e., with an extremely low resistance between emitter and
collector, or between source and drain, pass a lot of current and heat
up somewhat due to that current even though the voltage drop across
them was low due to the extremely low resistance?

[Phil Allison]

Post by unitron

Post by Phil Allison
** Power transistors fail short circuit after which they cannot generate
heat - so the ones you have removed are most likely OK.

Time for me to reveal my ignorance, I guess. Wouldn't transistors
that failed open, i.e., unable to pass current, stay cold, and shorted
ones, i.e., with an extremely low resistance between emitter and
collector, or between source and drain, pass a lot of current and heat
up somewhat due to that current even though the voltage drop across
them was low due to the extremely low resistance?

** The chip inside a power transistor ( fet or bipolar) fails when it
overheats, the silicon semiconductor material develops a hot spot that melts
and so a hard short forms. Unless the device has internal fusible links the
device is shorted C to E or D to S from then on. The QSC in question uses
bipolar transistors in plastic packages that have no fusible links.

The amount of DC current that would be needed to heat a shorted power
transistor is so large that an amp's supply fuses would blow first.




........... Phil

[BOB URZ]

Post by Phil Allison

Post by unitron

Post by Phil Allison
** Power transistors fail short circuit after which they cannot

generate

Post by unitron

Post by Phil Allison
heat - so the ones you have removed are most likely OK.

Time for me to reveal my ignorance, I guess. Wouldn't transistors
that failed open, i.e., unable to pass current, stay cold, and shorted
ones, i.e., with an extremely low resistance between emitter and
collector, or between source and drain, pass a lot of current and heat
up somewhat due to that current even though the voltage drop across
them was low due to the extremely low resistance?

** The chip inside a power transistor ( fet or bipolar) fails when it
overheats, the silicon semiconductor material develops a hot spot that melts
and so a hard short forms. Unless the device has internal fusible links the
device is shorted C to E or D to S from then on. The QSC in question uses
bipolar transistors in plastic packages that have no fusible links.

Can also fail from voltage breakdown. Anything out of the SOA is
a potential problem.

Post by Phil Allison
The amount of DC current that would be needed to heat a shorted power
transistor is so large that an amp's supply fuses would blow first.

Or the low value emitter swamping resistors would open up taking
the device out of the circuit (which may or may not happen). A defect in the
driver or pre driver stage could cause a otherwise good output devices to run
hot. Or oscillation for that matter.

Bob

Post by Phil Allison
........... Phil

[Phil Allison]

"BOB URZ" <

Post by BOB URZ
Can also fail from voltage breakdown. Anything out of the SOA is
a potential problem.

** Like some demented puppy Bob Errs just has to have a little piddle on
every word of mine he sees.

Post by BOB URZ

Post by Phil Allison
The amount of DC current that would be needed to heat a shorted power
transistor is so large that an amp's supply fuses would blow first.

Or the low value emitter swamping resistors would open up taking
the device out of the circuit (which may or may not happen). A defect in the
driver or pre driver stage could cause a otherwise good output devices to run
hot. Or oscillation for that matter.

** None of which is relevant to the question or my response.




.............. Phil

[boburz]

Post by Phil Allison
"BOB URZ" <

Post by BOB URZ
Can also fail from voltage breakdown. Anything out of the SOA is
a potential problem.

** Like some demented puppy Bob Errs just has to have a little piddle on
every word of mine he sees.

Post by BOB URZ

Post by Phil Allison
The amount of DC current that would be needed to heat a shorted

power

Post by BOB URZ

Post by Phil Allison
transistor is so large that an amp's supply fuses would blow first.

Or the low value emitter swamping resistors would open up taking
the device out of the circuit (which may or may not happen). A defect in

the

Post by BOB URZ
driver or pre driver stage could cause a otherwise good output devices to

run

Post by BOB URZ
hot. Or oscillation for that matter.

** None of which is relevant to the question or my response.

The question was failed output's in an amp. And heating of the outputs.
You don't think that an amp gone into oscillation could heat up the
outputs? Maybe amps are not allowed to oscillate down under.......
And you don't think a defective driver or pre driver could cause the
outputs to run warm under some conditions???


Bob

Post by Phil Allison
.............. Phil

[Phil Allison]

Post by boburz

Post by Phil Allison
** None of which is relevant to the question or my response.

The question was failed output's in an amp.

** WRONG !!!

The question being answered by me was that posted by "unitron" :

" Time for me to reveal my ignorance, I guess. Wouldn't transistors
that failed open, i.e., unable to pass current, stay cold, and shorted
ones, i.e., with an extremely low resistance between emitter and
collector, or between source and drain, pass a lot of current and heat
up somewhat due to that current even though the voltage drop across
them was low due to the extremely low resistance? "

Post by boburz
You don't think that an amp gone into oscillation could heat up the
outputs?

** I mentioned instability already in this thread - in a reply to Mr
Jones.

Post by boburz
Maybe amps are not allowed to oscillate down under.......
And you don't think a defective driver or pre driver could cause the
outputs to run warm under some conditions???

** None of which is relevant to my response to unitron.




............. Phil

[unitron]

Post by Phil Allison

Post by unitron

Post by Phil Allison
** Power transistors fail short circuit after which they cannot

generate

Post by unitron

Post by Phil Allison
heat - so the ones you have removed are most likely OK.

Time for me to reveal my ignorance, I guess. Wouldn't transistors
that failed open, i.e., unable to pass current, stay cold, and shorted
ones, i.e., with an extremely low resistance between emitter and
collector, or between source and drain, pass a lot of current and heat
up somewhat due to that current even though the voltage drop across
them was low due to the extremely low resistance?

** The chip inside a power transistor ( fet or bipolar) fails when it
overheats, the silicon semiconductor material develops a hot spot that melts
and so a hard short forms. Unless the device has internal fusible links the
device is shorted C to E or D to S from then on. The QSC in question uses
bipolar transistors in plastic packages that have no fusible links.
The amount of DC current that would be needed to heat a shorted power
transistor is so large that an amp's supply fuses would blow first.
........... Phil

If the transistor is shorted then it will pass current, right?
Could it be that something in series with it and mounted on the same
heat sink would be subjected to more current due to the transistor
being near zero Ohms instead of more Ohms than that and that that
other component would get hotter than it would normally, thus heating
the heat sink and thereby heating the shorted transistors mounted on
the same heatsink? 'Cause I don't see how open transistors, which
means an open circuit, which means no current flow, could heat up or
allow anything else in the circuit to heat up. Of course transistors
that haven't gone bad will heat up in use, otherwise they wouldn't
need to be heatsunk (heatsinked?).

[Phil Allison]

Post by unitron
If the transistor is shorted then it will pass current, right?
Could it be that something in series with it and mounted on the same
heat sink would be subjected to more current due to the transistor
being near zero Ohms instead of more Ohms than that and that that
other component would get hotter than it would normally, thus heating
the heat sink and thereby heating the shorted transistors mounted on
the same heatsink?

** Transistors always get hot first - then the heatsink gradually heats.
Is is easy to tell which devices are the source of heat with your fingertips
since they are hotter than the heatsink.

Post by unitron
'Cause I don't see how open transistors, which
means an open circuit, which means no current flow,

** Get off this mad idea.

Power transistors fail SHORT !!!

Flat pack devices have no internal fuses - they cannot go open after
failure.




............ Phil

[Hawker]

Why do people on this NG attack rather than educate?
Come on some of you.. Peter needs help and teaching not attacking. We
were all new at one time.

Peter.
The poster is probably correct. The hot transistors are probably the
good ones, since there is bias current on all the transistors. The load
is not shared equally and hence some are hot.

In a case like this you really need to check all the transistors with a
VOM not just trouble shoot with a Scope. I too am a design EE and not
that great at repair without a schematic, (I have great respect for true
repair techs who work in the dark all the time) but amps are usually not
that bad. Just check the output transistors and drivers as someone else
suggested.

As for a schematic. Just E-mail QSC with the model number and serial
number. You can download some of the schematics from the QSC website,
but for the rest they have never refused me when I e-mailed them. They
just sent them to me via e-mail and sometimes have given me the password
to the service tech area. It's all about asking nicely.

Hawker



On 4/5/2004 4:28 PM The digits of Peter Jones's hands composed the
following:<br>

Post by Peter Jones
Hi all,
I'm in possesion of a QSC RMX 2450 amplifier that has had a little
accident. The amp was on monitors when a digi desk recall resulted in
excessive feedback at about 1 - 2kHz. The HF diaphragms on channel B blew,
whilst the 15" on channel A remained intact. Channel A clip light remains
constantly lit, and no sound is heard when connected normally (I do not know
if this is true on channel B or not). I have tested the PSU and all rails
are correct. When powered up with no load and no input, the "A" heatsink
warms moderately quickly (about 10 - 20 C / minute). I measured the
temperature of the power transistors, and found the 4 to the rear of the
amplifier were responsible for the heating. I have now removed these.
What I am after is circuit diagrams for the amplifier (or links to them) if
these are available. And I'm wondering if anybody has dealt with this
problem before and can give me any advice on the issue? Also I cannot find a
supplier of the power transistors, which I am currently assumng to be at
fault.
Thanks for taking the time to help me.
Pete Jones
Freelance Live-Event Technology Engineer
BSC Live Performance Technology

[Todd H.]

Post by Hawker
Why do people on this NG attack rather than educate?

Lack of [education/confidence/social skills] (pick any one) on the
part of the attacker(s) who are attempting to compensate for this
lacuna in their life by tearing down anything possible. The
superiority rant makes them feel warm fuzzy and superior somehow.

But that's just a guess.

Best Regards,

[Ernie Garner]

In Phil's defense - can you believe it? <g> - there is a major
incongruity between the stated education level and the questions.

Ernie

Post by Todd H.

Post by Hawker
Why do people on this NG attack rather than educate?

Lack of [education/confidence/social skills] (pick any one) on the
part of the attacker(s) who are attempting to compensate for this
lacuna in their life by tearing down anything possible. The
superiority rant makes them feel warm fuzzy and superior somehow.
But that's just a guess.
Best Regards,

[Phil Allison]

Post by Ernie Garner
In Phil's defense - can you believe it? <g> - there is a major
incongruity between the stated education level and the questions.

** It is quite possible that Mr Jones has an electrical engineering degree
and experience with certain electronic devices items as he stated. The bit
that smelt off was the claim he had a degree level knowledge of power
amplifier design while appearing to be sadly ignorant of even the basics of
modern audio power amps.

The fact is that academic courses in electronics rarely delve into the
details of typical commercial electronic devices but rather give a broad
overview of the principles that lie behind all such designs. An engineering
graduate is ( hopefully) then in a position to understand the details when
and if they take up a career in that area of technology with some equipment
manufacturer.

It is assumed that manufacturers have experienced engineering staff from
whom the new graduate can learn the nitty gritty, non academic stuff that
the commercial world exploits for fun and profit ;-)

Of course this is not the only way for a person to become familiar with a
particular technology and a great many successful electronics designers have
not acquired an engineering degree but simply concentrated their time and
minds on one type of product and become very expert on that.



............ Phil

[Ernie Garner]

Ok, I spoke too soon so I apologize. Got messed following the thread
but that's no excuse.

I hate repairing power amps with blown outputs because you have to
test everything. Make a mistake and, if you are honest, your wallet
will feel it. Because most amps are closed loop DC coupled it's
easier to fix them "open loop", that is, test and fix everything that
might be damaged so that you don't have to do any serious
troubleshooting.

My generic procedure - open for criticism and additions.

I pull all the transistors and test them. The means outputs, drivers,
pre-drivers, and protection. Along with that I pull the output
emitter resistors and check them. Along the way I do a visual on all
the resistors to make sure nothing else is burned. It's also not a
bad idea to flick your thumbnail across all the resistors to make sure
none of them are cracked. Just takes a second. Turn it on with the
parts out and make sure that any driver IC isn't running hot.
Double-check IC rails. Reassemble, replacing any bad parts.

Get out a variac or the previously described poor man's variac - light
bulb in series with the "mains". Put a voltmeter on the output, 1k
load. Bring the output rails up to +/- 20 volts or so (enough for the
circuitry to kick in) via variac or appropriate wattage bulb and check
the output. Make sure it's basically zero. If not, you are going to
need a schematic and plan on "digging in". Isolating problems in
direct coupled amps is a major pain in the ass.

If the amp works, put a signal through, take it up to clipping and
make sure it's even. Also check out the zero crossings for crossover
distortion. If it is ok then slowly take the voltage up to the full
mains.

Now, go back down again and repeat the procedure with an 8 ohm load.
Start with a small signal. You're looking for any distortion in the
output sine wave. Once you've taken the power supply voltages up to
full rails with a load you can go balls out and run it into clipping.
Calculate the wattage at clipping based on the peak voltage and make
sure it it matches the amp specs. Drop it back to 2/3 power and run
it into a dummy load for a few hours.

If you have to replace one transistor in the output group, depending
on the economics, you may want to consider replacing them all. Not
because of stress but because you want them to be matched.

Again, sorry for opening my mouth without having brain in gear and if
anyone has anything to add to this, please do.

Ernie

Post by Peter Jones
Hi all,
I'm in possesion of a QSC RMX 2450 amplifier that has had a little
accident. The amp was on monitors when a digi desk recall resulted in
excessive feedback at about 1 - 2kHz. The HF diaphragms on channel B blew,
whilst the 15" on channel A remained intact. Channel A clip light remains
constantly lit, and no sound is heard when connected normally (I do not know
if this is true on channel B or not). I have tested the PSU and all rails
are correct. When powered up with no load and no input, the "A" heatsink
warms moderately quickly (about 10 - 20 C / minute). I measured the
temperature of the power transistors, and found the 4 to the rear of the
amplifier were responsible for the heating. I have now removed these.
What I am after is circuit diagrams for the amplifier (or links to them) if
these are available. And I'm wondering if anybody has dealt with this
problem before and can give me any advice on the issue? Also I cannot find a
supplier of the power transistors, which I am currently assumng to be at
fault.
Thanks for taking the time to help me.
Pete Jones
Freelance Live-Event Technology Engineer
BSC Live Performance Technology

[Chad Wahls lt]

Post by Ernie Garner
My generic procedure - open for criticism and additions.

Nobody has bothered to tell him that the amp has 2 sets of rails and a
diode between them on the heatsink. This diode can fail sinking the
whole big rail into the finals, it makes then quite hot! Also makes
the amp prematurely limit.

There is a tech bulliten for this as 2450's had a problem with this
device.

Chad

[Ernie Garner]

Thanks for the info.

Ernie

Post by Chad Wahls lt

Post by Ernie Garner
My generic procedure - open for criticism and additions.

Nobody has bothered to tell him that the amp has 2 sets of rails and a
diode between them on the heatsink. This diode can fail sinking the
whole big rail into the finals, it makes then quite hot! Also makes
the amp prematurely limit.
There is a tech bulliten for this as 2450's had a problem with this
device.
Chad

[Phil Allison]

Post by Chad Wahls lt
Nobody has bothered to tell him that the amp has 2 sets of rails and a
diode between them on the heatsink. This diode can fail sinking the
whole big rail into the finals, it makes then quite hot! Also makes
the amp prematurely limit.

** You must have missed this post 3 days ago from Mike Diack then:

" (BTW with 2450s , make sure you check the rail commutating fets
(IRFZ44N) and disconnect diodes as well) "


The damn silly OP informed us he had already removed half the output
transistors (the hot ones) rendering the amp inoperative - this kinda
stuffed up giving him any advice to test voltages etc.

He also maintained that only half the output devices became hot with no
load - indicating that the other half must be conducting current without
heating or there is a dead short on the output the amp is trying to drive DC
into.



.......... Phil

[Chad Wahls lt]

Post by Phil Allison

Post by Chad Wahls lt
Nobody has bothered to tell him that the amp has 2 sets of rails and a
diode between them on the heatsink. This diode can fail sinking the
whole big rail into the finals, it makes then quite hot! Also makes
the amp prematurely limit.

Oops, my bad! Point taken

Post by Phil Allison
" (BTW with 2450s , make sure you check the rail commutating fets
(IRFZ44N) and disconnect diodes as well) "
The damn silly OP informed us he had already removed half the output
transistors (the hot ones) rendering the amp inoperative - this kinda
stuffed up giving him any advice to test voltages etc.
He also maintained that only half the output devices became hot with no
load - indicating that the other half must be conducting current without
heating or there is a dead short on the output the amp is trying to drive DC
into.

Don't remind me. The bruise on my forehead is starting to go away :)

Chad