[Richard Rasker]

Hi all,

I'm trying to fix an old Heath amp, the AA-1600. One day, it blew up one
end stage, and as the original PNP transistors 2N6609 are no longer
available here, I replaced the transistors with MJ15022/MJ15023 pairs.
Further investigation revealed that all four(!) 4K7/2W resistors
(R728/R729) were overcooked, and had resistances between 80K and
infinite. All other components seemed fine, so I thought I had the
problem nailed.

For testing, I put a 150W halogen lamp in series with the mains supply,
and everything still went fine. The end stage bias current seemed
completely stable, the amp operated on nearly its full mains voltage,
and both end stages behaved identical, both on low and high frequencies.
Of course, the supply voltage dropped when I applied a load on the output,
but even then, I could find no difference between both end stages. They
delivered 40W each without a problem before the mains voltage dropped too
much for the amp to function properly.

Then I hooked up the amp to the 'hard' mains voltage (without a series
lamp), and still all seemed well. The end stage bias current was fine and
stable, and both end stages delivered the same signal without a load
connected.

But alas, with an 8 ohm dummy load connected, the very same end stage
fried instantly as the output power got past a few watts. I was actually
monitoring the oscilloscope and volt meters when it happened, and there
was no warning sign whatsoever. No sudden oscillation, no DC voltage
drift, nothing at all. One second the amp worked fine, an instant later an
end stage transistor shorted out, blowing the fuses. :-(

So before I replace the end stage transistors once again, I'd like to see
if anyone here has a clue ... I considered replacing both driver
transistors as well (Q711/Q712), but based on the original type numbers
(F-317-B respectively F-417-B), I can't find any suitable replacements.

A somewhat low-quality schematic diagram can be found he

http://web.archive.org/web/200106071...th/aa-1600.gif

Does anyone have an idea what might be the problem? Or how I could trace
down the problem without blowing yet another set of end stage transistors?
I'm particularly puzzled by the fact that the wretched thing only blows up
when loaded and at the same time connected to the 'hard' mains voltage
(230V AC). Even with a low-ohm load in series, the amp behaves just fine ...

Thanks in advance,

Richard Rasker

--
Linetec Translation and Technology Services

http://www.linetec.nl/

[Ken Weitzel]

Richard Rasker wrote:
Hi all,

I'm trying to fix an old Heath amp, the AA-1600. One day, it blew up one
end stage, and as the original PNP transistors 2N6609 are no longer
available here, I replaced the transistors with MJ15022/MJ15023 pairs.
Further investigation revealed that all four(!) 4K7/2W resistors
(R728/R729) were overcooked, and had resistances between 80K and
infinite. All other components seemed fine, so I thought I had the
problem nailed.

For testing, I put a 150W halogen lamp in series with the mains supply,
and everything still went fine. The end stage bias current seemed
completely stable, the amp operated on nearly its full mains voltage,
and both end stages behaved identical, both on low and high frequencies.
Of course, the supply voltage dropped when I applied a load on the output,
but even then, I could find no difference between both end stages. They
delivered 40W each without a problem before the mains voltage dropped too
much for the amp to function properly.

Then I hooked up the amp to the 'hard' mains voltage (without a series
lamp), and still all seemed well. The end stage bias current was fine and
stable, and both end stages delivered the same signal without a load
connected.

But alas, with an 8 ohm dummy load connected, the very same end stage
fried instantly as the output power got past a few watts. I was actually
monitoring the oscilloscope and volt meters when it happened, and there
was no warning sign whatsoever. No sudden oscillation, no DC voltage
drift, nothing at all. One second the amp worked fine, an instant later an
end stage transistor shorted out, blowing the fuses. :-(

So before I replace the end stage transistors once again, I'd like to see
if anyone here has a clue ... I considered replacing both driver
transistors as well (Q711/Q712), but based on the original type numbers
(F-317-B respectively F-417-B), I can't find any suitable replacements.

A somewhat low-quality schematic diagram can be found he

http://web.archive.org/web/200106071...th/aa-1600.gif

Does anyone have an idea what might be the problem? Or how I could trace
down the problem without blowing yet another set of end stage transistors?
I'm particularly puzzled by the fact that the wretched thing only blows up
when loaded and at the same time connected to the 'hard' mains voltage
(230V AC). Even with a low-ohm load in series, the amp behaves just fine ...

Hi Richard...

Happens that I still have the whole manual for the aa-1640, but no
idea at all how much it differs from the 1600. I'm also on the
other side of the puddle (Canada), though I doubt that would make
much difference other than to the power transformer.

If you think it might be similar enough to help, I'll be happy to
scan a few pages for you.

Take care.

Ken

[Jim Land]

Richard Rasker wrote in
news
A somewhat low-quality schematic diagram can be found he

http://web.archive.org/web/200106071...l.freeserve.co.
uk/heath/aa-1600.gif

Does anyone have an idea what might be the problem?

Did you measure any DC at the output?

[Richard Rasker]

Op Mon, 20 Nov 2006 22:02:55 +0000, schreef Ken Weitzel:

Richard Rasker wrote:
Hi all,

I'm trying to fix an old Heath amp, the AA-1600.

[snip end stage keeps blowing up]

A somewhat low-quality schematic diagram can be found he

http://web.archive.org/web/200106071...th/aa-1600.gif

Does anyone have an idea what might be the problem? Or how I could trace
down the problem without blowing yet another set of end stage transistors?
I'm particularly puzzled by the fact that the wretched thing only blows up
when loaded and at the same time connected to the 'hard' mains voltage
(230V AC). Even with a low-ohm load in series, the amp behaves just fine ...

Hi Richard...

Happens that I still have the whole manual for the aa-1640, but no
idea at all how much it differs from the 1600. I'm also on the
other side of the puddle (Canada), though I doubt that would make
much difference other than to the power transformer.

If you think it might be similar enough to help, I'll be happy to
scan a few pages for you.

Thank you for the kind offer, but I found someone here willing to copy &
send me the complete manual - although he said that it probably won't give
more clues as to what goes wrong. Also, I'm quite experienced in these
matters myself (I very rarely have to give up on something like this), and
the schematic is simple enough. I just hoped that someone here would
recognize these symptoms and tell me the "golden clue"; from searching the
Internet I already found that the AA-1600 almost never breaks in any
serious way, so that wasn't succesful either.

Thanks again all the same, regards,

Richard Rasker

--
Linetec Translation and Technology Services

http://www.linetec.nl/

[Richard Rasker]

Op Mon, 20 Nov 2006 23:24:09 +0000, schreef Jim Land:

Richard Rasker wrote in
news
A somewhat low-quality schematic diagram can be found he

http://web.archive.org/web/200106071...l.freeserve.co.
uk/heath/aa-1600.gif

Does anyone have an idea what might be the problem?

Did you measure any DC at the output?

Both end stages produced a near identical DC voltage of 0.135 volts (one
was 0.132 volts, the other 0.136 volts), which seems harmless enough.
As far as I could see, this voltage didn't drift with time, temperature,
or load until the amp broke down.

Richard Rasker

--
Linetec Translation and Technology Services

http://www.linetec.nl/

[default]

On Mon, 20 Nov 2006 22:55:33 +0100, Richard Rasker
wrote:

Hi all,

I'm trying to fix an old Heath amp, the AA-1600. One day, it blew up one
end stage, and as the original PNP transistors 2N6609 are no longer
available here, I replaced the transistors with MJ15022/MJ15023 pairs.
Further investigation revealed that all four(!) 4K7/2W resistors
(R728/R729) were overcooked, and had resistances between 80K and
infinite. All other components seemed fine, so I thought I had the
problem nailed.

For testing, I put a 150W halogen lamp in series with the mains supply,
and everything still went fine. The end stage bias current seemed
completely stable, the amp operated on nearly its full mains voltage,
and both end stages behaved identical, both on low and high frequencies.
Of course, the supply voltage dropped when I applied a load on the output,
but even then, I could find no difference between both end stages. They
delivered 40W each without a problem before the mains voltage dropped too
much for the amp to function properly.

Then I hooked up the amp to the 'hard' mains voltage (without a series
lamp), and still all seemed well. The end stage bias current was fine and
stable, and both end stages delivered the same signal without a load
connected.

But alas, with an 8 ohm dummy load connected, the very same end stage
fried instantly as the output power got past a few watts. I was actually
monitoring the oscilloscope and volt meters when it happened, and there
was no warning sign whatsoever. No sudden oscillation, no DC voltage
drift, nothing at all. One second the amp worked fine, an instant later an
end stage transistor shorted out, blowing the fuses. :-(

So before I replace the end stage transistors once again, I'd like to see
if anyone here has a clue ... I considered replacing both driver
transistors as well (Q711/Q712), but based on the original type numbers
(F-317-B respectively F-417-B), I can't find any suitable replacements.

A somewhat low-quality schematic diagram can be found he

http://web.archive.org/web/200106071...th/aa-1600.gif

Does anyone have an idea what might be the problem? Or how I could trace
down the problem without blowing yet another set of end stage transistors?
I'm particularly puzzled by the fact that the wretched thing only blows up
when loaded and at the same time connected to the 'hard' mains voltage
(230V AC). Even with a low-ohm load in series, the amp behaves just fine ...

Thanks in advance,

Richard Rasker

Suspect the bias diodes. It isn't uncommon to get a bad internal
connection in one. Replace them, even if you have to fabricate your
own from discrete parts, don't believe a meter.

To check the diodes, it helps to have some current flowing ~100 ma and
then apply heat and cool to them. The voltage change should be smooth
with temperature.

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[default]

On Mon, 20 Nov 2006 22:55:33 +0100, Richard Rasker
wrote:

Any number of places where a bad solder joint can also cause this
problem.

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[[email protected]]

Hi Richard,

I have an have an AR-1650 Receiver that I have the amplifiers removed
from to troubleshoot. I pumped the preamp into a Carver power amplifier
and the repair slid down my priority list. It has a similar blown
output, so I'm interested in your plight and what you find.

I started capturing the power amp schematic into Electronics Workbench
MultiSim 9 to start playing with updated replacement components and was
testing the model when I quit. I had been reading Randy Sloan's
amplifier book and thought I would learn from it. I want to get back to
it as time allows.

I would be willing to share the model, although I don't know if their
trial software is crippled beyond the point that it is useful. I have a
license for the software. (if time-no tools and if tools-no time,
seems always the way).

I don't know the specifics of your amplifier, but this was 125 per
channel and had a huge power and very heavy linear power supply. The
Carver is 225 watts per channel and weighs a fraction of what only the
transformer weighs. I would like to understand that. I wonder if the
Carver is a 'switcher'.

Good Luck!

Regards,
Jim Shedden

[Mark D. Zacharias]

Suspect the bias diodes. It isn't uncommon to get a bad internal
connection in one. Replace them, even if you have to fabricate your
own from discrete parts, don't believe a meter.

To check the diodes, it helps to have some current flowing ~100 ma and
then apply heat and cool to them. The voltage change should be smooth
with temperature.


Ditto the diodes. Also, there has been a number of fake, counterfeit
transistors sold in recent years. If your MJ series did not come directly
from Motorola, Onsemi, or B&D Enterprises they could be fakes. You can
Google the topic of counterfeit transistors for an eye-opener. I've had
exactly this happen. New parts with NO other circuit problems, fail within
minutes when playing into a load.

Mark Z.

[Richard Rasker]

Op Wed, 22 Nov 2006 12:22:38 +0000, schreef Mark D. Zacharias:

Suspect the bias diodes. It isn't uncommon to get a bad internal
connection in one. Replace them, even if you have to fabricate your
own from discrete parts, don't believe a meter.

To check the diodes, it helps to have some current flowing ~100 ma and
then apply heat and cool to them. The voltage change should be smooth
with temperature.


Ditto the diodes. Also, there has been a number of fake, counterfeit
transistors sold in recent years. If your MJ series did not come directly
from Motorola, Onsemi, or B&D Enterprises they could be fakes. You can
Google the topic of counterfeit transistors for an eye-opener. I've had
exactly this happen. New parts with NO other circuit problems, fail within
minutes when playing into a load.

I have found a source for more suitable power transistors (the recommended
2N3773/2N6609), and I hope to receive a set of these in de mail today.
Also, I'll check the diode combo again for unusual current/temperature
drift, although I couldn't find anything wrong during previous
measuerements.

The trouble is that the amp seems perfectly fine when operating at 90%
of the normal mains voltage. Only when it's hooked up to the full 230
volts AC, trouble hits almost instantly.

But first I think I'll whip up a sort of electronic fuse to completely
detach the end stage from the power supply as soon as the voltage of NPN
emitter relative to the PNP emitter exceeds a few volts (i.e. the bias
current exceeds 5 amps). This way, I should be able to provoke the
failure without having to spend $15 in transistors and 15 minutes worth of
work every single time. This little tool is something I've been planning
to build for some time anyway, and this is as good an occasion as any to
build it.

Thanks for your reply and suggestions (and the others too, of course);
I'll let you know if I find the cause.

Richard Rasker

--
Linetec Translation and Technology Services

http://www.linetec.nl/