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Old 12th June 2012, 11:42 PM   #1
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Default Proceed Amp troubleshooting / refurbish

Any Proceed Amp gurus out there?

Here's my problem: I'm lucky enough to own two Proceed Amp5 boxes. Unfortunately, of the 10 available channels, 4 of them no longer work. (They each trigger the protection circuit if left plugged in.)
I've read in a few places that these amps suffer from a bit of underdesign when it came to long term heat fatigue - and I suspect that is what is happening here.
I've checked the power supplies, they work fine.
I've pulled out every board, both good and bad, and looked them over carefully. I've noticed a couple of common problems / quirks:
  1. On every amp board there are severe indications of heat stress on several resistors. The symptoms are charred nylon spacers that hold the 3W resistors up from the board, cracking in the resisor, heat marks on nearby parts. The most consistently cracked is R506, (1K21, 3W IRC TO-3 metal film resistor) which sits next to a trim pot. The pot shows heat marks on the case due to the resistor nearby. The resistor's standoff nylon spacer is blackened at the top
  2. Q316 (ON Semi MJE 15030) reads as being shorted between pin 1 (base) and pin 2 (emitter) on all boards, good and bad. This seems wierd. Could the good boards be almost failing and this is an early indicator?
  3. I've read where some of the electrolytics fail due to heat prostration and short out as a result. I've found and measured a couple of those as shorted - but replacing them hasn't fixed the problem.
  4. On every board, the sixth pair of output stage transistors, and associated emitter resistors and base stopper resistors are left off the board. Not needed for stability at only 125W and would otherwise create too much heat?
  5. Several Caps (both elytic and film) and a couple of resistors not directly associated with the output transistors are also left off the board.
Other thoughts:
I've noticed via pictures on the web that the Proceed HPA amp boards look awfully similar (read that as nearly identical) with the exception of different values in all of the 3W resistors. Maybe when they cranked up the VA rating to put out more power they solved the reliability problem?

I have asked for a schematic and have been summarily dismissed by HK and one of the "authorized" repair shops; I guess it is super secret intellectual property - so be it. They need to make a buck, I understand.

So... I'm asking for help.

Any one have a list of parts that should be replaced, upgraded, or maybe changed in value, that would improve the reliabiltiy of these beasts? If so, you could always PM me so that we don't offend any of the dealers.
Thanks for reading.
Cheers.
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Old 13th June 2012, 07:17 AM   #2
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Location: Texas
Although I haven't worked on Proceed amplifiers, I would try to narrow down the problem. First, isolate the cracked/ stressed resistors and test them to make sure that their values are still correct. Then, try to remove from the equation any circuits that might be triggering the protection circuit. Finally, you can systematically work through each circuit from the input to the output or visa versa while testing every component. When you hit a problem, replace the component.

Did you properly remove Q316 from the circuit before testing? Not doing so will likely result in a bad reading.

Finally, if you didn't test these capacitors out of circuit, you will get erroneous readings. Replacing all old electrolytics is a good idea anyway if you amplifier is 20+ years old however.
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Last edited by techbiker; 13th June 2012 at 07:19 AM.
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Old 13th June 2012, 03:49 PM   #3
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Techbiker,
Thanks for the inputs.
I have replaced Q316 and all the elytics on one bad board. No satisfaction. A note: the removed transistor tested out fine with a diode tester once it is out of the circuit. That led me to replacing the elytics thinking one of them may be in the circuit with that transistor and it had shorted.
I am doing exactly as you suggest in trying to be methodical about testing.
I am first working with no power on the amp doing resistance measurements (to ground, to +V, and to -V) as a first pass - comparing a good board's values to the bad board values.
Without a schematic, and well over 400 components on the board, I am still hoping someone quite familiar with the amp will help out.
Thanks again for your thoughts.
JN
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Old 7th October 2012, 07:16 PM   #4
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Pyramid Audio, in TX, has repaired my failed AMP5 channels - none of them have had the issue recur. They are great, work by mail, etc. Pyramid Audio -- Audio & Video Repair Center
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Old 7th October 2012, 09:45 PM   #5
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Location: wigan
Hi JazzNut.

Testing components while in circuit can give you some missleading information as you have leant. Remember them elctolytic capacitors that read short in circuit , remove the new capacitor and mesure the resistance across the now vacant pads to se if the short is still there. if it is do the same with a working pcb as to compare .. if the pads read short on the faulty board it will at least give you a starting point and hopefully narrow down the faulty area.

Kind Regards Ian
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