Harvard Reverb II power amp issue

[Apexelectric]

Having an issue trying to get the output section of this amp back up and running. The preamp works fine when I inject signal into another power amp from the pre-out jack. I’ve been through a handful of component changes to get it to this point and now I’m at a loss. Solid state signal tracing is not my strong point so bear with me.

I originally had an issue with the +15v supply loading down. Found and replaced a bad diode that rectified that issue and got the preamp back on line. I started tracing signal in the power amp with a probe at that point trying to find  where it’s being lost. Was getting signal through Q4 but not Q5.  Replaced Q5 and got signal through it again but dropped out after that. Tried replacing Q6 and lost signal at Q5 and Q4 now.

Am pretty close to giving up on this as I’ve got a lot of hours into it already and am feeling like I should just cut my losses but I thought I’d put it out there before I give up.

I’m a little cloudy on the signal path in this amp which a clearer picture might help so any light shed on this would be appreciated.

[PRR]

This whole power amp is an integrated unit; you can't debug stage-by-stage like most tube amps.

Basics: what are the supply voltages? What is the idle DC at the speaker terminal? How hot does the heatsink run, cold warm or hot? Can you measure mV of DC across R72 R73?

But the most likely failure is one of the unprotected output devices has melted-short, which cascades as failure of most other devices. Diagnosis is either critical thinking, lucky guess, or very tedious. I would look to see if one of the LM3886 amplifiers would fit the heatsink.

[Apexelectric]

Thanks for the reply.

Supply voltages have been as per what the schematic shows at +-15V and +-25V. Overall current draw at idle is only about 9.5W at 120V.  R72 has about 5mv and R73 has 79mv with the speaker attached, 15-20 on both if I unplug the speaker. 1VDC at the speaker terminals. Nothing seems to be running hot anymore since I found the bad diode on the 15V supply. The 180R dropping resistor was cooking. I replaced that too.

[shooter]

see if one of the LM3886

+1

troubleshooting anything, needs to bring out a sense of adventure, of overcoming, of conquest.   frustration, aggravation, tool throwing causes the gremlins to get big and strong and.........

[Apexelectric]

see if one of the LM3886

+1

troubleshooting anything, needs to bring out a sense of adventure, of overcoming, of conquest.   frustration, aggravation, tool throwing causes the gremlins to get big and strong and.........

That’s kind of what I’m using this repair as, a lesson. I want to get a little better versed in solid state. The amp itself isn’t necessary to get back in working order but it’s become somewhat of a challenge that needs to be overcome. Thanks for the assistance!

[PRR]

> 1VDC at the speaker terminals.

Replace Q4 Q5. 30V NPN jellybeans.

The amp is almost working but one of those two parts (you did not say polarity) is blown open/short.

[mresistor]

PRR   very interesting idea, just wire in an LM3886 .  http://www.alltronics.com/mas_assets/acrobat/LM3886TF.pdf  they're only $7-$10.

[Apexelectric]

I did replace Q5 but not Q4. I’ll try that and see what happens. What should I look for if that doesn’t fix it? I have an Atlas tester for diodes, transistors, etc. it seems to be able to ID bad components but I’m not sure how trustworthy it is for verifying that the components are still good.

[shooter]

PRR passed this to me to jog memory cell that hadn't seen light in 30yrs!
good explanation on operation, basically a simplified version of your PA

[Apexelectric]

Thanks Shooter! Will give it a read.

[Apexelectric]

Well I pulled Q4 and tested it. It tests good with an HFE of around 800. Doesn’t seem like replacing it will resolve this issue. So to recap on the transistors, Q4, Q5 and Q6 have all been either replaced or tested good. I have not reinserted Q4 yet. All 3 transistors on the heatsink, the Q200’s have all tested good except for the one in the bias circuit and it has been replaced.

[shooter]

All 3 transistors on the heatsink

were they insulated originally with a Mylar backing, and plastic screw sleeve?
I've seen failures there where you have collector ground short, since the case of a lot of power transistors are also the collector.

[Apexelectric]

One appears to be shorted to ground. Let me check it further and I’ll get back to you.

[Apexelectric]

Put it all back together and cleared the short on the heatsink mounted transistor. Fired it up and got a hum out of the speaker but a surge in current draw. It will idle at 10W, but spiked to 50W.  Found that the collector on Q9 was temporarily shorting. Once it cooled down the short cleared. With the speaker output turned off it idles at the normal current draw.

[shooter]

fwd progress is good 
keep plugging, you're on your way to SS amp guy !

[Apexelectric]

Sound like an issue with Q9 or would it point to something else causing the intermittent short on Q9?

[shooter]

 
could be heat breaking down the PN junction, isolation stuff, ....
Q9 n 202 operate "like" one transistor, I'd have to confirm, but I think they are a Darlington pair.  in old stuff, if it's a keeper, I replace both, with new isolators and grease.
in my old life, quick was better than overthinking, so once I got it to one rail, inside the PA section, boom, 8 new transistors, 30 minutes later verify, go home.

[Apexelectric]

I’m with you on the time vs cost of parts thing. There’s this part of me that needs to know exactly where the problem lies so that there is a clear answer. Shotgunning entire sections leaves too many questions in my mind as to where the problem lies so it’s a hard strategy for me to embrace even though it makes better sense. Ill have to try to get better at that way of approaching these kind of issues.

Have any experience with the Atlas component testers? Are the test results dependable?

[shooter]

There’s this part of me

and that's what you need, that "spark"!  for me everyday is Saturday now so I can do the same, I spent 3hrs putting paint on a canvas, I only used 1pennies worth by volume!!

the "best" tester in this case, a breadboard, you take the "suspect" part, breadboard a little circuit that puts it under similar conditions and see how it reacts, even better, do it with a known good part, then compare. 
Another way, you have 2 "identical" sides + rail side, - rail side.  Ohm, E-C, repeat on other rails transistor.  = yes/no?  Yes, B-E.  =y/n?  repeat.
But at the end of day, you'll know all the failed components, but you can only "guess" at which one caused the "other" 3 to fail.
In other words, a complex circuit, gets complex.

most P/N junction "devices" fail short, with enough current and a sufficiently oversized fuse will change the short to open.

I have no doubt you'll get it working - as a Dilbert sketch says "don't worry mom, He's got the KNACK"
 

[shooter]

I like this site for answering some of my unsure of operation.  might help you with troubleshooting in a logical way, anyway, fwiw

https://www.electronics-tutorials.ws/amplifier/class-ab-amplifier.html

[Apexelectric]

Thanks again, Shooter! More good info that I’ll have to digest.

I wound up replacing all of the trannies on the board with no luck. I get the same hum out of the speaker with the same spike in current draw. The excess current draw will drop out if I switch off the speaker. Q201-Q203 are connected via a connector and if I connect all of the trannies one at a time, only Q203 spikes the current. It is a new replacement. I’ve tried installing a new trannie in Q201 and Q202 with no change.

I might put this one up on the shelf for a little while and revisit it a later date unless something jumps out at you.

Thanks again for all the help and info.

[shooter]

I mighta missed, but did you replace the Ecaps?  It's not your problem if a caps shorting there shouldn't be any current left for the transistors.

I got nothing yet, but my track record today hasn't been good with tubes so...

before you box it up, can you measure the +/-25 (blk wire to - , red wire on +).  clip on the leads then power up with speaker just long enough to get reading.

are you seeing the current spike measuring R72 -73?

[Apexelectric]

I did replace the two main filter caps and ESR checked the others on the board.

I have a current meter wired in with my variac so I am able to monitor the overall current draw of the amp.

I’ll check those other items and get back with you.