The first part we need to make sure is good is the power supply, which is shown in the block diagram on page 9 (?)

Q901, Q902, and a relay in that area that is clicking.

If it is NOT the power good relay (Do the rails show 30V between them, and ±15V to ground before it shuts off?)

Does the Relay that clicks off seem to be in a different area than the overvoltage/undervoltage protection area?


I have attached your full size pictures with black magic. They are the size we like to see, though some quality was sacrificed to make them fit, most everything is still readable.

 

Didn't get full pictures in last post..

Click on thumbnails for 2500x1200 sized images (LARGE) of board.

He's even cleaned off the flux so everything can be seen clearly!

Looks like a standoff footie or MOSFET is missing in the lower left of the Image that shows full bottom of board, toward the output wire edge, about 2" from each.

There is a matching one a few inches above it.

 

interesting...my area of expertise..

 

Make sure that when you connect GND to pin-2 and +12V POWER to pin-1 that pin-3 is also connected to pin-1.

Please provide the voltages at the three points below, B,C,E.

The other test we should try is to take measurements before the relay clicks. How many seconds transpires before the relay clicks? Can you measure Collector (C) reading from the first diagram and Collector (C) reading from the diagram below immediately on applying power before the relay clicks?

And finally, for this time around, we will measure the DC voltages at the output of the amps just before the relay clicks.
Measure the voltage with respect to GND at speaker output R- (pin-1) before the relay opens. Do the same at speaker output L+ (pin-4).

Thanks.

 

it clicks after 3 seconds. from the 1st diagram the collector is (-.88v) before and after the click.
from the last diagram:

before click: B=12.26v C=12.9v E=12.95v

after click B=12.23v C=12.86v E=12.94v

as far as R- and L+ i get the same readings for both. meter starts at .002v, then for half a second jumps to (-4v) then back to .002v

 

Thanks for this. I will have to study this over and get back to you. It may take awhile (hours) since I am busy at the moment.

I had misunderstood the purpose of the relay. The clicking of the relay is normal. The relay on power up disconnects the speakers and then connects after the 3-second warmup. This is to prevent popping sounds from the loud speakers on power on.

Looks like the problem is in the +15V switching supply.

 

If you pull the fans out and shine a bright white LED type flashlight down the rows of transistors inside the U shaped heatsink, are there any where the printing is dark brown instead of white?

Any with cracks or chunks missing?

When did the amplifier last work, and what were the circumstances when it transitioned from working to not working?

 

If you pull the fans out and shine a bright white LED type flashlight down the rows of transistors inside the U shaped heatsink, are there any where the printing is dark brown instead of white?

Any with cracks or chunks missing?

When did the amplifier last work, and what were the circumstances when it transitioned from working to not working?

not seeing any brown or discolored writing. no cracks or chucks missing. it actually looks pretty clean and neat in the heatsink area. other than the usual little dust. i dont know when it last worked. i was in the right place at the right time when the tech school electronics lab was cleaning out old junk. this is the condition i got it in. so i know nothing of the history.

 

You said you have several meters. So I take it you do not have access to an oscilloscope. My guess is that something is wrong with the circuit from IC901, UPC494 Switching supply and the drive transistors. IC901 is getting +12V but the transistors are not switching. So we are either going to have to replace IC901 or some transistors. I would like to nail it down a little better than that.

 

interesting...my area of expertise..

Feel free to start expertizing at any time.

The one thing I did notice is the wire-wrap size wire isn't going to carry as much current as the trace that fried used to. Though those two patches may point to where the fundamental issue is.


--ETA: Just read in OP that you have an ESR meter. Hop around all the electrolytic "can" caps and ensure their ESR is:

• <= 100mΩ for >= 500μF
• <= 1Ω for 100μF to 500μF
• <= 10Ω for 10μF to 100μF
• <= 50Ω for 5μF to10uF
• <= 100Ω for < 5μF

The ESR meter shouldn't have enough voltage to forward bias any P-N junctions, and runs at a high frequency so inductors have a higher resistance than the caps. So the in circuit readings should be close, usually when they are off, they are WAY off (factor of 10 to 100 higher) from list above.

 

Ok, getting close. I put my bet that IC901 is good. Then is down to one of four transistors.
Q905, Q911, Q903 or Q909.

OK, I've narrowed it down to:

Q903 2SB1277
Q909 2SB1919

place your bets.

Measure the voltages on Q909.

 

After looking at top of board, two caps do look a bit off to me, both near each other in about the middle left of this image, in front of the heat sink and to the right of the relay.

They are the only two smaller electrolytics visible near the center (not at the bottom of picture)

One is sitting maybe an inch to the right from the front of the black box (relay?), it is sitting a bit high off the board and doesn't appear to have a perfectly flat top.

The other, kind of in the shade of the white piece above it, under the dark "splash" on the heat sink, just above the direct center of photo, appears to have a bulged/discolored top, but is sitting on the board fine.

http://forum.allaboutcircuits.com/attachment.php?attachmentid=36999&d=1322441846

 

I'd just tag along.

Let's see how good you guys are in repair .

If you guys give up, I will start fresh. This will help you learn a lot.

 

ok, so the 2 caps thatoneguy mentioned appear OK. the one does look a little buldged because the plastic shell with the writing comes up a little higher than the other one. both of these caps had equal ESR at 4.5 ohms they are 10v 100uf. the rest of the electrolytics tested just a fraction of 1 ohm esr.

the voltages on Q909 are

e= 4.43v
b= 4.62v
c= 12.42v

before i hear the relay click the voltages are the same +/- .02v

 

ok, so the 2 caps thatoneguy mentioned appear OK. the one does look a little buldged because the plastic shell with the writing comes up a little higher than the other one. both of these caps had equal ESR at 4.5 ohms they are 10v 100uf. the rest of the electrolytics tested just a fraction of 1 ohm esr.

Those are suspect, since they are rated for 10V, the ESR should be around 1.2Ω maybe 2Ω worse case. Expected ESR goes up with rated voltage, hence the ranges in my list above.

If you have a couple 100μF 10V or 16V low ESR caps handy, replace those two. Measure their ESR before installing to see what a new-ish cap of same rating measures for ESR.

This most likely will not cure/fix the problem, though it might, it will save a replacement once it gets running again for a couple months.

Have you found the Rail voltages from ground yet? after the relay clicks on? Is there a + supply and a - supply, or is the 13.8V supply the highest you can find when probing the board? There hould be 30V between the emitters of Q908 and Q907, if not, the problem is in the power supply leading up to those transistors.

 

Hmm, I was hoping Q903 or Q909 would have blown. Not sure now.

At this point I would unsolder Q903 and Q909 and test them with an ohm-meter.
Perhaps replace them both.

 

Hmm, I was hoping Q903 or Q909 would have blown. Not sure now.

At this point I would unsolder Q903 and Q909 and test them with an ohm-meter.
Perhaps replace them both.

Getting them out will be a PITA, they are on that upside down heatsink.

I'd like to find where we lose rail voltage before yanking it apart to a non-measurable configuration. (RIFAA hasn't had a shot at it yet)

Perhaps an open inductor?

I forgot what the fried traced lead to now, but I'lll guess they are pointing in the general direction of the shorted transistor.

 

I have my eye's on you fellas.

I will be completely occupied for a few days.

 

Q903 and Q909 should be behind these capacitors.
Correction, those long black thingies are Q903 and Q909, the ones closest to the transformer. Don't get them mixed up because they are a complementary pair, Q903 is PNP and Q909 is NPN.

Any chance of unsoldering these and testing them?


Before we pull those I have another test for you coming up.

 

Measure the voltages at B,C,E shown. We are going to compare with the other supply.

At this point it would be nice if we had an oscilloscope.