G-K 700RB Power Amp Debug

[telecaster]

Schematic Attached. History: I replaced Q22, 23, and 24, which were shorted, along with R52, 53, 55, 56, 58 and 59 (open). Also Q7 (showed as shorted until Q22-24 were removed), Q15 (short), D8 (short) and R49 and R61 (burnt looking). All other diodes and transistors check fine with meter. After replacing all the above, amp powered up on light limiter fine with no speaker load. Bias set at 10mV per spec. When I attach a speaker load (still on light limiter), I get a hum through the speaker and the bulb glows fairly bright (150W bulb). I unplugged the speaker load and did some voltage measurements on the transistors that are accessible around the heat sink - they seem reasonable, but I will share them here when I get back to the shop.
One question I have: is it safe to power the amp on the light limiter and a load with a bigger bulb (300W)? I think with the 150W bulb the current is limited to a little over 1A
Also, should I have replaced Q28, 29, and 31 even though they weren't shorted?

[Jazz P Bass]

The 'hum' through the speaker indicates that you are not quite there yet with the repair.

Ideally, there should not be any voltage on the output buss.
AC or DC.

So you need to find out why there is a voltage there (the hum).

With the limiter connected start measuring base/ emitter voltages in the output section.
Something is off.

[telecaster]

Thanks Jazz P Bass, here are the measurements running on the light limiter (150W bulb), no speaker load

Q3 PNP	Q7 PNP	Q11 NPN	Q13 NPN	Q14 PNP	Q16 NPN	Q17 NPN
B -30.75	-1.127	-64.4	-0.557	+64.4	+1.202	+30.79
C -67	-30.78	-5.34	+0.556	+5.47	+30.73	+67
E -30.75	-0.525	-65.1	-1.124	+65.1	+0.599	+30.79

Q3 and Q17 are off (not forward-biased), might be ok?
Q7, Q11, Q13, and Q16 look to be properly biased (to me anyway)
Q14 is not quite biased on (only 0.3V), maybe ok?

Please let me know what I might check next

[J M Fahey]

Maybe you are turning amp ON with load attached and then remove it, always with bulb limiter inline?

Many amps don´t like that.

First turn amp ON with limiter without load, measure what you have, then connect load and remeasure, not turning it OFF in the meantime.

[telecaster]

Thanks J M Fahey - the measurements I took above were turning the amp on with NO load.
I followed your suggestions with very interesting results - power on with no load yielded similar measurements to above. Then I connected the load (still powered up), the light bulb did not glow bright, and the measurements were again consistent with the above. So I plugged in a bass and played at low volume and the amp worked fine!
If I power it up on the load limiter with the load attached, it lights the bulb bright and hums, so I agree the amp doesn't like that. So I am wondering if it's ok to try powering up off the light limiter - your thoughts?

[The Dude]

Check first that there's no DC on the output before connecting a speaker.

[Jeema]

I've worked on a 1001RB-II so I'm familiar with this circuit.

Q14 should be forward biased at all times. Check to make sure it and Q10 are not shorted with the power off. Also check that R31 and R33 are not open. My guess is that maybe Q14 has gone bad.

Also don't run the amp too long without the heat sink because both Q11 and Q14 will dissipate about 2.6W at idle (assuming they're working), which is more than they're rated for without a heat sink.

EDIT: The power capacitors C30, C38, C40, & C47 could also be causing increased power draw due to damage caused by the original shorting.

[telecaster]

Update: Actually Q14 is forward biased, 64.4 - 65.1 = -0.7 for a pnp, I measured right but miscalculated the bias math so I think that is ok too.
I took Jeema's advice and checked Q10 and Q14, both ok. R31 and R33 are both good (1K and 47.4). I learned to check the fuse resistors while working on a Backline600.

So I am left with no apparent component issues (except for maybe the filter caps Jeema mentioned). Does it make sense to bring it up on a Variac, with load attached, and watch the current draw ? Or do I change the filter caps first?

[Jeema]

Update: Actually Q14 is forward biased, 64.4 - 65.1 = -0.7 for a pnp, I measured right but miscalculated the bias math so I think that is ok too.
I took Jeema's advice and checked Q10 and Q14, both ok. R31 and R33 are both good (1K and 47.4). I learned to check the fuse resistors while working on a Backline600.

So I am left with no apparent component issues (except for maybe the filter caps Jeema mentioned). Does it make sense to bring it up on a Variac, with load attached, and watch the current draw ? Or do I change the filter caps first?

Variac is probably a good idea. If the capacitors are shot, you will likely have audible hum, uneven rail voltage, and/or 120Hz ripple on the output.

Also I recommend you reinstall and rescrew the heat sink before trying this or you risk thermal runaway with a load attached.

The heat sink also needs to be attached with an electrically isolating pad and you probably wrecked the original in the process of getting the heat sink off if your experience was anything like mine.

Come to think of it this amp is kind of a pain in the ass...

[Jeema]

Actually now that I'm reading your voltages more closely, did you check Q12? That went bad on the unit I was testing. It should be on at all times and the C-E voltage should be around 2.6V, maybe lower.

[telecaster]

Update: Jeema, I do have the heatsink attached, and I do know what you mean about the insulating pad.
I tried the Variac, and I can only get up to 25VAC or so before there is significant DC (couple volts) on the output.
I rechecked Q12 - it tests ok on the meter (not shorted). Here are the DC voltages on Q12: Emitter +1.253, Base +.577, Collector -1.17. So it's biased at 0.577-1.253 = -0.676 which should be on for a pnp.

[Jeema]

Update: Jeema, I do have the heatsink attached, and I do know what you mean about the insulating pad.
I tried the Variac, and I can only get up to 25VAC or so before there is significant DC (couple volts) on the output.
I rechecked Q12 - it tests ok on the meter (not shorted). Here are the DC voltages on Q12: Emitter +1.253, Base +.577, Collector -1.17. So it's biased at 0.577-1.253 = -0.676 which should be on for a pnp.

What do the collector voltages of Q11 and Q14 look like as it's coming up? They should be symmetrical around 0v. If they aren't symmetrical then there's likely something else going on in the bias part - maybe with Q8 or Q10.

[telecaster]

Collector Voltages of Q11 and Q14.
Measured as power comes up on light limiter with DC voltmeter (rounded to nearest volt):
Q11 starts around -19V, jumps to zero, then jumps/settles at -5V
Q14 starts around -9V, jumps to zero, then jumps/settles at +5V

[Jeema]

The bias portion of the circuit seems good from what you're saying. However you say there's still a DC offset on the output.

Have you checked the power transistors on the other side for shorts (Q25-Q27)?

What voltages are you seeing across R15 and R16?

[telecaster]

Jeema, I checked all transistors and diodes for shorts, all ok. I measure 2.8 volts across R15, and R16. I checked R24, R31, R33, R21, R22, R20, and R26 against voltages on the schematic, all checked ok.

Question: since I replaced Q22, 23, and 24 (shorted) and their emitter resistors (open), should I have also replaced Q28, 29, and 31 since they were in series with the failed transistors on the one side (negative supply)? They tested ok, maybe they were stressed and are causing the problem?