Gallien-Krueger 700RB II powers on but no sound

[Tone Meister]

Amp powers on and fuse holds but no sound from the speakers. CLIP light at INPUT emits steady yellow, fan is not working, pressing the TUNING MUTE does not light its red LED. POWER/PROTECT light at POWER switch should be blue, but instead emits steady red and does not change colors under any conditions thus far.

0.00 VDC at speaker outputs
0.001 VDC on FX Send
0.002 VDC on FX Return.

Injected signal into INPUT, get signal at TUNER OUT on patchbay, but no signal from FX SEND jack.
Injected +4 signal into FX RETURN jack and get no sound from power amp to speakers.

On the power amp board, someone has replaced D7 at one point as evidenced by it standing off the PCB by the full length of its leads. Measured 87.0 VDC across D7 in one direction and -86.9 VDC in the other direction. Can hear audible amplifier noise through power amp to speaker when measuring D7 with amplifier on.

UPDATE: Received current schematics from GK which cover this board revision. Correct schematics attached

[g1]

How about the other power supply voltages, +/-40V, +/-35V, +/-15V ?

[Tone Meister]

Make sure to check out the updated schematics attached below.

OK, we have power supply voltages out of whack all over the power amp. Since I have a number of measurements, this is a great opportunity to learn to organize it effectively in a post so its easy to follow for those helping out. Feel free to tweak my data organization, or lack thereof.

We have +86V at R70 (expected +86V)
We have -86V at R71 (expected -86V)
We have +40V at R70 (expected +40V)
We have -40V at R71 (expected -40V)
We have +40.3V at R78 (expected +35V)
We have -35V at R75 (expected -35V)
We have -40.1V at junction of pin#1 of Q20, R41, R36, R40, R44 (expected +15V)
We have -38.6 at junction of pin#3 of U4, R73, and neg side of C33 (expected -15V)

DC MEASUREMENT POINTS

Component ___________________ Expected _________ Measured
Across R15 _______________________ 2.6V _______________ 0.013V
Across R16 _______________________ 2.6V _______________ 0.018V
Across R24 _______________________ 2.1V _______________ 0.000V
Across R33 _______________________ 1.55V _______________ 0.000V
Junction D7, pin#2 Q14, D9 __________ 4.9V ______________ -0.001V
Junction D5, pin#2 Q11, R25, D11 _____ 4.9V ______________ -0.001V

[The Dude]

Presumably, you have 40V supplied to U5 since it's there on R78, but check it right on the leg of the regulator. You could have cracked solder or a burnt trace. Also, what are the other voltages right on that +15 regulator (U5)? Obviously, there's something radically wrong with voltages in that +-15v regulator circuit. Also wondering if we have a burnt or missing ground there somewhere.

[Tone Meister]

Presumably, you have 40V supplied to U5 since it's there on R78, but check it right on the leg of the regulator. You could have cracked solder or a burnt trace. Also, what are the other voltages right on that +15 regulator (U5)? Obviously, there's something radically wrong with voltages in that +-15v regulator circuit. Also wondering if we have a burnt or missing ground there somewhere.

Finally a chance to get back on it.

On U5 I get +39.9 VDC on the input and -39.9 VDC on both the output and adjust

[Tone Meister]

I'm lost with the voltage regulation, so I definitely need some help sorting it out. Posting all the numbers again for simplicity.

U5 LM317 +15 voltage regulator
input = +39.9V
adjust = -39.9V
output = -39.9V

Spot Measurements
+86V at R70 (expected +86V)
-86V at R71 (expected -86V)
+40V at R70 (expected +40V)
-40V at R71 (expected -40V)
+40.3V at R78 (expected +35V)
-35V at R75 (expected -35V)
-40.1V at junction of pin#1 of Q20, R41, R36, R40, R44 (expected +15V)
-38.6 at junction of pin#3 of U4, R73, and neg side of C33 (expected -15V)

DC MEASUREMENT POINTS
Component ___________________ Expected _________ Measured
Across R15 _______________________ 2.6V _______________ 0.013V
Across R16 _______________________ 2.6V _______________ 0.018V
Across R24 _______________________ 2.1V _______________ 0.000V
Across R33 _______________________ 1.55V _______________ 0.000V
Junction D7, pin#2 Q14, D9 __________ 4.9V ______________ -0.001V
Junction D5, pin#2 Q11, R25, D11 _____ 4.9V ______________ -0.001V

[The Dude]

I'm thinking there has to be an issue with a missing ground- either burnt trace, or if you have a board out and something needs to be grounded. Otherwise, I'd expect C37 to be exploding. Can you check the ground side of R76 and make sure it is indeed ground? Also check values of R76 and R77. Not telling you anything you don't already know, but there is definitely something funky with your +15 regulator circuit.

[Tone Meister]

No, the board is still mounted fully. So you are thinking circuit and not the regulator itself?

I will go now and check the places you suggested and report right back.

[The Dude]

My logic, however flawed, is that if the - side of C37 was at ground potential and the + side had -40V on it, it would be exploded- or at the very least, really hot. This makes me think the - side is not grounded.

Edit: Could be cracked/cold solder in the circuit somewhere, also.

[Tone Meister]

My logic, however flawed, is that if the - side of C37 was at ground potential and the + side had -40V on it, it would be exploded- or at the very least, really hot. This makes me think the - side is not grounded.

Edit: Could be cracked/cold solder in the circuit somewhere, also.

Bingo. No continuity to ground from R76. Both R76 and R77 are within spec cold.

Gonna try to re-flow the solder from the top and see what that does. If not, then I'll pull the board.

[The Dude]

I would probably pull the board anyway and give that area a thorough inspection. There may be cold solder on the regulators, possible burnt traces, and God knows what else. You'll want to make sure it's reliable.

[Tone Meister]

I would probably pull the board anyway and give that area a thorough inspection. There may be cold solder on the regulators, possible burnt traces, and God knows what else. You'll want to make sure it's reliable.

R76 opened completely with just a small amount of heat from the iron. Pulled the board and found two compromised traces. So something is pulling current in a couple traces.

• One curled trace runs from the cathode of D20 directly to the input (pin 2) of U4. There is continuity, but I will reinforce the trace to be sure.
• The second compromised trace runs from R76 to the 4th pin of J20 with the brown wire. That trace is open at the end right near the J20 connector.
• Pulled both C38 and C47 15K/50V filters to get a better look and both caps are leaky at <10 volts. They're so leaky that I made sure by testing my Eico 950B with another known good cap to be sure I wasn't being lied to. Nope, both caps are leaky as a screen door
• BR3 bridge rectifier tests ok out of circuit

In looking at the schematic, I don't understand what happens in the circuit between R76 and the brown wire at J20. What would/could have taken out those traces? What's drawing the current?

[The Dude]

It seems the power supply as a whole took a hit. Speculation of course, but my bet would be a supply overvoltage condition. I would fix the obvious things you find and carefully bring it up on a limiter with no speaker load. Continue to do so until you verify voltages are good and no DC on the output.

Edit: Do check U5 and make sure it's not shorted.

[Tone Meister]

Wow, ok. I plan to pull those other two filters and check underneath them as well. Will probably replace all four of the 15,000uF/50V caps while I have it torn down. Those aren't that easy to find from what I'm seeing.

[The Dude]

Digikey has a bunch. You'll have to measure hole spacing, diameter, height, etc. to make sure they fit. Also check their 63V ones if you can't find 50V the right size.