1981 Roland Jazz Chorus 120 CH1 Problems - Solved

[phatt]

I'd Check that R61 (68k) is in fact going to ground.
The Base of Q11 needs a common (ground) reference, otherwise the circuit won't work.
Verify Common at R68 also.
Phil.

[substatica]

I'd Check that R61 (68k) is in fact going to ground.
The Base of Q11 needs a common (ground) reference, otherwise the circuit won't work.
Verify Common at R68 also.
Phil.

R61 measures 67k with 0.2 ohm to ground, R68 measures only 500 ohms when it's a 1k resistor so I'll swap that out -- again 0.2 ohm to ground on the ground side. I swapped out R68, no change.

What signal do you have at the collector of Q13?

No signal at Q13, the scope loses trigger and the line drops off.

[substatica]

Seems like it may indeed be a grounding issue. When the pots are secured to the chassis the behavior changes, and that strikes me as something that shouldn't happen if the board is properly grounded.

[phatt]

Seems like it may indeed be a grounding issue. When the pots are secured to the chassis the behavior changes, and that strikes me as something that shouldn't happen if the board is properly grounded.

I agree it is likely part of the issue,,,When I said check ground/common at R61 & R68 Check they are all connected to each other. You should have continuity between all points that go to common.

Then,,, check DC voltages at points shown.

As you have had bad connections, drifting resistors then check resistors at those nodes for drift or open circuit.
R63 & R65 supply power to the emitters of Q11 & Q12.
Obviously if no voltage at emitters then the amp won't go.
Use the working amp as cross reference.

[substatica]

I agree it is likely part of the issue,,,When I said check ground/common at R61 & R68 Check they are all connected to each other. You should have continuity between all points that go to common.

Then,,, check DC voltages at points shown.

As you have had bad connections, drifting resistors then check resistors at those nodes for drift or open circuit.
R63 & R65 supply power to the emitters of Q11 & Q12.
Obviously if no voltage at emitters then the amp won't go.
Use the working amp as cross reference.

I'll do that tonight, yes, I've been comparing with the working channel which is a great resource.

[substatica]

I get 0.6 VDC coming out of R1, 16 VDC out of Q1 and 25 VDC coming out of Q2, not the output Q1/Q2, the 2SK117's near the inputs, that's very early in the signal path no?

Replaced all the electrolytic caps on that board, C32 was definitely burnt, but replacing it (and the others) hasn't changed the behavior.

R61 and R68 are connected and have 0.5 ohms to ground, same on the good channel.

[phatt]

Please forget the input section as it obviously works.
Just fix the power amp *First*,,, don't mess with other stuff until you have that pwr amp working otherwise it's all just way too hard to follow. xP

The idea is to find the problem ,,Then replace as needed, just replacing parts will just complicate the situation.
Use a radio output or Signal inject into the *Main in* socket thereby bypassing any other issues that may cause confusion.

If C32 (power amp) is burnt then check the voltage at ***R65, C32 and R63. Then check you have voltage at the emitters Q11 & Q12.
Cross reference them with the working amp.

If voltages are ok and still no signal at base of Q11 then check the pinout of the transistors,, you may have replaced them without checking the pinout and that will stop it dead. 8|

All done with light bulb until you have it stable.

Please don't change anything until you establish what voltages are present in the circuit. :trouble
Phil.

[substatica]

R65 reads 30 VDC, C32 reads 0 VDC on the negative side and 30 VDC on the positive, R63 reads 6 VDC on one side and 30 VDC on the other.

I've also verified that the pinouts are the same Q11 and Q12, (replaced 2SA970GR with 2SA970GR) they're in the correct orientation and their connections to the next components in the circuit are good. Could it be that Q11 went bad as soon as I replaced it due to the bad cap on C32?

[phatt]

Yeah,,, BUT How do those readings compare with the working amp voltages?

I assume you mean that the 6 Volt side of R63 is the Emitters of Q11 & Q12?
6VDC on the emitters seems high,, I'm guessing it should be less that 1Volt.
If Q11&12 are working you should have about ~100mV on the bases of those two transistors.
Phil.

[substatica]

Yeah,,, BUT How do those readings compare with the working amp voltages?

I assume you mean that the 6 Volt side of R63 is the Emitters of Q11 & Q12?
6VDC on the emitters seems high,, I'm guessing it should be less that 1Volt.
If Q11&12 are working you should have about ~100mV on the bases of those two transistors.
Phil.

CH1 (bad)
R65 30 VDC
C32 0 / 30 VDC
R63 6 / 30 VDC

CH2 (good)
R65 31 VDC
C32 .001 / 32 VDC
R63 .8 / 30 VDC

[phatt]

Then Q11 & Q12 are likely the problem.  Check the pinout, the replacements might be different to the originals.
Phil.

[substatica]

Then Q11 & Q12 are likely the problem.  Check the pinout, the replacements might be different to the originals.
Phil.

Unfortunately the pinout is the same and the transistors are all good (old ones and new ones). Haven't found any additional failed components on that board.

[phatt]

Then print out that power amp schematic you posted and write down all the voltages from the working circuit and then cross reference those voltages with the faulty circuit. Somewhere in that first section you have a failed part or broken track or even a cold solder joint.
Hairline cracks in the copper tracks can be right next to the solder and are hard to see. Bright sunlight and a magnifying glass will help.

Could even be a tiny fray of wire bridging a track or solder point which can't be seen by eye, I've had that happen before.
A good gentle scrub of the track side with a tooth brush can dislodge those offending little critters. :tu:
Phil.

[substatica]

Then print out that power amp schematic you posted and write down all the voltages from the working circuit and then cross reference those voltages with the faulty circuit. Somewhere in that first section you have a failed part or broken track or even a cold solder joint.
Hairline cracks in the copper tracks can be right next to the solder and are hard to see. Bright sunlight and a magnifying glass will help.

Could even be a tiny fray of wire bridging a track or solder point which can't be seen by eye, I've had that happen before.
A good gentle scrub of the track side with a tooth brush can dislodge those offending little critters. :tu:
Phil.

I've done the toothbrush thing, but yeah there's been a lot of debris with all the de-soldering. I'll try more voltage comparisons as you suggest.

[substatica]

Here are the voltages in the area, the bad channel in red, the good channel in green.

Update: Added more measurements.