Suggestions for no sound from effects unit

[dtone]

Reached a troubleshooting dead-end (for me), so hoping to get some advice. Initially thought it might be a muting circuit problem (see other thread), but no dice.

Problem: Reliable 1990s era Roland GP100 unit that recently stopped producing sound from any of the outputs. Functions properly in every other way.

Background and troubleshooting done thus far:

• Note: Link to service manual
• Powered on the unit one day and was working as normal. Couple days later, a high pitched signal noise began (link to sound file). Note that the sound worsens as I change to a higher gain patch on the unit. Normally, the unit is silent when powered on and idle.
• Tried moving to a different outlets/circuits, but little to no change.
• Noise was present with or without guitar plugged into input. Would sometimes go away temporarily if the input volume was quickly adjusted up/down a few times.
• Noise worsened and became more prominent over the course of a couple days.
• Unplugged unit and removed cover to visually inspect inside. Saw nothing obvious - No burn marks on PCB, broken traces or terminals, etc. Very clean inside the unit.
• Detached Input Board (containing input jack and input volume pot) from volume knob/front plate and visually inspected (it remained connected to Main Board). Nothing obvious.
• Reassembled the unit.
• Sound output stopped completely. No noise or signal. Dead silent.
• Swapped Input Board Assemblies with another fully working GP100 unit. No change. Bad unit didn't work, good unit continued to work. Replaced boards back into their respective GP100 units.
• Swapped Power Board Assembly and transformer with another working GP100 unit. No change. Bad unit didn't work, good unit continued to work. Replaced board and transformers back into their respective original units.
• Swapped Main Board Assemblies with another working GP100 unit. Bad unit now worked perfectly. Other unit now exhibited the same no sound issue. So, something on the Main Board is faulty. Kept boards in their new units.
• Inspected Main Board again and reviewed schematic. Noticed some slight white corrosion on a transistor (Q50). Per the schematic, this is part of the Muting Circuit. Cleaned the transistor with alcohol/q-tip. Still no sound.
• Measured Q50, Q46, and Q47 in the muting circuit with DMM on diode setting. All measured as expected, except for Q47 which indicated non-conducting B-E and B-C. I removed both Q50 and Q47 from the board and re-measured. Q50 was fine. Q47 was still non-conducting on B-E and B-C, but turns out it’s a BRT (bias resistor built-in transistor), so the DMM voltage was too low to measure.
• Replaced both Q47 and Q50. Still no sound.

Suggestions for possible causes and next steps for troubleshooting? Any feedback is appreciated.

 

[baconbadge]

You checked voltages everywhere on the main board? Assuming the service manual has voltages on the schematic. Don't start replacing things until you do that. I'd also look for/reflow any bad solder joints on all pots, jacks & switches. And if it's early 90s, could be a bad cap.

 

[dtone]

Copying sgregor suggestion here from the muting circuit thread...

Start with the res signal, and voltages on the 3 transistors in the muting circuit. It looks like the res starts at IC12 just above the muting transistors in the schematic. Not familiar with it, but am guessing it is a turn on delay for the res signal, which also goes to some of the ICs on the schematic. Good luck. View attachment 2506211

Voltage measurements (DMM negative to chassis ground):
Q50 (PNP): B: 4.91V, E: 4.7V, C: -14.93V
Q46 (NPN): B: -14.79V, E: -15.25, C: -5.18V
Q47 (NPN): B: -5.18V, E: -15.25, C: -15.25
Res signal out of IC12: 4.91V

 

[sregor]

[DELETE]

 

[dtone]

You checked voltages everywhere on the main board? Assuming the service manual has voltages on the schematic. Don't start replacing things until you do that. I'd also look for/reflow any bad solder joints on all pots, jacks & switches. And if it's early 90s, could be a bad cap.

Thanks. Definitely trying to check and find the issue before replacing anything. Checked muting circuit voltages and a few other areas. Any in particular you'd suggest?

Visual inspection didn't reveal anything except slight corrosion on Q50. Pots/jacks/switches good. Sound issue affects all paths - headphone jack, output-A, output-B, send-1, send-1, which seems like a centralized issue and why I initially suspected the muting circuit. Unit functions 100% otherwise.

 

[dtone]

Res needs to be low (less than 4 volts) to unmute outputs. Probably needs to be lower for other chips to run. (You can verify by checking a working unit.)
Caveat - because other chips are connected to the res line, it may be one of them causing problem, but not as likely)

I thought it was the other way around - Res high (~5V) to unmute and Res below 4V to mute? Copying your quote below from the other thread for convenience.

I was also able to check a working unit and got similar measurements. Voltage measurements don't change or fluctuate when tuner button is engaged (which mutes sound). Confused. Appreciate any insight.

Welcome to AK.
I'll give it a shot - the circuit appears to be controlled by a signal called "res" which I guess is reset. When res is 5 volts, Q 50 base is raised high enough so that it doesn't conduct. (assuming the small solid up arrow is 5 volts...) This will allow the voltage to base of Q46 to go down and shut the transistor off. This allows the voltage to the base of Q47 to go up, turning Q47 on and pulling the collector voltage low. That low voltage goes to q7,8,11,12,13,14,20,22 which keeps them from conducting, and there is output (not muted).
If the res line goes below 4 volts, Q50 will conduct, Q46 will conduct an Q47 will not conduct. Voltage at the collector of Q47 is allowed to go up, which causes q7,8,11,12,13,14,20,22 to conduct, muting the output. There are other resistors connected to the bases of Q13,14,20 and 22, so there are other routes to turn those transistors on and mute their outputs. on and mute different outputs.
Whew. my 2 cents

 

[sregor]

I thought it was the other way around - Res high (~5V) to unmute and Res below 4V to mute? Copying your quote below from the other thread for convenience.

I was also able to check a working unit and got similar measurements. Voltage measurements don't change or fluctuate when tuner button is engaged (which mutes sound). Confused. Appreciate any insight.

DOH You're right - I was right the first time.
As to insights - I would guess one of the big ICs is issue, but first I would look at the clocks (X1 and X2) and for activity on the inputs and outputs of the big chips. Maybe freeze spray or heat might get something going.

 

[baconbadge]

I don't know anything about the unit, so just giving general troubleshooting advice. The "main board" that works is going to have different voltages (somewhere) than the board that doesn't work. Just systematically go through the good board, check voltages, and notate them on a printout of the board layout diagram (probably easier to do it there than on the schematic). Then put the bad board in and do the same. That should lead you to the problem area.

 

[dtone]

DOH You're right - I was right the first time.
As to insights - I would guess one of the big ICs is issue, but first I would look at the clocks (X1 and X2) and for activity on the inputs and outputs of the big chips. Maybe freeze spray or heat might get something going.

No worries. Appreciate the feedback thus far.

Question for all - Any ideas on why voltage measurements are the same as a working unit and why the values don't change or fluctuate when turning the tuner/mute button on and off? I feel like I'm missing something fundamental.

 

[dtone]

I don't know anything about the unit, so just giving general troubleshooting advice. The "main board" that works is going to have different voltages (somewhere) than the board that doesn't work. Just systematically go through the good board, check voltages, and notate them on a printout of the board layout diagram (probably easier to do it there than on the schematic). Then put the bad board in and do the same. That should lead you to the problem area.

Thanks. I'll measure additional points to compare.

 

[cademan]

It might be a good idea to go through and resolder all connections and joints.

 

[dtone]

It’s now working!

I checked a few more voltage points per suggestion, but I wanted to check strategically/logically since there are a lot of points to check in this unit.

I was previously focused on the muting circuit since none of the outputs were working and seemed to be a centralized issue. This had me down a path. I decided to back-up and re-check my logic. The unit has a meter function to check/set input and output levels. I’d checked it before and saw zero level on both input and output. But, looking at it again, I realized that if only the muting circuit was the issue, then the input meter would still display a level even though sound would be muted. So, I revisited the input signal path in more detail.

Hit the schematic again and started checking additional voltages along the input path. Found an issue with a connector pin that should’ve been on the -14V rail, but was slightly positive and a few other pin readings were off. Measured several negative rail points along the audio input/output paths until reading were normal again. This led me to a shorted diode (Q42) handling the -14V. Swapped it out for a new one (along with a still good cap that was in the way) and all is well again.

I learned a lot about this unit. Thanks again for the help and info.

 

[baconbadge]

Nice job! Figuring out that kinda stuff is a wonderful feeling.

 

[sregor]