Peavey Classic Chorus 130 - No Sound From Left Speaker

[Jazz P Bass]

The layout image from Post #5 clearly shows the gate pin.
It should orient the same as Q3.
Check the pinout of the component that you received.
Unpowered, there should be a lowish resistance from D to S .
The pinout diagram is from OnSemi. (Post #5)

[mrkeetz]

The layout image from Post #5 clearly shows the gate pin.
It should orient the same as Q3.
Check the pinout of the component that you received.
Unpowered, there should be a lowish resistance from D to S .
The pinout diagram is from OnSemi. (Post #5)

Q1 is definitely installed in the same orientation as Q3. Resistance between D & S is around 320 ohms. I'm thinking there's a short somewhere in that part of the circuit. Why it's affecting the other side is confusing. When using the doide check function on the multimeter, most of the diodes in the area of the inputs and the Lead Gain pots are activating both directions. Am I getting this wrong?

[The Dude]

"most of the diodes" doesn't tell us which ones, but in places in the circuit there are 2 diodes paralleled and oppositely oriented. If you test in circuit you are measuring both and you will read a junction either way with diode check because one of the two diodes will be forward biased. That's not a problem.
Do you have both your +15 and -15 supplies?

[mrkeetz]

"most of the diodes" doesn't tell us which ones, but in places in the circuit there are 2 diodes paralleled and oppositely oriented. If you test in circuit you are measuring both and you will read a junction either way with diode check because one of the two diodes will be forward biased. That's not a problem.
Do you have both your +15 and -15 supplies?

Thanks for clarifying that for me. Logic told me that about the diodes, but the little voice in my head...
As far as the +15v supplies, I have +15v on pin 8 of U1 & U2. I also checked the source legs on Q1 and Q3 for voltage and there's none present. I checked other 5461's on the board, and they all have around 13 - 14v on the source leg.
Thanks again for taking the time to help.

[Jazz P Bass]

Do you have both your +15 and -15 supplies?

"As far as the +15v supplies, I have +15v on pin 8 of U1 & U2"

What about the -15Vdc supply.

[mrkeetz]

What about the -15Vdc supply.

I have -15.8Vdc on both U1 & U2

[g1]

The gate pin on the Fet's is where you should see voltages change when switching.

[mrkeetz]

The gate pin on the Fet's is where you should see voltages change when switching.

Apologies G1. I guess I don't understand that. When switching from what? I'm trying to learn as I go along.

[g1]

You had mentioned switching the Lead/normal.
Sorry if I'm not up to speed, is the one speaker still dead, and engaging 'lead' kills everything?
Q1 gate appears to be connected to point 'Y', which I have yet to locate.

[mrkeetz]

You had mentioned switching the Lead/normal.
Sorry if I'm not up to speed, is the one speaker still dead, and engaging 'lead' kills everything?
Q1 gate appears to be connected to point 'Y', which I have yet to locate.

When I got the amp, there was no sound from the left speaker. I swapped the wires from the right side and confirmed that the speaker itself is working. I opened up the amp and started looking over the board. There are several resistors that have been snipped and re-soldered (obviously for troubleshooting). I found that (Q1) (2N5462) had it's drain leg snipped and removed. I ordered replacements (2N5461). and replaced Q1. That led to sound from both speakers, albeit at quite a low volume, heavily clipped and distorted.
As for the switch, it's the Lead/Norm button on the LEAD GAIN controls. When in the NORM position, there is no sound from the amp. This is happening before and after the Q1 repair.

On another note, would an oscilloscope (I have collected 3 over the past few years) or a Signal Tracer be of any use in diagnosing this?

[g1]

If you haven't used the scope already, there is a bit of a learning curve. The signal tracer will probably help once the problem(s) is narrowed down a bit more.
Q1 and Q3 are power-up mutes to avoid any big thumps when turning the unit on. Point 'Y' connects their gates together, and to C40, which is charged up by the +15V supply. The time it takes to charge that cap allows the supplies to stabilize before allowing sound through, thus avoiding a 'thump' when powered up.
You should check for positive voltage at Q1 and Q3 gates, probably in the +10 to +15V range.

Also try plugging signal into the 'pwr amp in' 1 & 2 jacks. It should be clear and undistorted but will take a fair bit of signal to be loud.
If that is good then the problems are likely in the preamp.

The lead/normal switch toggles point 'x' between +14 and -14 volts. That 'command' goes to the gates of Q2, Q7, Q12, Q13, and Q15. Check those gate voltages in 'lead' then in 'normal'.

[mrkeetz]

Wow. Thanks for taking the time to explain this to me g1. I think I'm learning more playing with this than what I've learned this past few years. As for voltage at Q1 and Q3 gates, it's not present. I even went so far as to snip the source line on Q1 to see if I got voltage at Q3, but it made no difference. Still no voltage present (I do have 15v at R86 and CR11), and I again have full sound from the one speaker. I also checked the resistance on board of R86 and it was 4.7k (supposed to be 470k). I disconnected a leg, and got the 470k then.
I also checked voltages at Q2, Q7, Q12, Q13, and Q15 in that order. In lead I got 31.8, 0, 0, 13.8, 0. In Norm I got 0, -12.5, -12.5, 0, -12.5.
I'll try plugging a signal into the 'pwr amp in' jacks and see what happens. Thanks again. I really appreciate it.

[mrkeetz]

I just wanted to check back in and say thanks for the advice folks. I finally got around to putting a signal into the 1 & 2 "pwr amp in" jacks. Pwr Amp in #2 was scratchy, and #1 was non-existent. I tapped the chassis a couple of times and could hear it trying to start. I pulled the board out and reflowed all the solder points at the jack. Put it back in. #2 worked fine, but still nothing from #1. So I got a wooden spoon and started tapping around the board until I located the problem. Bad solder joint on 2R112 resistor. Also retouched a couple of other dodgy looking solder joints while looking over things. Put it back together and now I have sound from both sides.
So now the only problem I have left is the LEAD GAIN Controls. When the button is in the NORM position I have no sound, but I do get sound when in the LEAD position.

[mrkeetz]

I'm posting another update to see if I can get some advice from anyone. So after another day of troubleshooting, I found that Q11 (J111) was switching from both polarities. (ie: when using the multimeter in diode mode, it switches as both N channel and P channel). I swapped it with Q7 (another J111) and the situation remained the same. I then started tracing it further back, and have discovered that when I removed CR32 Q11 tested as N channel only. I replaced CR32 and the problem came right back. Am I barking up the wrong tree here? Could Q11 prevent the amp from working in NORM? I also removed Q14 to see if it had any effect, but it had none. I've tested Q9 and Q10 for the foot switch and they are working as they should. I even swapped IC U5 with U2 to see if the chip was bad, but everything remained the same. What could cause Q11 to behave like this?
Thanks in advance for any help.

[Helmholtz]

Jfets are typically symmetrical, meaning that they perform the same with drain and source reversed.

Doesn't mean they behave as N channel as well as P channel devices.