Fender Rumble 500 Power Supply Issue

[Bryan]

I'll check the values on R252-254 again.
Since the voltages on the rails are incorrect what voltage do you think I should I be looking for on the op amp outputs? I assume you are specifying the audio outputs of the op amps.

[Bryan]

I'll check R252-254 again. With the DC rails out of balance, what voltages should I be expecting at the op amp outputs........I'm assuming we are talking about the audio outputs.

[x-pro]

Yes. The audio outputs should be at or near zero.
If any of them has a different value, it should be unsoldered. After each unsoldering operation, check the supply voltage +/-15. At the outputs of 13700 and detector chips the voltages may be very different from zero. Do not touch them yet.

[Helmholtz]

Make sure to measure close to zero Ohm between grounding points of power supply and amp.

[g1]

Yes, ground points are critical as Helmholtz mentioned. Where are you connecting your black probe? You might want to recheck with black probe connected to control board ground.
Also, was the Rumble 100 you compared to also a V3? The older versions gain pot worked different like x-pro stated.
In any case, the Rumble 500 V3 gain should not kill the signal when turned all the way down. The Master does that job. The owners manual (attached) explains what the gain pot should do.

[Bryan]

I locked myself out of the forum for a bit.
Thanks for the manual. Not sure of the version for my working Rumble 100.
I have the control board ground strapped to the chassis ground with the black probe on the control board ground.
After some pretty whacked readings on the op amps I pulled U1-U11 and still got unbalanced supply voltages............until I pulled the fan circuit. Then the supply voltages came back to +/-15v.
I should have pulled the fan in the first place.
I still have some components to put back in. I'll let you know if I manage to get all of chips soldered properly. Should be fun. Then I can check the supply voltage with and without the fan circuit.

[nevetslab]

I was about to suggest it could be our old nemesis solder joint issues lurking about but lifting the fan load on the supply sounds like you re on the right track. I’d have to dig thru my past maintenance records to find when and what I had written in my service notes. I know there are a couple of the Fender rumble 500’s there and they have come thru my shop.

watching with interest!

[Bryan]

I'll try to get time tomorrow. As for my Rumble 100 turning off the audio, you are completely correct. On closer inspection the R23 does not turn the audio off although I thought it did.

[Bryan]

So now I have the clean channel working. No sound from the overdrive channel. Switch seems to work. LED comes on. The fan circuit still throws the voltage out of balance, so I have it disconnected for now.
Any suggestions where to start checking the overdrive channel?

[g1]

You had a cracked pot issue earlier. Often if there is one, there may be others so I would inspect very carefully. Usually due to mishandling at some point, whether in shipping or usage. Also Q3 and Q11 are doing switching to engage the OD circuit.

As far as the fan circuit, it is a bit odd. The fan circuit runs on the negative rail only. Normally I would expect a fault there to pull down the negative rail, but in this case it seems to be moving the common/ground instead.

[Bryan]

The cracked pots were unusual. They made contact intermittently. When I took them out the legs just fell off. The rest of the pots seem to be working.

[Bryan]

Is there a reason I only get signal on one side of C18? I get signal between R37 and C18 but not C18 and R43. I'm getting the correct voltage reversals at D13. and R42 is 100k.

[g1]

It may be due to 'virtual ground' at input to following opamp. Check at next stages, pins 1 and 7 of U4.
See note 6 of schematic for correct test conditions.

[Bryan]

Thanks for the reply.
That's what I am thinking. I get nothing at Pin 8 on U3 or Pins 1 and 7 on U4.
Do you think puling U3 and U4 would give me a signal at C18-R43 with the overdrive switch engaged? Maybe one of them is blown?

[Bryan]

Do I need to move my ground probe to Ground-C?