Fender Rumble 500 Power Supply Issue

[Bryan]

Hi, I have a Fender Rumble 500 with a DC rail issue. I have 15.8 +/- at the regulators but when I plug in the control board it drops to +14.8v and -17.6v. Something on the control board is pulling it down. The symptom is the first gain stage won't turn the signal completely off. Any ideas on how to isolate the component/s that might be causing this? Thanks.

[TomCarlos]

I found this schematic on another MEF post, where Jazz P Bass posted the file. I hope it matches your amp. Please confirm.

Rumble_500_V3_Schematic.pdf

[g1]

If the -15V rail is changing to -17.6, that is an increase not a drop. Recheck the (-) rail when properly loaded.
Not sure what you mean about the first gain stage. If it is a control called 'gain', it may not turn the volume off (assuming there is another volume control).
If it is the V3 like Tom posted, the 'gain' pot will not kill the sound when set to zero.

[Helmholtz]

Yes, gain pot R23 just varies the amount of negative feedback around the first opamp, not allowing for zero gain.

[Bryan]

Thanks for the replies. I do have the circuit schematics. I have 7 pages rather than 4 and it covers the Rumble 500, 100 and 150 as well. I can't seem to load them all here. If you need them I can try to find another way. I'll check for any differences that there may be.

You are correct that it is a jump on the negative rail to 17v but it is a drop on the positive rail to 14v. I happens when the control card is connected. Since the voltage should be 15v, (16v on the schematic), I suspect there is a component causing a problem.

Interesting that R23 does not bring the level to zero. I've never seen an amp that didn't turn down to zero. That is the problem I am having now. How do you think this amp achieves zero gain? It's not going to be the master gain. I have a few friends with Rumbles and they all turn to zero gain with R23 turned down. I think the incorrect rail voltage is keeping the op amp more open than it should be.
The amp wasn't working at all when I got it. R23 had cracked along the legs breaking the traces to the potentiometer.

Thanks for all of the help. With your ideas it will work again, I'm sure.

[Helmholtz]

The way R23 is wired in the schematic it is not intended to bring the signal level to zero.
Rather it should be used to adapt different instrument levels.
Why is that a problem as you still have the level control R68?

[Bryan]

R68 is for the overdrive circuit. I'll double check, but i'm very sure that does turn the volume completely off. R23 is the initial gain pot and does not turn the signal completely off. So when not using the overdrive the guitar is always coming through. I also have a Rumble 100. R23 turns off the signal on that amp.

I still think the faulty rail voltages are the symptom and the op amps are remaining slightly open. I've tested all of the diodes. Replaced U1, (TL074). Looked for damaged tracks or debris between components. A leak to ground?

Thanks again.

[g1]

The problem with the voltages on the control board is they have lost their balance. Maybe a grounding issue. They still add up to a difference of 32V, which is correct.
Can you confirm this unit is the V3 version?

[x-pro]

Check it out R252, R253, R254, C233, C234

[x-pro]

I also have a Rumble 100. R23 turns off the signal on that amp.

In rumble_100 the “Volume” control works differently. It is connected differently in the “Feedback” circuit of the operational amplifier. Its minimum position causes the latter to become a repeater of the input signal, which will be very small at the input. The magnitude of the input signal here is determined by the high-pass filter on U1-A.
In the rumble_500, the operational amplifier, with the minimum regulator resistance, will still have some signal gain at low frequency. Especially since the input signal comes to this amplifier without attenuation.

[Bryan]

I will confirm it is a V3 when I get back to the shop.
I will also compare schematics. I may be looking at the schematic for the Rumble 100.
I previously checked the voltages on TP48-51 but I will check out the values of R252, R253, R254, C233, C234 as well.
Thanks for the help.

[Bryan]

Schematics are the same. I can't see how the input stage is different between the 100 and the 500.

[Bryan]

Definitely a V3. Any suggestions on a sequence to fault find the voltage imbalance? All voltages are good on the Rear IO PCB until the Control board is plugged in.

[x-pro]

Sorry. I was looking at the Rumble 100 REV-A pattern.

[x-pro]

The values of these R252-254 could change upward, which leads to a current limitation for powering the elements and this can lead to misalignment.
It is also possible that there is a faulty operational amplifier. Check DC voltages at all microcircuits outputs.