Fender Rumble 100 low voltage supply

[Mr_bibbles]

Hey y’all, got something in thats a pain. I think I have it dissed out but wanted to check with the Old heads if I’m on the right track:

Fender Rumble 100, schem attached at bottom. Low voltage supply being pulled down to -1/2 vdc instead of -16/16. Disconnected preamp board from the supply, it reads fine at supply diodes d35/d34.

So, something on the preamp board is pulling it down. I don’t see any transistors, just 4x tlo72cpu and a BA4560. What is the recommended way to track down this voltage loss beyond “it’s in the preamp”?

Thanks as always for your experience and generosity.

https://p13.zdusercontent.com/attach...b9uGeSLVHBos0g

[Dave H]

So, something on the preamp board is pulling it down. I don’t see any transistors, just 4x tlo72cpu and a BA4560. What is the recommended way to track down this voltage loss beyond “it’s in the preamp”?

Test the temperature of each of the op-amps in turn with a finger. If one of them burns off your fingerprint replace it.

[Jazz P Bass]

Looking at the schematic I notice there are a number of 0.1uf bypass capacitors on the IC power rails (C42-C45).
One or more may have gone short.

[nickb]

and adding...The resistors R101 and R102 run quite hot and the solder joints can fail. Look at them closely, remove the old solder, clean and resolder.

[Mr_bibbles]

Thanks y’all-

Dave, tried it but no luck! None of them get warm enough to be considered “hot to the touch”, they all stay about the same temp.

Jazz,

Good looking out! Measures the 4 ou mentioned, nothing was shorted but I shotgunned then anyways as I have 100x .1’s on hand. No luck

And nick, that was my first thought too, solder is good and with the preamp board disconnected, I get 16/-16 at the junction between the diodes and resistors. This must mean the supply is good, but a component is eating the voltage?

I noticed some more bypass caps similar to c42/45, I’ll do some more measuring. Is there a way to test an IC in circuit though when they all show bad supply voltage?

[nickb]

Thanks y’all-


And nick, that was my first thought too, solder is good and with the preamp board disconnected, I get 16/-16 at the junction between the diodes and resistors. This must mean the supply is good, but a component is eating the voltage?

Well, no... If the joints are bad they will be high resistance. That means with preamp connected it will read low and when disconnected it will read higher.

The way to check is to measure the voltage drop across the resistors and then divide by the resistor value to calculate the current with and without the preamp connected. Note, it's important to measure on the resistor leads, not the joints. While you are at it you should check the resistors are the correct value, unlikely but needs to be checked.