Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Help with fender rumble 100 bass amp. Power supply troubleshooting?

Status
Not open for further replies.

dirtyb15

Member
Hello all. Last time i had a problem with a fender amp, i was able to fix it with help from members here. Hoping I can fix this amp as well. It is a fender rumble 100. The only schematics i have been able to find are here.
https://www.fmicassets.com/Damroot/Original/10002/Rumble 100 V3 Schematics-Diagrams.pdf

So when i plug it in, the two red led's just flash and I get no sound. I did not have much time last night, but was able to at least put a meter on VDD and VSS which are supposed to be 24 and -24 Volts. With the rear I/O board unplugged was was measuring about 16V and -16Volts. When I plug the I/O board in, it measures at 3 and -3 volts. Just wondering where to even start. Any help would be appreciated.
thanks!
 
Doesnt sound good, maybe the tda8950 is shorting the supply rails.
 
What voltage do you get on U303 pin 6 (the switcher chip's Vcc)?
 
Hmm. Should be 15V. I think the next thing I'd check would be the resistors defining the Vdd/Vss output voltage feedback fraction to the opto-isolators. If that fraction is wrong then U303 won't get 15V, so it may not work properly, so the Vdd/Vss voltages will be wrong, so the feedback will be wrong, so ........
 
Hmm. Should be 15V. I think the next thing I'd check would be the resistors defining the Vdd/Vss output voltage feedback fraction to the opto-isolators. If that fraction is wrong then U303 won't get 15V, so it may not work properly, so the Vdd/Vss voltages will be wrong, so the feedback will be wrong, so ........

So prolly have to pull these surface mount components then?
 
Well you could try checking them in situ. That won't give wholly accurate values, but hopefully should identify an open or short condition or a totally unreasonable value.
 
Hmm. Should be 15V. I think the next thing I'd check would be the resistors defining the Vdd/Vss output voltage feedback fraction to the opto-isolators. If that fraction is wrong then U303 won't get 15V, so it may not work properly, so the Vdd/Vss voltages will be wrong, so the feedback will be wrong, so ........

Sorry, but almost certainly not the case - it would be EXTREMELY rare for such a fault to occur.

The voltage is low and fluctuating because the PSU isn't 'running', either because there's an excessive load on it - so it's repeatedly shutting down due to over current - or the start-up circuits aren't working correctly.

C381 and C379 would be common components that cause this type of fault - however, it's rather vague about where the start-up supply actually comes from?, perhaps it's in the chip?.
 
Sorry, but almost certainly not the case - it would be EXTREMELY rare for such a fault to occur.
I bow to your greater experience, but thought it was something relatively easy to check.
 
I bow to your greater experience, but thought it was something relatively easy to check.

I've repaired thousands of SMPSU's, the faults that occur are relatively similar across all types - although a lot depends how catastrophic damage is after a failure - and this varies greatly in different circuits, even those using the same IC.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top