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

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dirtyb15

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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.
http://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!
 

dr pepper

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Doesnt sound good, maybe the tda8950 is shorting the supply rails.
 

alec_t

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What voltage do you get on U303 pin 6 (the switcher chip's Vcc)?
 

alec_t

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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 ........
 

dirtyb15

Member
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?
 

alec_t

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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.
 

Nigel Goodwin

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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?.
 

alec_t

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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.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
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.
 
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