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grommeteer 6th June 2012 09:12 PM

Fender sidekick reverb 30 noise problem
I have this problem with the Fender i am trying to fix for a friend,
The power amp makes a noise like a 2stroke engine. On the scope I see very short bursts of oscillation. I know it is the power amp because the noise continues if I short the input of the diff amp. It even continues after switching the set off as long as there is still energy in the caps.
There must be some kind of stability problem.
Two things i would like to hear from the community:
There is a capacitor Depicted on the board, but not soldered. C18. Any ideas why that is so?
Is there a known fix to this stability issue?
I will try to scan and post the schematic tomorrow.

Enzo 6th June 2012 11:58 PM

Are all your power suplies up to voltage and clean of ripple? Is your oscillation appearing on your power supplies?

My schematic is also not nearby, so I cannot look up C18 at the moment, but often times certain parts are found to be unnecessary, or are only installed for certain model configurations - in other words the same board is used in more than one model amplifier, so they do not install the part sometimes. That may or may not apply to your amp.

Does it look like that C18 was NEVER installoed? Or does it look removed?

grommeteer 7th June 2012 11:31 AM

I do not think this is a power supply problem. The voltage looks clean with very little ripple, the oscillaton does not show either. To be sure a I paralleled the original 3300F caps with two 4700Fs. No change.
C18 has never been in the amplifier. The value in the schematic is " - ".
My agenda for tonight includes posting the schematic and some more details.
I could replace some electrolytics (not my preferred method of repair..) and if nothing helps, I think I will have to open the feedback loop and see how the amp behaves then.

grommeteer 7th June 2012 05:21 PM

1 Attachment(s)
Here is the schematic. The actual hardware is a bit different. I found R30 to be 820R instead of 100R and the bias pot is a 4k7. So there will be less curent through Q5 and Q7. Looking at Q5. I should think it is a normal voltage amplifier. When I tried to estimate its gain using the formula A=Rcoll/Remit I got stuck. With Rcoll being a current sink, an extremely high impedance, the gain of this stage will be very high.
The nfb will take care of that - but how is that going to affect stabilty?
This is going to be a long journey, unless someone out there knows a short cut.

grommeteer 24th June 2012 06:39 PM

Just in case someone has been following:

I replaced C22 in the feedback loop with a new 100F and the permanant noise came down to cracks every few seconds.
Removing C23 made the amplifier (with its shorted input) silent. The frequency response ist not exactly flat now. I do not think this is going to show with the built in speaker and the intended use.
The amp is very susceptible to emc on the AC input. Even switching it on and off gave a very loud bang. So I added a mains filter and now the Fender is much better behaved. A real improvement- at least here in 230V country.
I would really like to hear if you have similar experience.

turk 182 6th November 2012 03:31 AM

i've encountered all kinds of problems with the 1/4 inch jacks in these amps with symptoms ranging from completly dead to microphonic squealing the oem jacks are real bunk and in this amp the speaker output is routed through one of these so watch out.consider jumpering the contacts that switch the speaker out or replace the jack altogether sometimes trying to find something that will mechanicaly fit can pose a real challenge
while your in there it wouldn't hurt to do the same to the input

grommeteer 6th November 2012 07:02 PM

Thank you turk 182.
I wont be able to make use of your advice, though. The Fender is back with its owner since June. He is quite happy with his amp. My patient had the noise problems withour any of the jacks. While I was hunting the fault I took the pcb out of its surroundings. Just power supply, power amp with shorted input an my crappy test speaker on the bench.
The signal I was trying to get rid of was in fact induced by a power line networking device I had in my home. I was able to measure this signal literally everywhrere in the house and garden with my DSO. But the fender was the only piece of audio equipment that would make it audible.
My very private little "emc lab" ;-).
I have started a thread in "everything else" about this powerLAN issue.

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