Fender SRM 6302 Trouble

[Clint_309]

Hey folks first post for me so i thought id make it a good one for you all. 

I was shopping around my local music stores looking for some old amp chassis that i could use for my LM3886 guitar amp project.  I found a shop that had a few "junk" combo amps and they also gave me a Fender SRM 6302 powered mixer.  The Fender was completely intacked and the store owner said he doesn't even remember why they had it.  I thought i would see if i could get it working again.  So i plugged it into a junk speaker i have and a loud buzz around 100hz or so (maybe a bit higher) was all it would do.  Until the speaker coil got hot enough to start to distort.  I disconnected the preamp section and the buzz is still there.  So my question is where should i start my diagnosis?  If i could get this thing working it would be a nice practice head for my friends band.

Thanks in advance for any and all help.

Clint

[teemuk]

My first suspect would be either failed output transistor(s) or failed power supply filter capacitor(s).

Next time you power it up, hook it up to a device that limits current (e.g. light bulb wired in series with the device in the mains line).

[Clint_309]

I removed all the Transistors from the circuit and tested them with my multimeter and they check out.  What would be the best way to test the power supply filter Capacitors?  And will they need to be removed from the circuit also?

Thanks

Clint

[Enzo]

This is solid state, so it doesn't need a speaker load, so no need to burn up speakers.  If there is DC on the output, then something in the amp circuit is bad, if not the output transistors, then open bias or bad driver/predriver or something like that.

See if the filter cap or caps has cracked its solder.  Otherwise, if the cap is not filtering there will be tons of ripple instead of smooth DC.  Set your meter to AC volts - or scope it -  and measure the DC voltages.  The reading should be pretty low for the higher voltages and almost non-existent for the 15v rails.  If there is like 20VAC where 30VDC should be, then I'd suspect a filter cap.
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