Fender Delux 90 amp

[tinpan]

Hello, I have this Fender Delux 90, type PR402. Does anyone have any history on this amp. It just went to a hum and there was a burning smell, As I was about to shut it down the audio came back on, I still shut it down. I have electronics repair background so I opened it up and can see nothing burned on the circuit board. The fact that it came back on indicates to me that the output transistors
are good, the only other thing that makes sense to me considering no visible charring on the circuit board is the transformer is burning. At the time when I was playing I was not pushing the volume, I was under 2 in the clean channel  with a Boss GT10 effects board hooked up with heavy OD distortion.
Any help is appreciated
Don

[Roly]

As you...

have electronics repair background

...you will know that...

a hum and there was a burning smell

...strongly suggests excessive current somewhere.

the transformer is burning

...is generally the last thing that happens in chain of failure, and it generally has a pretty characteristic hot varnish smell.  I would also expect the mains fuse to blow under these conditions.

Something that can get hot enough to smell but not show any signs later are the emitter power resistors in the power amp, typically 0.2 to 0.5 ohm, 5 watt or more.  Even so I would expect anything that got hot enough to produce a burning smell would leave some sort of trace of it overheating, so I'd go over the PCB again with a magnifier and bright light.

On the minimal information so far a possibility might be the bias going open for a short period, because excessive current is typically a one way trip, amps generally don't smoke then repair themselves.


First up, please find the circuit and post it here.

Second, disconnect the speaker, power up via your Limiting Lamp, and measure the voltage coming out of the amp.

The voltages on the mains supply rails would also be useful.