Fender Frontman 65R popping when switched on

[jonberrier]

I have a Frontman 65R on my bench that pops loudly for about 2 seconds when it is switched on and then works properly. I checked for DC on the output and there was none. Switched over to ac to see what was going on and there is about 45v on the output for about 2 seconds when the amp is turned on. It is after the preamp because I opened the pwr amp in circuit to see if the preamp was causing the problem. Any Ideas?

Thanks,
Jon B

[Enzo]

Just to be sure, unplug the amp, turn the power switch on, then plug it back in. Act the same? That eliminated any sparking as the switch was turned on. Some switches do that.

You may have a soft power supply filter. First resolder the main filter caps, just to see. After that, monitor the voltage on each as you power up to see if one of the supplies comes up slower than the other.

[mozwell]

are the +/-42VDC and +/-15VDC supplies stable during this time.
Is the current source transistor Q8 stable, measure TP23, is this stable during this time.
Lets divide & conquer, is it preamp, or power amp.
Short out R64 (input to the power amp), does teh problem go away, if so, it may be the mute circuit, or the preamp, if not, the problem is in the power amp.

[jonberrier]

Thanks for the help so far. I am going to get this one back out tomorrow and do some measuring on the power supply. I had already re-soldered some cracked joints and reflowed the filter caps for good measure. When I first looked at the schematic I suspected the mute circuit might have something to do with it because it comes directly after the pwr amp in (which I had open just to make sure it wasn't the preamp). I'll let you guys know what I find.

Thanks,
Jon B