Fender Super 90, cuts out at high volume.

[m.edwards]

I've got a friend's Fender Super 90 (solid state) in my shop now, which I've fixed before (loud popping due to a faulty solder joint on a cap). Now it has a separate issue that I'm having trouble diagnosing. The amp performs well until it is overdriven, and turned way up (almost to max volume). At this point it cuts out completely for a split second at a time, but repeatedly. There are no popping or scratching noises, just a loss of signal. I've ruled out the power cable, the speaker and it's connections, and the input jacks. The LEDs are still lit while it cuts out, which leads me to believe the issue occurs after the power transformer. I can't find any burned or loose components, and the amp is not very old, so I wouldn't think any of the caps or resistors are worn out. I've blown off the PCB, touched up most of the solder joints, and used contact cleaner on all the pots and switches. I'm not very experienced, but so far these things have fixed most of the issues I've been confronted with. At this point I'm stumped and don't know what else to try. So...

If anyone has any ideas, I'd really appreciate the help!

Thanks

[52 Bill]

I'll have to look up the schematic for that amp, but in the mean time try and isolate the problem to the different sections. Is the signal cutting out in the power amp or in the preamp? What about the break jacks? Finding out where the signal stops is the first step.

If the problem is power supply related, the leds may not be a good indicator as they will stay lit until the voltage drops below the turn on voltage which is fairly low. And they are only monitoring one side of the power supply, so the other polarity could be cutting out and the side that they are connected to could be fine.

[g1]

Can you double check that model name? There doesn't seem to be such a thing as a Fender "Super 90" amp.

[m.edwards]

Sorry, Fender Deluxe 90. the schematic is here:

http://support.fender.com/schematics..._schematic.pdf

It turns out that resistors R144 and R145 are getting really hot really quick. they seem to break the solder joints, which would account for the intermittent loss of power. I resoldered them and it doesn't cut out anymore, but it keeps happening, so there is something wrong in the circuit before this. They're the big ceramic buggers, so i figure they're supposed to be able to handle some heat, but they get silly hot after playing for a few minutes at a comfortable volume. any ideas as to why this occurs?

[m.edwards]

Sorry, the amp is actually a Fender Deluxe 90. The Schematic is here:

http://support.fender.com/schematics..._schematic.pdf

It turns out there was a resistor that came unsoldered. It is the same one I touched up the last time I fixed this guy's amp. R144 and R145 seem to get dangerously hot very quickly. Within 5 minutes of playing at a normal volume they are untouchable. They are the big ceramic buggers, so i figure they can handle some heat, but this doesn't seem normal, so I figure there is something wrong with the circuit before this. Any ideas as to what could cause this overheating? I cleaned the heat sink and reapplied thermal paste, so it should be dissipating heat correctly there, doesn't seem to be any issue with that.

[eschertron]

Sounds like something downstream of the +/-16vdc is shorted. are the zeners or the caps in that part of the PS blown and shorting to ground? if not, it'll be harder to find as it could be anywhere in the low voltage circuitry. Do you have an IR heat gun (sensor)? scan the PCB to find the other hot spots. that'd be my humble suggestion.

[Enzo]

Those resistors provide current to the 16v zeners. So the first question is this: do you indeed have +16 and -16 at the end of those resistors? Zeners will hold the voltage from going higher than their rating (assuming they are OK) but they can't hold the voltage up if it wants to fall. I'd think if an op amp were shorted and dragging down on the 16v rails, then they would measure lower.

You could increase those resistors to 470 ohms, I;d think and save a little heat.

Mount new resistors up of the board a little ways for ventilation. Support them with a bead of silicone underneath or some other method. ALSO, the wire leads act as heat sinks. The additional altitude helps, but when you poke the wires through, don;t trim them off, scrape away the board coating to expose copper, and lay the excess resistor lead wire flat along the copper traces, then solder the emtire length of that excess wire. It will help dissipate heat.

The Hot Rod series of amps has a problem with the equivalent resistors. One brute force method is to mount a pair of aluminum power resistors on the chassis and run wires back to the board. They will never overheat. For example, Mouser stock 71-RH010470R0FE02 $2.50 each




Certainly interruptions in 16v rails will interrupt the signal. But so can other things. This is a combo amp, so speaker vibration can be a factor. Pull the chassis and sit it on the bench. Does it still cut out? Whack the end of the chassis with your fist, or use a rubber mallet or a hunk of 2x4. Does sending that little mechanical shock through it cause the sound to interrupt? If so, you have a loose connection.