Fender Super 60 Red Knob Diagnosis (Blowing Fuses)

[Matt D]

I agreed (reluctantly) to take a look at my buddy's circa-'89 Fender Super 60 red knob era amp. I guess this amp was their attempt to compete with Mesa-Boogie. Its a fixed bias amp (with no bias control adjustment!) with 2 6L6gc's and Bassman-looking iron in a Princeton-sized cab. I replaced the fuse a couple of times and tried switching on the amp with the power tubes both in and out. The amp comes on with a new fuse, but as soon as I bring it off standby the fuse blows, even with the power tubes out. Any clues about where to start? Getting the PCB out of this thing is a B@#ch! The PCB is put in components down and I tested the power resistors and screen grid resistors from the back-side of the PCB.

[rdjones]

http://ampwares.com/schematics/super_60_rack.pdf

Is the blowing fuse "F4", the HT fuse ?

I'd be looking first at the bridge rectifier, and then the flyback snubber diodes (on the power tube plates).

rd

[Matt D]

Thanks for the reply. It is blowing F1, the 2.5A fuse on the mains and the only one accessible from outside the amp.

[rdjones]

Thanks for the reply. It is blowing F1, the 2.5A fuse on the mains and the only one accessible from outside the amp.

The schematic indicates F4 may not be present on all versions.
In which case the mains fuse would blow if any of those diodes shorted.
(typical diode failure mode is a short)

rd

[Matt D]

Cool. I'll open it up tomorrow and test the diodes for shorts.

[Matt D]

http://el34world.com/charts/Schematics/ ... _60All.PDF

Here is a parts list and layout as well as the schematic.

[Stevem]

Even circa 1989 amps I have gone thru have had bad power supply filters, even if they are not bad yet, while your in the boiler room change them out!
In face change out the first 2 factory 100 uf caps for 220 uf and be sure to check the diodes on each primary leg of the OT.

[Matt D]

Working on getting the PCB out. It sucks. There are these little ribbon wires that connect the separate control PCB to the main PCB. As I try to work the solder-less connector terminals that that tie everything together in this amp loose these ribbons are disintegrating. Looks like I have to remove the front PCB too and replace the ribbons with actual wire. Anyone know if I can replace diodes 101-108 with 1n4007s?

[Stevem]

Yes 1n4007 is fine.

[Firestorm]

D101 and D102 are flyback diodes on the ends of the OT. If you use 1N4007, you should put three in series.

[Matt D]

Thanks for the replies. I'm going to try to rule out the power transformer today and then order some parts.

[vibratoking]

Those brown burned spots on the PCB don't bode well for the future. I wonder what the glass transition and decomposition temperature for the those boards is. Anyway, good luck with the repair. It seems the guys here have you on the right track.

[Matt D]

Anyone have any tips for checking the PT? I can unhook all the leads from the board I believe. The fuse only trips with the HT on so I'm assuming I could potentially have a shorted HT winding? The brown spots are definitely not a mark of quality!

[rdjones]

D101 and D102 are flyback diodes on the ends of the OT. If you use 1N4007, you should put three in series.

Correct, according to the parts list D101-102 are 3KV rated.
The idea is to damp any flyback on the OT primary without shorting.

reddog

[Matt D]

The power transformer is good. Time to order some parts.