Tweet
I'm currently repairing a Fender Super Champ XD that had a failure of R79 - this resistor drops the high voltage supply for the screens and is apparently meant to fail when there is a problem with an output tube.
R79 was glued to the filter caps next to it so it appears that it dissipated quite a lot of heat before it failed. It read open on my DMM. I replaced a bunch of components in the power supply section that looked like they sustained a bit of heat damage.
The trace from R79 to R47 lifted while I was removing the pool of hot glue that enveloped the board so that trace was epoxied down and the through holes for R79 replaced with eyelets. Any exposed trace I gave a coat of conformal coating.
The board is slightly discoloured around 12V section of the power supply (under D34 & D35, see pic). The neutral wire(P13) on the primary side from the transformer has some slight discolouration as well.
In the HV section, I've replaced R79, R4, R25, C38, C40, and C41.
In the 5V section, I've replaced U10, C56, and R64.
in the 12V section, I've replaced D34, D35, R87, and R88.
The stock tubes were replaced with a set of matched JJs.
After the repair, I powered up the amp to do some testing. It sounded normal, CH1 and CH2 work, and the effects work.
I felt some heat coming from the discoloured part of the board and with my IR gun I measured it at 75C(167F). It seems to stabilize around that temperature but the hottest I've measured was the LM7805 peaking at 150C(302F) but that high heat doesn't seem to happen regularly. I replaced the vreg but no change.
The heating happens with the tubes in or out. The output transformer windings check out okay and the heating happens whether or not it or the tubes are plugged in.
It seems to only happen when 5V and +/-12V are present. If just the 5V windings are plugged in, no heat. If just the 12V windings are plugged in, no heat but I read 2.3V on the 5V rail and 1.5V on the HV rail. Same reading if I inject +/-12V from my bench PSU and current draw is around 70mA.
I've frozen the board, run through every component with a curve tracer, reflowed solder joints, and yanked and double checked a bunch of components. Everything appears to be running correctly and within spec but I feel I'm overlooking something obvious.
The power transformer reads okay. Winding resistances and voltages are within spec.
The localized heating is messing me up as I haven't experienced this type of problem before.
Could the traces in the discoloured section of the board be damaged?
Here's some relevant forum posts on this amp, mostly talking about R79:
Fender Superchamp XD, no sound http://http://music-electronics-forum.com/t19899/
NO CHANNEL 2 on MUCH BELOVED FENDER SUPER CHAMP XD. http://http://music-electronics-forum.com/t33292/
Fender Super Champ XD - Dead https://forums.fender.com/vie...p?f=13&t=65120
The Super Champ XD club http://www.tdpri.com/threads...67992/page-100
Fender Super Champ X2 smoking resisitor http://forum.ampage.org/forum...d=vt&tid=35338
Fender-Super-Champ-XD_Schematic.pdf
R79 was glued to the filter caps next to it so it appears that it dissipated quite a lot of heat before it failed. It read open on my DMM. I replaced a bunch of components in the power supply section that looked like they sustained a bit of heat damage.
The trace from R79 to R47 lifted while I was removing the pool of hot glue that enveloped the board so that trace was epoxied down and the through holes for R79 replaced with eyelets. Any exposed trace I gave a coat of conformal coating.
The board is slightly discoloured around 12V section of the power supply (under D34 & D35, see pic). The neutral wire(P13) on the primary side from the transformer has some slight discolouration as well.
In the HV section, I've replaced R79, R4, R25, C38, C40, and C41.
In the 5V section, I've replaced U10, C56, and R64.
in the 12V section, I've replaced D34, D35, R87, and R88.
The stock tubes were replaced with a set of matched JJs.
After the repair, I powered up the amp to do some testing. It sounded normal, CH1 and CH2 work, and the effects work.
I felt some heat coming from the discoloured part of the board and with my IR gun I measured it at 75C(167F). It seems to stabilize around that temperature but the hottest I've measured was the LM7805 peaking at 150C(302F) but that high heat doesn't seem to happen regularly. I replaced the vreg but no change.
The heating happens with the tubes in or out. The output transformer windings check out okay and the heating happens whether or not it or the tubes are plugged in.
It seems to only happen when 5V and +/-12V are present. If just the 5V windings are plugged in, no heat. If just the 12V windings are plugged in, no heat but I read 2.3V on the 5V rail and 1.5V on the HV rail. Same reading if I inject +/-12V from my bench PSU and current draw is around 70mA.
I've frozen the board, run through every component with a curve tracer, reflowed solder joints, and yanked and double checked a bunch of components. Everything appears to be running correctly and within spec but I feel I'm overlooking something obvious.
The power transformer reads okay. Winding resistances and voltages are within spec.
The localized heating is messing me up as I haven't experienced this type of problem before.
Could the traces in the discoloured section of the board be damaged?
Here's some relevant forum posts on this amp, mostly talking about R79:
Fender Superchamp XD, no sound http://http://music-electronics-forum.com/t19899/
NO CHANNEL 2 on MUCH BELOVED FENDER SUPER CHAMP XD. http://http://music-electronics-forum.com/t33292/
Fender Super Champ XD - Dead https://forums.fender.com/vie...p?f=13&t=65120
The Super Champ XD club http://www.tdpri.com/threads...67992/page-100
Fender Super Champ X2 smoking resisitor http://forum.ampage.org/forum...d=vt&tid=35338
Fender-Super-Champ-XD_Schematic.pdf