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Hi everyone, I'm having a bit of trouble with a Super Champ XD that I'm struggling to diagnose. After repairing an R79 failure, the 5V regulator is dissipating quite a lot of heat. I'll do my best to describe the repair and when and how the overheating happens but I'm fairly to new to amp repair so I feel I'm overlooking something because of inexperience...that or this amp has gremlins in it! A million thanks for any help, hints, or tricks on how to diagnose this issue!
The Repair
The amp came in to me with the issue of not having any output.
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. R4 and R25 looked a bit cooked as well but their resistance was okay - replaced anyway.
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 coat of conformal coating.
The board is slightly discoloured around 12V section of the power supply (under D34 & D35). P13 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 JJs. Power tubes are matched.
The Issue
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 board and with my IR gun I measured that the LM7805 and PCB around it were at 75C(167F). It seems to stabilize around that temperature but the hottest I've measured it was at 150C(302F). I replaced U10 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's plugged in.
It seems to only happens 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.
Is this just normal for this amp? Does the voltage regulator just run hot and I've been stressing out over a bad design?
Since then I've frozen the board to see if anything else is heating up, nothing. I've run through every component with a curve tracer, nothing. Everything appears to be running correctly and within spec but I feel I'm overlooking something obvious.
I'm having a bit of trouble reading rail to rail resistances. They seem to rise depending on the range. On the curve tracer I see a narrow vertical oval between all the rails.
I had a working theory that there was a shorted transistor in the power transient muting section but everything in that circuit checks out.
I've yanked the all the filter caps that I didn't replace but that doesn't seems to change anything.
Power transformer resistances:
P7 to P6 0.3Ω
P11 to P12 0.5Ω
P9 to P10 62.5Ω
P21 to P16 7.5Ω then either to P120 is 3.7Ω
P13 to P18 6.6Ωp
Here's some relevant forum posts on the R79 topic:
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 http://https://forums.fender.com/vie...p?f=13&t=65120
The Super Champ XD club http:// http://www.tdpri.com/threads...67992/page-100
Fender Super Champ X2 smoking resisitor http://http://forum.ampage.org/forum...d=vt&tid=35338
Here's the service manual and schematic:
Fender-Super-Champ-XD-ServiceManual.pdf
Fender-Super-Champ-XD-Schematic.pdf
And finally some pictures:
https://drive.google.com/open?id=1sf...aQy_OAl9_kiyjq
The Repair
The amp came in to me with the issue of not having any output.
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. R4 and R25 looked a bit cooked as well but their resistance was okay - replaced anyway.
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 coat of conformal coating.
The board is slightly discoloured around 12V section of the power supply (under D34 & D35). P13 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 JJs. Power tubes are matched.
The Issue
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 board and with my IR gun I measured that the LM7805 and PCB around it were at 75C(167F). It seems to stabilize around that temperature but the hottest I've measured it was at 150C(302F). I replaced U10 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's plugged in.
It seems to only happens 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.
Is this just normal for this amp? Does the voltage regulator just run hot and I've been stressing out over a bad design?
Since then I've frozen the board to see if anything else is heating up, nothing. I've run through every component with a curve tracer, nothing. Everything appears to be running correctly and within spec but I feel I'm overlooking something obvious.
I'm having a bit of trouble reading rail to rail resistances. They seem to rise depending on the range. On the curve tracer I see a narrow vertical oval between all the rails.
I had a working theory that there was a shorted transistor in the power transient muting section but everything in that circuit checks out.
I've yanked the all the filter caps that I didn't replace but that doesn't seems to change anything.
Power transformer resistances:
P7 to P6 0.3Ω
P11 to P12 0.5Ω
P9 to P10 62.5Ω
P21 to P16 7.5Ω then either to P120 is 3.7Ω
P13 to P18 6.6Ωp
Here's some relevant forum posts on the R79 topic:
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 http://https://forums.fender.com/vie...p?f=13&t=65120
The Super Champ XD club http:// http://www.tdpri.com/threads...67992/page-100
Fender Super Champ X2 smoking resisitor http://http://forum.ampage.org/forum...d=vt&tid=35338
Here's the service manual and schematic:
Fender-Super-Champ-XD-ServiceManual.pdf
Fender-Super-Champ-XD-Schematic.pdf
And finally some pictures:
https://drive.google.com/open?id=1sf...aQy_OAl9_kiyjq
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