Fender Super Champ XD - PCB HOTSPOT

[nicbul]

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?

[schmitt trigger]

Do a simple test;
with all tubes installed, measure the voltage across R79.

With the formula P = V^2/R, calculate how much power this resistor is dissipating.

Do the same with other power resistors.

Eyeballing the photo, it appears that the resistors are 2-watters. Correct me if I'm wrong.
Compare your calculated dissipation with that power.

[nicbul]

Do a simple test;
with all tubes installed, measure the voltage across R79.

With the formula P = V^2/R, calculate how much power this resistor is dissipating.

Do the same with other power resistors.

Eyeballing the photo, it appears that the resistors are 2-watters. Correct me if I'm wrong.
Compare your calculated dissipation with that power.

Right on.  I'll get it back in the chassis again and see if anything is out of whack.

Those are 2W resistors which on some guitar centered forums raised some questions.  Being that it's a common failure point, why wouldn't it be a 5W resistor?  Those resistors are marked FR on the schematic, I assume for fusible resistor, and are raised a fair bit off the board.

Anyways, I'll double check when I have a chance and I'll report back.

Thanks for the input!

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[schmitt trigger]

Very likely are fusible resistors.

You could replace them with 3 or 5 watters, but before you do you have to ask yourself, what else would burn during an overload?
Perhaps something that does catch fire.

[nicbul]

Very likely are fusible resistors.

You could replace them with 3 or 5 watters, but before you do you have to ask yourself, what else would burn during an overload?
Perhaps something that does catch fire.

That's correct, it's meant to fail of there's an issue with the output tubes.



I ran through all the test points with a 1kHz 50mV sine wave.  They all are within spec. 
R79 drops 380V to 373V so nothing funny there.

***EDIT - I'm gonna have to modify this post: I originally replied that the heat was coming most from the B+ quick connect. 
I negelected to realize that my IR gun was over the 12V Zeners and lm7505 and it turns out those are what's heating up.

Those are the components above the discoloured section of the PCB.

I'll do some more digging and report back...









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[nicbul]

Very likely are fusible resistors.

You could replace them with 3 or 5 watters, but before you do you have to ask yourself, what else would burn during an overload?
Perhaps something that does catch fire.

That's correct, it's meant to fail of there's an issue with the output tubes.



I ran through all the test points with a 1kHz 50mV sine wave.  They all are within spec. 
R79 drops 380V to 373V so nothing funny there.

***EDIT - I'm gonna have to modify this post: I originally replied that the heat was coming most from the B+ quick connect. 
I negelected to realize that my IR gun was over the 12V Zeners and lm7505 and it turns out those are what's heating up.

Those are the components above the discoloured section of the PCB.

I'll do some more digging and report back...









Sent from my LG-H933 using Tapatalk

So the heat is coming from the the 100ohm resistors before the zeners on the 12V rails(R87 and R88).  Under the test conditions,  they're seeing 20V and getting 80mA through them but heating up to 75C(this time measured with a thermocouple and emissivity corrected on the IR  gun).

Those were replaced...80mA doesn't seem very high for 6 op amps, led indication, and other devices doing their business.

Another funny thing popped up:  the signal from the DSP board has a fair amount of noise in it and is affecting the amplified signal. It appears to be white noise in the audible band. Even funnier,  at U2 pin 1 the signal is noisey and then splits off to the power amp and other side of that opamp.  On the other side of U2 at pin 7, crystal clear signal to the line out jack.  I guess I'll yank more components to see if they're okay.

This amp was left for dead and I'm beginning to see why...





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