Repairing and sealing burned PCB

[Tone Meister]

OK, the charred area(s) cleaned up well with acetone then lightly grinding with a Dremel. Replaced all the burned resistors, including all those in the doghouse. When inspecting that area, I found the 4.7K at R68 had been toasted in the process, so I replaced them all. All high voltages are now restored and the amp is stable with regard to the high voltages.

But there are audio problems now. The amp passes a low volume signal that is very distorted. In going through the signal path test points I've measured some signal voltages that are way out of whack.

TEST CONDITIONS
All tubes in
All controls set to "5"
Bias -52Vdc at TP36
NO SIGNAL inserted when testing DC voltages
1K @ 1.0Vac sine wave signal inserted into J3 (vibrato channel input 1)

All numbers are rounded to 2 places

Numbers in RED are of concern. Numbers in (parentheses) are expected voltages.
At V2
TP12 1.02Vac (20 mVac)
TP13 40.2 Vac (610 mVac)
TP13 2.19 Vdc

At V4
TP22 2.2 Vdc
TP24 1.63 Vac (31 mVac)
TP25 42.8 Vac (680 mVac)

At V6
TP31 0.04 Vac (2.4V pk to pk)
TP32 0.04 Vac (1.0V pk to pk)
TP33 57 Vdc (+93.6 Vdc)
TP34 0.003 Vac (5.5 Vac)
TP35 0.003 Vac (5.9 Vac)

Can someone shed some light on what is going on here?

[g1]

1VAC in is very hot. Try injecting whatever signal gives 20mV at TP12 and then compare the numbers.

[Tone Meister]

OK, adjusted the input signal to where TP12 (pin 2 of V2) was getting 23 mVac.


Numbers in (parentheses) are expected voltages
At V2A
TP12 23mVac (20 mVac) (signal input - pin 2)
TP14 36 mVac (610 mVac) (signal output - pin 1)
TP13 1.89 Vdc
At V2B
TP15 37 mVac (12 mVac) (signal input - pin 7)
TP16 32 mVac (690 mVac) (signal output - pin 6)

At V4B
TP22 2.0 Vdc (2.01 Vdc)
TP24 45 mVac (31 mVac) (signal input - pin 7)
TP25 32 mVac (680 mVac) (signal output - pin 6)

At V6
TP31 41 mVac (2.4V pk to pk) (signal input - pin 2)
TP32 42 mVac (1.0V pk to pk) (signal input - pin 7)
TP33 53.9 Vdc (+93.6 Vdc)
TP34 82 mVac (5.5 Vac) (signal output - pin 1)
TP35 36 mVac (5.9 Vac) (signal output - pin 6)

[vintagekiki]

Supposes that the measured voltages on TP1..TP4 have value as written on schematics.

When repairing before any measurement with a signal source (tone generator) should be measured all DC voltages on the tubes, and if they are not within the given limits, replace defective parts and bring the tubes into working condition.

Regardless on set the Bias -52Vdc at TP36, it is necessary to measure the idle current the output tubes V7/V8 (6L6GC), measuring the voltage at TP37/TP38 (33mVDC = 33mA)

Question
Is the signal distorted on both (normal and vibrato) channels?
If distorted is on both channels, the fault is in output stage V6 (12AT7), V7/V8 (6L6GC)


The sine wave test voltage on both channels has a value of 20mVAC measured at TP6/TP12

EDIT 190827
Orientation values of DC voltages

https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-CBS-45W-Super-Pro-Bandmaster-Reverb-Schematic.pdf

[Tone Meister]

Low volume on both input channels
Tried fresh 12AT7 in V6 phase inverter
The DC voltages at TP1 (454 Vdc)and TP4 (407 Vdc) are within specs.
Verified 33 mVdc bias at TP37 and TP38.
All DC plate voltages on V1 - V8 are within spec
B+ to OT (P10) = 454 Vdc
Bias voltage is reaching each power tube (pin 5)
Lifted D2, D3, D4, D5 diodes and all test good

I'll continue using the schematic you linked and verify all the DC voltages.

[The Dude]

I'm not sure the measurements can be trusted- maybe a meter issue or something the meter doesn't like. The reason being that, as you follow through the gain stages, your plate signal readings are low. Then, at the next grid test point, your reading is near correct again. Things just don't jive. That said, since both channels are low volume, I would think the problem is at or after the PI. Are your screen resistors good, or what voltage are you measuring at the screens of the output tubes?

[Chuck H]

I'm not sure the measurements can be trusted- maybe a meter issue or something the meter doesn't like. The reason being that, as you follow through the gain stages, your plate signal readings are low. Then, at the next grid test point, your reading is near correct again. Things just don't jive. That said, since both channels are low volume, I would think the problem is at or after the PI. Are your screen resistors good, or what voltage are you measuring at the screens of the output tubes?

THIS!^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Do you have a meter that is set strictly for something like 200mV perhaps? So it can't properly read anything higher.?.

[vintagekiki]

If all DC voltages are in the filter supply chain and at the output stage, within normal range and the sound is still low power, distorted (most likely no bass) check the output transformer.

The fastest way to check the correctness of the output transformer is to bring an AC voltage of the order 5-10V (I use 6.3V from some other amplifier) to the SPKR jack (J8) for measure its transmission ratio.
The transmission ratio depends on which value of Raa is the designed output transformer.
For 2 ohms it's about 43 - 54
For 4 ohms it's about 30 - 38

EDIT 190828
Question
If disconnect C25 (.001) from a 220k resistive network, and at C25 (.001) enter a signal from some source (tone generator or guitar) what kind is signal low volume and distorted or clean signal?

If the output stage is OK and entered the signal from the guitar through C25 (.001) should be obtained a 5-6W clean signal.

[Tone Meister]

OK, got back on the amp this morning (I'm a full time pro musician) and now have it all sorted out.

NOTE: I have Fluke 8050A and Tenma 1020 benchtop meters and know how to adjust range and how to read them.

• Found the 12AX7 at V2 had severe grid leakage, even though it had passed earlier tests. A new tube did away with the wacky signal voltages.
• Isolated the issue to the phase inverter.
• Found R54 (82k) PI plate resistor open and the .1/630 PI coupling cap was open as well
• The 100K PI plate resistor at R55 measured over 400K and the .1/630 PI coupling cap at C28 tested in the MILLIfarads! (B&K 885 for those who won't accept my measurements)
• the .1/400 at C26 also measured excessively high

Replaced all those parts and let the circuit run for a couple hours then final adjusted the bias and all is well. These repairs are in addition to the PCB repair above that included replacing R60, R61 (1.5k), R62, R63 (470R), and R70, R71 (100R), and the 6L6GC power tubes.

I believe if the previous repair was done right the amp would not have failed in this manner. Thanks for all your help fellas, you guys really do know your stuff.

Tom: thanks for the safety tip when dealing with the toxic PCB material.

[Chuck H]

Just wanted to say that it's unusual for a regular capacitor to fail open. Good on you for finding this. Preamp tubes don't often become grid leaky, but it has happened to me too and it makes for some strange troubleshooting. So you had two somewhat anomalous issues that could confuse symptoms and found them both Good work.

[Tone Meister]

Seems like the high voltage had a field day in places before taking out the power tubes and mains fuse. The one .1 failed open and the other measured excessively high capacitance. Odd thing about the 12AX7 was all preamp tubes tested fine initially. Unless I just happened to miss checking that triode first time around, but it definitely had major grid leakage.