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Thread: Repairing and sealing burned PCB

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    Repairing and sealing burned PCB

    I have a Super Reverb reissue that appears to be a victim of a previous repair. The R60, R61,
    grids, the R62, R63 screens, and R70, R71 heater false center tap 100R resistors are all fried. I believe that high voltage arced on burned carbon that was left and not cleaned and sealed during the previous repair.

    Once I remove all the burned material, what product do I need to seal this area to prevent further damage?
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    Sealant, like conformal coating, is to prevent oxidation and to prevent arcing from component to component...but it will not help or prevent conduction WITHIN the fibreglass material.

    Your PCB needs careful investigation, both sides, to see if it's even a candidate for repair.

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    The burn is on the surface only. I believe I can remove all the bad material and preserve the solder paths. I think.

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    In that first photo, that bad circular burn is deep. Flip the board and look at the side with the traces. If any go near any browned-out parts of the PCB, there will be trouble.

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    Supporting Member The Dude's Avatar
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    It looks repairable to me (without seeing the underside). Just cut out the burnt board. It appears that the burnt part will be between traces.

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    Quote Originally Posted by The Dude View Post
    It looks repairable to me (without seeing the underside). Just cut out the burnt board. It appears that the burnt part will be between traces.
    It doesn't go all the way through, and luckily it is between the traces and solder pads. I've been up since 3 am so I'm gonna wait till morning to break out the Dremel.

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    My original question was, do I need a specific sealer or "paint" to seal the affected area, or is removing the charred material enough?

    I thought I remember a discussion awhile back about a particular product made for just that.

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    Old Timer Tom Phillips's Avatar
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    If you grind away all of the carbonized PCB material then there is no need to apply any sealant. It also does not even matter if you end up with a hole in the PC board.

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    Quote Originally Posted by Tone Meister View Post
    My original question was, do I need a specific sealer or "paint" to seal the affected area, or is removing the charred material enough?

    I thought I remember a discussion awhile back about a particular product made for just that.
    Super Corona Dope
    The source for the MG Chemicals Super Corona Dope P/N 4226-55ML is Mouser Electronics, their P/N 590-4226-55ML @ $11.06 55mL bottle. thank nevetslab for this info

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    Old Timer olddawg's Avatar
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    It can affect the integrity of the board. In the past I have used a piece of scrap board, even perf board, and 5 minute epoxy to patch an area.

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    Sometimes I'll leave a neat hole if it's between pads - say, where a resistor body burns through the board. Just like how Mesa does the resistor cutouts on power amp PCBs. If warranted I'll epoxy a piece of bare board over the area. In some cases I'll let a piece in so it's flush. It all comes down to how it's burnt, what's affected and how the components need to be reinstalled. I have some tiny eyelets (about 1mm bore when fitted) that I use to mount parts on the new piece of board.

    I always take pictures of the original board on both sides as a reference.

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    All very good information folks. Thank you.

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    Bent Member Chuck H's Avatar
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    I'm going to back what Tom said because it's important and warrants repeating. Grind away any PCB material that shows ANY discoloration. You may yet end up with a hole.

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    Old Timer Tom Phillips's Avatar
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    It's also worth noting that, when you grind away PCB material, you are generating fine dust composed of epoxy and glass particles so use appropriate protection. For a small job I wear a dust mask and hold the shop vac hose next to the spot being worked.

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    That also reduces the risk of carbon contamination elsewhere on the board from the dust. I always do a cleanup of the area first with acetone - sometimes the residue is slightly oily and a good deal of surface contamination can be removed to get to the actual burnt fabric of the board. Where there surface is burnt and grinds off down to good substrate I just flow epoxy in to level the crater, though over the past few years I've been using UV cure epoxy for this type of work as it gives near instant results.

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    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?
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    Last edited by Tone Meister; 08-27-2019 at 08:52 PM.

  17. #17
    Don't forget the joker g1's Avatar
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    1VAC in is very hot. Try injecting whatever signal gives 20mV at TP12 and then compare the numbers.

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    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)

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    Last edited by Tone Meister; 08-27-2019 at 08:49 PM.

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    Senior Member vintagekiki's Avatar
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    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)

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    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

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    Last edited by vintagekiki; 08-27-2019 at 10:16 PM.

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    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.

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    Supporting Member The Dude's Avatar
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    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?

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by The Dude View Post
    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.?.

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    Senior Member vintagekiki's Avatar
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    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.

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    Last edited by vintagekiki; 08-28-2019 at 08:26 AM.

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    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.

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  25. #25
    Bent Member Chuck H's Avatar
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    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.

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    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

    "Back to the amp. It makes horrible sounds when I play my guitar thru it... because I suck at playing guitar." Mike6158

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    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.

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