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Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

Did my first PCB repair today and discovered an old repair

Discussion in 'Amp Tech Center' started by Bones, Oct 12, 2017.

  1. Bones

    Bones Doctor of Teleocity Ad Free Member

    Dec 31, 2005
    Luddite Island, NY
    I ordered a Blues junior kit from Fromel for my CS Two-Tone and it arrived today, so I decided to have a crack at it. My main goal was to replace the failing input jack and from there I would see how things went. The amp sounds good, so I wasn't too anxious to do a wholesale change of caps or resistors just yet.

    Having done minimal work on PTP eyelet boards , I was super paranoid about burning up the pads on the PCB, but once I got the old switch out and had clean pads, I felt better about it and wired up the new switch.

    After this success, I decided to reassemble the chassis and try it out and save the rest of the kit for another day. I was hungry and I learned that my work station was not really optimized to go onto the next part of the kit and I didn't want to push my luck.


    While I was dissembling the chassis for the switch repair, I did notice this resistor, apparently the original resistor blew out and was replaced with this one, soldered to the cut off legs of the old one. This seems like a slip-shod type of repair, next time I'm in there, should I replace it properly or is it OK?

    Also, what would cause a resistor to blow out like that with so much scorching around it?

    I'm kinda proud of myself for even seeing it and sort of knowing what had occurred :) There was a repair shop label on the transformer side of the chassis, so i knew I was probably going to find something.

    20171012_135516.jpg
     

  2. Preacher

    Preacher Tele-Afflicted

    Apr 17, 2007
    Big D
    I had a valve Jr that had the same type of blow out on a resistor. On mine it was someone plugging in a "powered" speaker feed from an amp into my input jack of my Valve Jr. The Epi made a horrible noise and then poof.
    I turned around and saw what had happened, and told the guy thanks for blowing up my amp. He was to plug the other 1/4" jack that came from the guitar into the amp, not the speaker cable that was powering the front of house speakers that had not yet been connected.

    As far as the fix, with a PCB that is probably what I would have done. It looks bad, but it is the safest way to not overheat the board and add the resistor back in.
     

  3. VintageSG

    VintageSG Tele-Afflicted Ad Free Member

    Mar 31, 2016
    Huddersfield, UK
    Personally, I do not like that kind of repair, but I understand why it's done. Taking a PCB amp apart enough to get to the solder side of the board can be a pain in the fundamental orifice. If the bodge method of cutting the leads and soldering to the leads is done, the leads should be formed into a hook or 'J' to provide mechanical coupling, not just abutted and soldered. Solder makes rubbish glue, solder is surprisingly not that good in terms of vibration resistance and shear strength either when used like that.
    'J' hooked then soldered is fine.
    As to what cooked both it and the board, depends what the resistor is connected to. It's quite beefy, so I'm guessing it's in the power stage somewhere. Valve went haywire and drew too much current?. Is it one of the bias resistors?. Another gripe is fitting power resistors, expected to dissipate heat, so close to the PCB. The things have legs!. Raise them off the board!. It isn't tricky to do, just put a crimp in the legs before insertion, but I guess the machinery costs too much to bother with these days.
     
    ponce likes this.

  4. Bones

    Bones Doctor of Teleocity Ad Free Member

    Dec 31, 2005
    Luddite Island, NY
    Just did a little research, turns out that R32 is a 2 watt voltage dropping power resistor. So I guess that would explain that. I'm really starting to like learning about this amp stuff. I can build a guitar in the dark, but this is a whole different thing.
     

  5. Bill Moore

    Bill Moore Tele-Holic

    On many of the PCB Fenders, many lift those, (and other), power resistors higher off the board to avoid the heat damage. I believe the newer incarnations are built that way.
     
    Bones likes this.

  6. ranjam

    ranjam Tele-Holic

    568
    Dec 29, 2015
    Canada
    Because a Blues Junior is notorious for running output tubes hot, a tube likely went south, and took out R32, which is a dropping resistor between the plate and the screen grid. What's more scary is the carbon left on the circuit board. Not a good repair at all.
    Clean up the carbon, and replace either R37 or R31. Try 27K for R37 or for R31. Which ever one you replace will bring up the bias voltage. I can't remember the last one I did, but I went from -15VD (or something like that) to -18VDC (or something like that).
    If you change R32, you can play with value to either run the tube cooler yet again, or a little harder (now that the bias is 'better') for an extra watt or two. It's 2.2K now, and you can try 2.7K to give the tube more relief, or 1.8K and get some more power tube drive. There's plenty of preamp tube distortion, but some people like power tube distortion. It's fun to experiment, so try it.
     

  7. Finck

    Finck TDPRI Member

    Age:
    52
    55
    Oct 11, 2017
    São Paulo - Brazil
    Sometimes that kind of repair is adopted simply to minimize risk of damages in the PCB, specially when the solder points and paths are too thin.
    I have damaged a few PCB's trying to put new components into the old holes.
    So, that's ugly, but safer.
     
    LudwigvonBirk likes this.

  8. Bones

    Bones Doctor of Teleocity Ad Free Member

    Dec 31, 2005
    Luddite Island, NY
    The kit I have has a 33k resistor for R31, what to you think about that?

    What can I use to clean up the carbon without damaging anything?

    Thanks
     

  9. ranjam

    ranjam Tele-Holic

    568
    Dec 29, 2015
    Canada
    33K is the stock value. Some people put a bias pot in their Blues Junior. If you don't want to drill holes or shoehorn in a pot, try 27K for R31. The bias voltage should go up a few volts, and run the tubes a little cooler. I believe you'll find here that the Blues Junior runs the 6BQ5s very hot. I doubt anyone will disagree.
    I start cleaning carbon from circuit boards with a simple 99% isopropyl alcohol (not the 70% kind) and a Q-Tip. Sometimes a shot of solder flux remover and a toothbrush gets the carbon off. If the board itself is burned, you may have to just scrape gently to get the carbon away from the fiberglass board. But it looks like there is just carbon on the board, and not a pit of where the fiberglass was scorched. You won't know until you try.
     
    Andy B likes this.

  10. LudwigvonBirk

    LudwigvonBirk Tele-Meister

    Age:
    116
    150
    Aug 26, 2017
    Madison
    For 20 millenia at minimum, there have been pockets of smart humanoids across all our cultures who have been obsessively working on executing the well-crafted splice. :)
     

  11. Bones

    Bones Doctor of Teleocity Ad Free Member

    Dec 31, 2005
    Luddite Island, NY

    Thanks, it's weird that Fromel would replace the stock bias resistor with one of the same value, that makes no sense to me at all.

    The carbon is definitely just on the surface, didn't see any signs of pitting.

    From Fromel's site...... "Cooler Bias - You are cooking the output tubes in your amp. This mod lets the amp run cooler without sacrificing tone or drilling holes in your PCB. Your tubes should live a long and happy life after this mod. I include a new bias resistor that sets the idle on the tubes to about 8.5w which is right where it should be."
     

  12. Finck

    Finck TDPRI Member

    Age:
    52
    55
    Oct 11, 2017
    São Paulo - Brazil
    And a larger bunch of lazy technicians... unfortunately...

    To clean the PCB carbon residues, isopropyl is really the way. If the q-tip doesn't do the job, an old toothbrush will do. But use it gently. Isopropyl is good to remove the excess of solder resin too.

    To clean the hole, in order to allow new component be installed, I usually use a solder extractor (not sure what's the English word, I'm talking about that device that looks like a syringe). A toothpick, can helps, eventually, use it from the copper side, never the opposite, while solder is melted. Then, try to put the terminal wire through the hole. If it doesn't go easily, don't force it, or the copper track can break. This case, do the cleaning again!
     
    Last edited: Oct 13, 2017

  13. waparker4

    waparker4 Doctor of Teleocity Ad Free Member

    Nov 9, 2011
    Philadelphia, PA
    In my experience, these Fromel mods can be pretty old. When I was working on my Pro Jr., I did not buy a kit but found the mods on a forum. The mods were based on the Pro Jr. V1, and some of the changes were eventually adopted by Fender into I think V3 of the Pro Jr. It could be the case that you have a mod designed for an old Blues Jr. version that you are applying to a newer version.
     

  14. cap47

    cap47 Tele-Meister Silver Supporter

    433
    Jul 27, 2014
    bushnell, fl
    I would change that resistor and solder it in correctly instead of half assed. So what if you have to flip the board. If you know how to solder, it should go smoothly. A solder sucker before you put the new one in. I myself would put a Bias Pot in instead of a resistor probably a 50K Bias pot. You can hook up a meter to the wiper terminal and the end terminal and set the pot at 33K which is the original resistor value before you even solder it in. That gets you close to where it will be finally set and will not red plate a tube. You won't have to try different resistors. You can then adjust the Bias any time you want .
     
    asnarski and Bones like this.

  15. Bones

    Bones Doctor of Teleocity Ad Free Member

    Dec 31, 2005
    Luddite Island, NY
    I have an old green board USA made one.
     

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