How do I go about diagnosing a broken pedal?

Discussion in 'Pickups, Electronics & General Tech' started by mnemonic, Aug 25, 2017.

  1. mnemonic

    mnemonic Custom User Title

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    Hey guys, I think my TC Electronic Integrated Preamp is broken, I was using it today, and it set the eq flat and volume at unity gain on my clean channel, and there is a gross distortion added by the pedal. I've narrowed it down to something in the pedal, as I've tried different cables, different guitars, different amps (even direct into a PA), and using a battery (I only have one adapter that fits it).

    Basically its kind of a gross clipping that abruptly stops when the signal goes below a certain volume. The position of the knobs on the pedal doesn't affect it. I can be boosting, cutting, or at unity gain and its still the same.

    I guess this might go a way to explaining why I haven't been using it much, I remember it being totally awesome a couple years back when I got it, but lately every time I use it, I end up switching back to a tubescreamer after a while as the feel isn't right. I just chocked it up to changing tastes and different gear from way back when, but maybe not!

    I'm handy with a soldering iron, and I can borrow my dad's multimeter, so how would I go about diagnosing what is wrong? Alternatively, if anyone knows a better place to ask, or someone who does these kinds of repairs in the north-west of England, I'd be happy to have the details.

    Lets help make this poor little guy kick ass again

    TQ5CU0f.jpg
     
  2. bostjan

    bostjan MicroMetal Contributor

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    It's tough, but I'd take the next step as looking inside to see if anything looks burnt. I believe those preamps use an IC chip along with a buffer transistor. If the IC is messed up, you might be SOL, but if there is just a problem with the transistor or something in the circuit biasing the transistor, you might be able to patch it up, but even then, you might run into some other issues.
     
  3. mnemonic

    mnemonic Custom User Title

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    I had a look at the schematic and parts list and it looks like I can still get the IC chip for a few cents, so that's good at least.

    I had a look inside but only the bottom of the board is visible. No burns or missing parts or disconnects. Looks like I'll have to take off the knobs to get the pcb out and see how the other side looks.

    Do pcb's get brittle as they age? I imagine this thing is likely well over 30 years old, and I don't want to break it worse.
     
  4. bostjan

    bostjan MicroMetal Contributor

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    Heh, I'm not going to say for sure. Typically not, but you never know what weird stuff they might have used. Anything made in the 80's should be fairly safe to assume is G10, which should be relatively unchanged material properties for 100 years or so.

    If you do go poking around in there with a multimeter, just be very careful. Also, be careful handling any IC's, since they are usually easily damaged by static charges so small you can't even feel them. If you do mess with the IC, make sure you are properly grounded and be extremely gentle. Heat can also damage IC's, so you may want to install a socket if there isn't one already.
     
  5. mnemonic

    mnemonic Custom User Title

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    I'm finally home again and I have some spare time, so I opened her up. once I got the knobs off, the PCB came out easy. The knobs are kinda weird, red cap on the top of the knob has to be removed, then you can undo a screw, and off comes the knob.

    Does anything look amiss from these shots? It looks okay as far as I can tell. Is there any way to test if an OP amp or transistor is functioning correctly without removing it?

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
  6. bostjan

    bostjan MicroMetal Contributor

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    Don't see anything obvious. Not sure why the input jack looks burnt. Maybe check it with an ohm meter. With a cable plugged in, there should be no resistance from sleeve to ground (the bare wire should work as a probe point) and from the tip to the input capacitor (there is a large solder well near the back end of the jack on the circuit board). You might want to also hit those pots with some contact cleaner and then turn them for 30-60 seconds. If they are dirty, it could cause a lot of crackling and possibly some drop in volume.
     
  7. rami80

    rami80 SS.org Regular

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    How old is the pedal? I'd be good if you change out the caps if its over 10 years old as you already have it open. A cutting signal is usually a dead cap so start from there. Also measure the resistance on the resistors and the pots and make sure they are within 10 to 25% of their value. Finally check for continuity along them.

    Also change the wires on the power pin. One of them looks toast.
     
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  8. bostjan

    bostjan MicroMetal Contributor

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    While those electrolytic capacitors do have a limited lifespan, most of the older designs were prone to failure because of leaking. I would recommend testing them before replacing them. Resistors only generally fail from abuse, in my experience.

    I agree that the power input component looks a little scruffy, along with the solder joints there. I would test that for continuity as well, while you are at it. If anything seems sketchy, i.e. you are not getting solid continuity, simply replace the component - the only trouble is if there was some sort of power surge blowing up resistors and capacitors, then certainly the IC would be fragged, so you might well want to go ahead an order that one you found for a few cents, if it comes to that.
     
  9. mnemonic

    mnemonic Custom User Title

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    I'll grab my dads ohm meter tomorrow, he has a nice one.

    I can test parts with a multimeter without removing them right?

    Since it's open and the power Jack looks dodgy, I might as well replace it with a modern one so I can use other power supplies.

    There also seems to be no consensus on the IC chip. The clones I've seen use the TL071, the schematic I found on Google calls for LM741A, and mine is using UA741CP. I would assume these all do the same thing in slighty different ways? Maybe they just used whatever they had on hand. It looks like I can still get them all, so I guess I might as well socket it, so I can easily swap. Assuming it's the busted part.

    The caps are all probably very old, I believe they stopped making these in the 80's?
     
  10. wakjob

    wakjob SS.org Regular

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    Start with the simple stuff,
    Clean all the jack AND pots really well, as well as the power supply input.

    After that, because I'm psycho, I'd shotgun all those electrolytics like others have said.
    I'd use UCC (United Chemi-Con) because I love them.

    Also, because I'm crazy, I'd swap that rectifying diode out with the UF (Ultra Fast switching) equivalent.
    I've yet to find a circumstance where they didn't improve things.

    Definitely grab an 8-pin adaptor socket and experiment with a handful of the equivalent IC's. They're cheap.
     
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  11. CrazyDean

    CrazyDean SS.org Regular

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    I concur about the capacitors. A couple of them look bloated which means they have failed. I'd start with replacing all of those blue electrolytic capacitors. Should be pretty cheap place to start.

    P.S. You can clean up the board with some rubbing alcohol and q-tips or soft cloth.
     
  12. mnemonic

    mnemonic Custom User Title

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    I got my dads multimeter but I've been busy all this week tiling my kitchen, so I haven't had a chance to use it.

    I've had a think about it, and while I do have plenty of experience soldering and it's easy enough for me, I don't have experience working with PCB's, so rather than ruin this old pedal, I've bought the BYOC FET Preamp kit and I will practice with that.

    Once I feel I've got a handle on it, I'll swap the caps in the TC Pre and go from there. Hopefully I don't need to swap the IC, unsoldering 8 connections sounds hard!

    Also the Rullywow chuggapre pcb is only $10 so I may buy one of them and source the parts for it while getting parts for the TC preamp. It will be cool to compare the two.
     
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  13. bostjan

    bostjan MicroMetal Contributor

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    To desolder an IC like that, you'll need a solder wick or a squeeze bulb, then it shouldn't be too difficult. If you do take the IC out, though, I would highly recommend replacing it with a socket.
     
  14. mnemonic

    mnemonic Custom User Title

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    Yeah I'll definatley socket it.

    Last thing I need to do is burn out a new IC when soldering it in.

    I have a solder sucker gun thing, but I havent used it before.
     
  15. bostjan

    bostjan MicroMetal Contributor

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    Do you have any finely stranded copper wire? One trick it to hold a piece of stranded copper wire against the solder joint, then hold the soldering iron against the stranded wire. The hot wire will attract the solder once it melts, and if there are enough strands, it'll soak up the solder like a sponge soaks up water. I've had very good luck with the technique. Just keep in mind that there will be basically no way to remove the solder from the fine strands of copper, so it'll be there for good (in case you were thinking of reusing the same wire).

    The desoldering tools with the iron and sucker-bulb take a little practice to get used to.
     
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  16. macgruber

    macgruber cat enthusiast

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    i'd say that corroded looking socket would be my first suspect in this situation. then maybe rework the solder on the components. all the bare metal exposed looks quite corroded too. best of luck :)
     
    Last edited: Sep 1, 2017
  17. Shask

    Shask SS.org Regular

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    I agree with many others. My first thought it replace those light blue electrolytic caps. Some of them do look a little bulged, especially the ones near the battery clips/plug. Usually a crackling sound below a certain volume is related to the voltage being too low, and that could be caused by bad filter caps.

    I would also clean the board with alcohol and a Q-tip, especially the contacts in the jacks. Make sure they are making a good connection.

    Honestly, I wouldn't mess with the IC Chip. There is usually a very low chance those are going to go bad. The 741 was/is a super-cheap chip you could buy anywhere for like $.03. Nothing special.
     
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  18. CrazyDean

    CrazyDean SS.org Regular

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    I just noticed in the first inside pic that there is some white stuff inside the case. Is that corrosion from an old battery?
     
  19. mnemonic

    mnemonic Custom User Title

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    I belive so, yes, it's about where the battery would be when its assembled.

    It wasn't me I promise, it was there when I got it.
     
  20. CrazyDean

    CrazyDean SS.org Regular

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    With the board out, now is a good time to clean it up!
     
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