Help finding fault with old bass amp that blows fuses

Discussion in 'General Electronics Chat' started by permanentrecordstudios, Jan 11, 2018.

  1. permanentrecordstudios

    Thread Starter New Member

    Jan 11, 2018
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    Hi folks!

    I'm trying to find fault in an old Yamaha bass amp from the very early 80s, but I'm struggling a bit because of the fact that the fault is intermittent, sometimes instantly blowing the two 4 amp fuses on the main circuit board for the amp (secondary power side), sometimes working fine for extended periods.

    I've disconnected everything I can, tested as much as I can with a meter.

    I think I could probably find the fault quite easily if I could just power the board and trace it (I have the schematic), but obviously I can't do that because the fuses instantly blow when they do blow.....otherwise all is fine.

    There is no short (no continuity) between B+ and B- anywhere on the board.

    Does anyone have some good advice on how best to proceed, without unsoldering every single component and testing it?

    P.S. This amp has worked fine for 20+ years. This first started recently when the terminal on the speaker frame broke, and the speaker wires shorted, blowing the fuses for the first time.
     
    Last edited: Jan 11, 2018
  2. permanentrecordstudios

    Thread Starter New Member

    Jan 11, 2018
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    Sorry, meant to mention the fuses that are blowing are F2 & F3.
     
  3. dendad

    Well-Known Member

    Feb 20, 2016
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    I would really look closely at the repaired speaker connections to see if there is a problem there. Even a fine piece of wire or suspect insulation. It does sound like one problem could be intermittent speaker wiring shorting.
     
  4. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    I would say if there is no shorts between B+ and B- to each other and Ground, then also check the Bridge Rectifier, try powering up the amplifier without any speakers, and measure the DC supplies B+\- and the output of the speaker + at the Relay terminals.
     
    Last edited: Jan 12, 2018
  5. dl324

    AAC Fanatic!

    Mar 30, 2015
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    That's the way it is with most things. They work until they don't.

    Intermittent is difficult to troubleshoot. Without further information, I could only guess at what could be wrong; so I won't.

    Relevant portion of the schematic:
    upload_2018-1-12_10-46-24.png

    PDF of complete schematic attached minus irrelevant pages.
     
  6. sghioto

    Member

    Dec 31, 2017
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    Check the insulators on the output transistors on the heatsink.
    Steve G
     
  7. permanentrecordstudios

    Thread Starter New Member

    Jan 11, 2018
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    Thanks for the reply. I keep going back to the speaker, since it seems to have started the problem, but...I replaced the speaker terminal, and have double and triple checked everything, and it is all clean & solid.
     
  8. permanentrecordstudios

    Thread Starter New Member

    Jan 11, 2018
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    Thanks for the reply. The bridge rectifier is one of my suspects. But my main problem is with it blowing fuses, I can't keep checking things while powered (I'm going broke buying fuses).

    Here's an update on what I've done:
    I've checked transistors (Tr120-121 & Tr123-1234). Each falls within specs.
    I can't check Tr122 because I can't find specs on it (my amp actually has a C3298B rather than the C1953 listed on these schematics).

    Could a faulty cap (C181 &/or 182) cause an intermittent excess current draw on power up & blow these fuses? Couldn't a faulty Cap here also cause excess current draw at the bridge rectifier?

    But it looks like Tr122 draws from both B+ and B-? Since both fuses are blowing, would I not be looking for a common link between B+ and B-? Can this be a clue? I can read schematics to a point but I'm certainly no expert.

    What else would cause such a current draw?
     
  9. sghioto

    Member

    Dec 31, 2017
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    Speaker cable OK? Tr122 controls the bias on the output transistors, I don't see any problem there that might blow the fuses. Certainly an intermittent short on either filter capacitor will blow a fuse. An old trick is to run the amp connected to a variac. The idea is to slowly raise the AC voltage to a point where you can get it to work and troubleshoot. At some point you may have to just start replacing components.
    Steve G
     
    Last edited: Jan 12, 2018
  10. permanentrecordstudios

    Thread Starter New Member

    Jan 11, 2018
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    Thanks for the reply. So I've removed the output transistors to test 'em (well, tested 4), and the insulators underneath seemed fine. But I didn't see any rips or tears or problems. Not sure what exactly to look for in good insulation, but it seemed surprisingly clean & intact.

    There are two fuses on this particular circuit, one for the B+ side, one for the B- side. Both are blowing. Seems that's got to offer some clue. If only one were blowing that would be different, but both are blowing.
     
  11. permanentrecordstudios

    Thread Starter New Member

    Jan 11, 2018
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    Pretty sure. I keep going back to the speaker area, since that seems to be where it all started, but all seems ok. I've connected an Ohm meter to the plug (amp) side of the cable, while connected to the speaker, and jiggled the cable to check for intermittent shorts, but can't find one.
     
  12. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Yes caps can cause the fuses to blow, but my money is on the four output transistors, or the two drivers Tr118,119,,, Tr122 sets the bias current through the 4 output transistors,.
    Since you already said you had a previous speaker board problem, i would look there, if it helps remove the collectors from the output transistors and power it up without speakers , and measure the voltages and currents through the fuses.
     
  13. wayneh

    Expert

    Sep 9, 2010
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    I’d take a hard look at the bridge rectifier D115 and by that I mean removing and inspecting it if on-board testing doesn’t give clear results. It’s a likely suspect and I wouldn’t spend time looking elsewhere until I had ruled it out.

    And yes, replacing electrolytic caps is a good policy on old electronics. They can fail with no outward appearance. You can mess around testing them with an ESR tester, but it’s just about as easy to simply replace them, and then you get a bit of insurance against future failure.
     
  14. permanentrecordstudios

    Thread Starter New Member

    Jan 11, 2018
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    I've removed & tested Tr120-121 & Tr123-124, and those measured within specs (my meter has special slots for transistors to measure the frequency).

    Since both fuses are blowing, B+ and B- sides, shouldn't that offer a clue? The output transistors are connected in opposing pairs to either B+ or B- (not both), so there'd have to be an intermittent problem in two opposing transistors for those to be the source. Tr122 seems to have a common link to both B+ and B-. But I can't find the specs (my amp has a c3298b rather than the c1953 shown on these schematics) to test it.

    Not much else uses both sides, that I can see.

    Everything on the speaker side is solid. I replaced the speaker terminal, and have double & triple checked it.
     
  15. LesJones

    Well-Known Member

    Jan 8, 2017
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    You could connect a 12 volt 20 watt bulb in series with each fuse. This would not allow you to get the full output from the amplifier but would save on fuses while testing. You could also measure the current through the fuses. The current should be about the same through both fuses and should be quite low with no drive to the amplifier. This current will rise as the drive is increased. (But it's value should be similar on both rails.) If the speaker had been dropped it is possible that the magnet had moved and was rubbing on the voice coil. This may cause the fuses to blow. The fact that both fuses blow suggests that the fault is on the output of the amplifier. If you had another speaker of suitable rating it would be worth trying it with that speaker. It would also be worth looking closely at the output jack sockets to make sure there was no damage that could cause a short.

    Les.
     
  16. permanentrecordstudios

    Thread Starter New Member

    Jan 11, 2018
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    So the circuit in question (B+ B-) runs at about 30v (at last check), so is a 12v bulb enough? I already know a 120v60w bulb will light dimly on this circuit (I've checked), can I use the 120v60w bulb instead?
    But that's the kind of idea I need...how to power this thing without blowing fuses so I can check voltages.

    The speaker hasn't been dropped (it initially shorted, the speaker terminal just broke and the wires crossed), and everything on the speaker side seems solid now that I've repaired it. Plus the fuses have blown only on power up, that I know of. I've had the amp on, for extended periods, playing, and it's been fine. The next time I tried to power it up, the fuses blew.

    "The fact that both fuses blow suggests that the fault is on the output of the amplifier"
    Why is this? I read this on another post, but it was closed so I couldn't ask.

    And what else on the output side could it be? I've checked the transistors.
     
  17. permanentrecordstudios

    Thread Starter New Member

    Jan 11, 2018
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    Yeah, people & logic keep pointing to the output side. But I've tested the four output transistors and they test within hFE tolerences. Can they still cause fault?

    "if it helps remove the collectors from the output transistors and power it up without speakers"
    I hadn't thought of that. That could save me some time.
    Are you saying Tr122 is not a likely source of the problem? It is the only transistor with both B+ and B- in common (from my limited knowledge of schematics). I've read that bias problems can cause power issues in transistors, but I don't know enough.
     
  18. LesJones

    Well-Known Member

    Jan 8, 2017
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    I had missread the +B and -B as +8 and -8. I thought at the time it was rather a low voltage for an amplifier. I just assumed it was a very low power amplifier. If you use 120 volt bulbs they will probably need to be a higher power than 60 watts. (I was aiming at about 3 amps which is just a bit less than the fuse ratings. So if the bulbs were120 volts you would need about 300 watts.) As has been suggested replacing the smoothing capacitors would be worth while. the fact that it is an intermittent fault makes it difficult to use a totaly logical approach so it may also be worth replacing the bridge rectifier at the same time as the capacitors as it will probably be quite cheap. I have never seen a rectifier or capacitors cause this type of fault but that does not mean it could never happen.

    Les.
     
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