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Another amp blowing fuses... but why?

Discussion in 'Amp Tech Center' started by MrCoolGuy, Jul 10, 2020.

  1. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    So I'm working on a 1982 Mesa Son Of Boogie, the one with two 6L6GC's. Classic story: it powers up on stand-by just fine, but when you switch it off stand-by, POP goes the fuse. I've switched out the power tubes (I have a tube tester and other amps... they are good tubes). No luck. I am using the correct fuse, 2.5A slo-blo... the power tube sockets are a little loose, but I stabilized them best I could and all pins are making good contact. The amp is NOT tube rectified.
    4 little diodes look ok but haven't been tested (how would I?)... I'm running out of ideas. Oh, all the filter caps seem ok. None are shorted Hard to test them in circuit... but I took one out and it was dead on perfect. Even with the power tubes out, fuse blows when stand-by is switched off.
    Any ideas?
     
  2. Paul G.

    Paul G. Friend of Leo's

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    Check the transformers. I believe your amp has plug-in connectors, so disconnect the PT primaries and measure the resistance across them. If you see zero ohms, you have a short. If you see some resistance, you're ok there. Try the secondaries one at a time, again, some resistance is good. No resistance, bad.

    Remove your power tubes and measure resistance across the OT primaries and CT. Measure pin 3 to pin 3. You should have some resistance. Measure pin 3 to CT on both sides. The resistance should be split more or less evenly. Check the secondaries for shorts.

    Look around for signs of burned components.

    If you find nothing and want to continue diagnosing, you need to build a current limiter so you can run for a bit without blowing fuses.
     
  3. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Yeah, I was afraid I'd have to do this. The leads are all soldered. And while Mesa's boards are pretty good, I still hate desoldering and resoldering on these circuit boards. You gotta use a foot of wick for each connection. They really puddle it on. I'll Get back with you this afternoon. Thank you!
     
  4. Paul G.

    Paul G. Friend of Leo's

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    Well, thinking about it, the PT primaries are probably fine if you don't blow fuse until HT is switched in.
     
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  5. King Fan

    King Fan Poster Extraordinaire

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    I always quote @jhundt who quoted G. Weber about using a LBL to diagnose fuse-eating...

     
  6. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    The LBL will tell you that you still have a short, but won't tell you much about where.

    You've already isolated the issue to *something* in the B+ string. If it's an amp from 82 it needs a cap job no matter what. I wouldn't even bother troubleshooting until that's been done. Then if they are still issues, pull tubes one at a time - no reason it couldnt be a preamp tube. Still issues? Start disconnecting things from B+, or at the least measuring B+ connections to ground to see if there is a short.
     
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  7. tubegeek

    tubegeek Friend of Leo's

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    I think that's mistaken, unless the preamp tube's plate resistor has failed short.

    If a preamp tube itself is shorted you still have a pretty big plate resistor (or cathode resistor on a cathode follower) in series with the tube. Can't pull enough current through that to pop the fuse.
     
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  8. Engine Swap

    Engine Swap Tele-Holic

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    As suggested, new filter caps before anthing else. They can measure perfectly for capacitance, but leak dc blowing the fuse.

    The rectifier diodes are before the standby switch, so they are probably OK.
     
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  9. jhundt

    jhundt Doctor of Teleocity

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    isn't everything in the amp eventually "something in the B+ string"?
     
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  10. jhundt

    jhundt Doctor of Teleocity

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    I don't understand why people want to work on amps without a light-bulb limiter. They'll spend a hundred bucks on a computer-controlled 'soldering station', but not $5 on the most useful troubleshooting tool in the shed.

    BTW, a 60W Weller solder gun works just fine on every amp I ever dealt with!
     
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  11. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    @tubegeek - good point on the plate resistors, I neglected to think of that. Still, pulling a preamp tube is a quick enough test to be extra sure.
     
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  12. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Oh perfect. Thanks!
    By the way... the bias supply is good. I forgot to mention that. But ill try these steps today. I'll keep yall posted!
    Thanks again!
     
  13. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Ht?
     
  14. jhundt

    jhundt Doctor of Teleocity

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    The troubleshooting procedure that King Fan printed uses a light-bulb limiter to determine where the fault is, if you follow the steps.
     
  15. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Preamp Tubes won't pop a fuse usually, but thanks.
     
  16. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    I have a limiter.
    My 70w weller works fine. Does't change anything I said.
     
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  17. tubelectron

    tubelectron Tele-Afflicted

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    IMHO :

    - It can be located in the PSU or PA stage.
    - It can also be a short-circuit somewhere in the wiring (involving HV or maybe heater)

    NOTE : think to permute your probes polarity when performing DCR test, because of possible diode presence... ;)

    1 - Amp off, SBY ON : did you checked with a DMM the DCR between CT of OPT primary and GND ?
    2 - Same question, but SBY OFF ?

    A steady low value (less than say 1K) indicates a short. If you reach over 100K, that could be considered normal.

    This is more annoying, in the sense of the OPT transformer could be damaged...

    3 - Power tubes removed, check the DCR between each plate and GND : you should find at least 100K.

    4 - If not, disconnect the 3 wires of OT primary and test the DCR vs GND of each wire : you should find infinite value.

    If not : the OPT transformer is damaged. Even if an infinite value is read, the transformer can show an insulation leak that make it defective too (insulation of a PT or OPT is way over 1000M usually). So :

    5 - Power your amp with the PT disconnected as above : the fuse shouldn't blow.

    If it blows, then your OPT is probably not the cause of the problem (rather a good new) and the problem is in the PSU (circuit, transformer). If the fuse doesn't blow, then :

    6 - reconnect only the primary CT of OPT to B+ and redo the test.

    If the fuse blow, then the OPT is the culprit because of an insulation problem (it could be also a connection wire which has a defective insulation vs. the cover hole : it often arise on old OPT).

    If the fuse dosen't blow, then your OPT is safe : good point. There maybe special diodes tied to the plates pins of the power tube sockets : remove them and see if there is an improvement (sometimes they blow or are leaky).

    You can perform the same disconnection method with the power transformer, etc... In order to locate the fault.

    OK, but you will tell me : how many fuse will I need ? Not economical ! You're right... :D

    So I usually replace the fuse with an ammeter and use a variable circuit breaker or a Variac : any supect increase of current past the PWR ON / SBY OFF / reaching the nominal mains value at the Variac, moreover accompanied by adverse noise, and I stop immediately.

    It's not very easy to help at distance... :oops: But nonetheless, you have some additional ideas here... o_O

    Good luck in your troubleshooting ! If you're not confident enough, stay wise for you and your amp and better see a competent friend or tech. ;)

    But it's me, OK ? :D

    -tbln
     
    Last edited: Jul 10, 2020
  18. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Thanks everyone
     
    Last edited: Jul 10, 2020
  19. jhundt

    jhundt Doctor of Teleocity

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    and - the advice to replace all filter caps is probably the key to your problem.
     
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  20. radiocaster

    radiocaster Poster Extraordinaire

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    Happened to me when I replaced the filter caps but wired them backwards.
     
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