1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Peavey Classic 100 (Tweed Look - mid 90s) - Popping, then cuts out!

Discussion in 'Glowing Bottle Tube Amp Forum' started by japanvintagefx, Aug 8, 2020.

  1. japanvintagefx

    japanvintagefx TDPRI Member

    Age:
    40
    Posts:
    7
    Joined:
    Nov 21, 2018
    Location:
    Japan
    Hello all,

    I wonder if anyone had experience with this particular amp?

    For reference it is the larger Tweed effect head of 100w, incorporating 3 x 12ax7 preamp valves and 8 EL84s in the power section (in four pairs). Very nice in-between Fender/Marshall sound wise, quiet, nice reverb, plenty of power.

    Had mine at least ten years but it had been rested a long time, and has recently developed a fault. It will play nicely for anywhere between 10 minutes up to and hour, but then intermittently makes a loud 'pop", then some crackling, further pops then cuts out.

    So far some preliminary checks:

    • Sprayed switch contacts on front panels and jacks (very brown and gunky switches)
    • Bridged the FX loop at the rear with a patch cable
    • Observed the valves/tubes during said "popping", no flickers, no indication of obvious failures.
    • Read up on the fiendishly difficult to repair circuit board!
    Any advice or troubleshooting points much appreciated!

    Thanks!
     

    Attached Files:

    Last edited: Aug 8, 2020
  2. BigDaddy23

    BigDaddy23 Tele-Holic

    Age:
    46
    Posts:
    639
    Joined:
    Jan 4, 2019
    Location:
    Australia
    Photos of the circuit and schematic posted in the thread would help folks troubleshoot with you.
     
    japanvintagefx likes this.
  3. tubegeek

    tubegeek Friend of Leo's

    Age:
    60
    Posts:
    3,879
    Joined:
    Jan 31, 2020
    Location:
    Brooklyn, NY
    Problems like that - especially in a circuit board amp - often are traceable to a resistor that has cracked or the solder joint that connects it. Resistors are the components that tend to move as they heat up. They also can crack and become intermittent.

    This problem is exacerbated by the use of lead-free solder in manufacturing (not sure if that's the case for this amp though.) Modern PCB construction gets this problem a lot.

    The diagnosis and repair starts with 1) if it's a multi-channel amp, does the problem affect all channels? (Then you can say that the problem is in - most likely - the power supply, which is common to the whole amp) 2) a close look at all your solder joints, particularly at resistors, and re-flowing anything that looks sketchy, 3) use of freeze spray to try and cool off selected components to try and find the problem one.

    2) can be made more complicated if the lead-free solder was used in the original build - you'll want to clean it all off of any joint you're working on and re-solder with lead-bearing solder ideally.

    The prime suspects are large power resistors in the power supply. Don't mess with any of this process if you don't have basic tube amp safety FULLY understood, bring it to a tech.
     
    japanvintagefx likes this.
  4. japanvintagefx

    japanvintagefx TDPRI Member

    Age:
    40
    Posts:
    7
    Joined:
    Nov 21, 2018
    Location:
    Japan
    Hello, thank you for your reply.

    I have attached photos of the accessible areas on the rear of the amplifier (by unscrewing the rear access panels). Unfortunately this series employs a quite complex hidden folded pcb board which is very difficult to access without full disassembly.

    A google search has yet to reveal an exact schematic (the "50" is different), but there surely must be one out there).
     
    Last edited: Aug 8, 2020
  5. japanvintagefx

    japanvintagefx TDPRI Member

    Age:
    40
    Posts:
    7
    Joined:
    Nov 21, 2018
    Location:
    Japan

    Thank you for your detailed replay, very much appreciated. Photos now added of the observable portion of the circuit board. (The rest being quite hidden). Those resistors do look rather old, could they possibly be the culprit?

    The problem occurs across both channels, though (with limited playing time) I feel it is more prevalent when pushed with a pedal or on higher gain.
     
    Last edited: Aug 8, 2020
    tubegeek likes this.
  6. Nick Fanis

    Nick Fanis Poster Extraordinaire Ad Free Member

    Posts:
    6,040
    Joined:
    Mar 3, 2003
    Location:
    Athens-GREECE
    So most probably your electrolytic caps are shot.
    Never "rest" an amp.
    It needs juice at least a few hours per months.
     
    japanvintagefx and tubegeek like this.
  7. tubegeek

    tubegeek Friend of Leo's

    Age:
    60
    Posts:
    3,879
    Joined:
    Jan 31, 2020
    Location:
    Brooklyn, NY
    Oh man, that's a tough one. And those grey ribbon cables are another likely point of failure. The solder joints are not visible from that side of the board, but I don't see any obviously burnt components in a quick look.

    You can also provoke components with a chopstick to see if you can get them to act up, after the heat-up time of course.
     
  8. tubegeek

    tubegeek Friend of Leo's

    Age:
    60
    Posts:
    3,879
    Joined:
    Jan 31, 2020
    Location:
    Brooklyn, NY
    No law that says he only has one problem, either!
     
    japanvintagefx likes this.
  9. japanvintagefx

    japanvintagefx TDPRI Member

    Age:
    40
    Posts:
    7
    Joined:
    Nov 21, 2018
    Location:
    Japan
    Thank you for your reply.

    You're quite right, those ribbon cables are not the best for reliability long term. I gently tweaked them while in action with no direct result (yet!).

    There are many threads on the difficulty of repairing the later Classic 30, and this shares some of the same commonalities design wise that make it awkward, tubes on the PCB, a bizarre 3 sided folded triangular PCB hidden away and ribbon cables! Still I would guess a step-by-step approach may prove dividend in the end.
     
  10. japanvintagefx

    japanvintagefx TDPRI Member

    Age:
    40
    Posts:
    7
    Joined:
    Nov 21, 2018
    Location:
    Japan
    Indeed! Could be the "buses" scenario, only I wasn't waiting for one (or three)!
     
  11. JohnnyJumpUp

    JohnnyJumpUp TDPRI Member

    Posts:
    55
    Joined:
    Aug 13, 2019
    Location:
    California
    If this is built like a Classic 30 it is a very special kind of circuit board(s) cat-hair arrangement. Shame on Peavey for building in this manner.
    Maybe you can tell that I am not enamored with this series. Open one up, you will see why.
    I am not saying that it can't be fixed. I will never touch one again.
     
  12. japanvintagefx

    japanvintagefx TDPRI Member

    Age:
    40
    Posts:
    7
    Joined:
    Nov 21, 2018
    Location:
    Japan
    Yes, I quite agree, a real challenge!
     
  13. vanr

    vanr Tele-Afflicted

    Posts:
    1,206
    Joined:
    Oct 20, 2003
    Location:
    AZ
    Classic 50 and 100 don't have the folded PCBs like the Classic 30. I'll bet this is a tube problem.
     
  14. BigDaddy23

    BigDaddy23 Tele-Holic

    Age:
    46
    Posts:
    639
    Joined:
    Jan 4, 2019
    Location:
    Australia
    ^ A good place to start. Have you tried gently rocking the tubes (circular motion) when it it operating? Those PCB sockets or the tubes themselves would be an easy plave to start. I've seen a few vids on Classic 30 repairs and they did look like a massive PITA to disassemble to troubleshoot. Could be tech time.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.