Peavy JSX head... R77?

Discussion in 'Amp Tech Center' started by MrCoolGuy, May 31, 2021.

  1. MrCoolGuy

    MrCoolGuy Tele-Holic Silver Supporter

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    I may be in over my head on this amp...
    It's a 120watt Peavey JSX head. It was blowing fuses. The owner replaced the fuse, turned it on, sparks, smoke... brought it to me along with this picture...
    received_892966474599713.jpeg
    The landmine in question is R77...
    As seen here...
    Screenshot_20210531-155625_Acrobat for Samsung.jpg
    R77 is... was a 4k7 1/4w carbon film resistor. I replaced it with a 4k7 1w metal film resistor. I also replaced those two diodes (d16 and d20) due to collateral damage... 1N4007's...
    I powered up with my lightbulb limiter...
    The bulb did dim... but not much, but regardless... the new R77 started smoking.
    I forgot to mention, I also tested the 4 EL34's with a basic emissions tester... no shorts or leaks...
    So obviously R77 is not the issue, but a symptom of whatever is the real issue.
    Any ideas here? I'm not even sure what R77 is for? I see it's off one side of the filaments... off the positive of C55...
    Should I replace those E Caps? I guess the amp is about 15 years old...
     
  2. MrCoolGuy

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    Here's the Schematic.
     

    Attached Files:

  3. Refugee

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    Never overlook the obvious. Is there a speaker cab plugged into it? Are you sure it's an un-shielded speaker cable, not an instrument cable? That is a frequent fuse fryer.
     
  4. MrCoolGuy

    MrCoolGuy Tele-Holic Silver Supporter

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    Thank you. It's just a regular speaker cable I made for testing amps... 6 foot long, unshielded. Two 14 Guage wires twisted together, 1/4 phono jack on one end, spade connectors on the other.
     
    Last edited: May 31, 2021
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  5. Jon Snell

    Jon Snell Tele-Holic

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    What are the voltages either side of R77 with the output valves removed.
    Check R121 & 122 as they may but not likely to have duffered.
    The heaters are normally held at around 20 volts positive to ground. To burn up R77 there is probably a high DC voltage on the heaters caused by a faulty valve or damaged track on the board.
    Has P1 been inserted incorrectly?
     
    Last edited: Jun 1, 2021
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  6. Lynxtrap

    Lynxtrap Tele-Afflicted

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    That should be an elevated heater supply for the PI and the output tubes. The AC heater supply for the output section is elevated by about 24 VDC from the rectifier for the preamp DC heater supply. Down by the output tubes in the schematic, there is an artificial center tap with 100R resistors and a cap.

    I would check the artificial CT, the output tubes and the PI tube, and probably also the pilot lamp. Take voltage readings.
    Which fuses were blowing?

    DISCLAIMER: I haven't had my morning coffee yet!
     
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  7. MrCoolGuy

    MrCoolGuy Tele-Holic Silver Supporter

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    I kinda thought so... because there is an actual CT coming out of the PT... but it's unused. So I figured there was an artificial somewhere. And I saw the 24v check point there on the filament...
    Anyway... I'll check when I get home today. Thank you.
     
  8. MrCoolGuy

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    Thanks, I'll let you know.
     
  9. MrCoolGuy

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    F1 the 2a fuse, at least.
     
  10. Lynxtrap

    Lynxtrap Tele-Afflicted

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    It might be wise to start the investigation with no tubes in the amp. Check voltages at all relevant points.
    I agree with Jon Snell that it could be a short between B+ and heater supply.
     
  11. MrCoolGuy

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    Yeah, that's the plan. I'll keep ya posted.
     
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  12. MrCoolGuy

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    Ok, I feel dumb for posting this thread before I pulled the power board up...
    There's a blown screen grid resistor I couldn't see before (and some casualties in its proximity)...
    This brings up a question though...
    Screen resistors usually blow due to bad power tubes, correct? So when I test these power tubes on an old emissions tester (eico 625), they test fine. No shorts, no overload, decent emissions.
    I understand the scope of these old emissions testers is limited, but what's going on here? Is it the tube shorting under high voltage but not under the conditions of the tester? Just curious.
     
  13. MrCoolGuy

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    Actually, the more I look at this...
    That screen resistor didn't blow...
    The trace beside it did... the trace goes to the plate (pin 3) of V6...

    Both those artificial CT 100r resistors measure infinite resistance, which explains the 4.7k resistor that feeds the elevation blowing.
    20210601_143014.jpg
    The resistor still tests right at 700r...
     
    Last edited: Jun 1, 2021
  14. MrCoolGuy

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    Do you think a bad tube did this?
    Can I simply replace all the damaged components and use jumpers in place of the bad traces?

    It looks like the trace for pin 2 shorted to the trace of pin 3...
     
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  15. MrCoolGuy

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    ...
     
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  16. Jon Snell

    Jon Snell Tele-Holic

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    Yes, flashover in the valve caused your problem. Possibly shorted momentarily causing that sort of damage,
    I did say about R121 & 122.
    If the EL34s run too hot they die most spectacularly as you have witnessed.
    I recomend 26SWG tinned copper wire to repair the tracks if you do not have PCB Track material.
     
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  17. MrCoolGuy

    MrCoolGuy Tele-Holic Silver Supporter

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    So, is that the probable order of events:
    Tube shorts
    Traces melt/short
    Artifical CT resistors fail
    B+ to filament elevator resistor fails...?
     
    Last edited: Jun 1, 2021
  18. MrCoolGuy

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    By pcb trace material you mean those repair pens? I've never tried those.
    I always just make jumpers from whatever point on the damaged trace has a component soldered in to the next. I think I've been using 22awg tinned copper wire.
    Which is pretty close to I think 24swg...
     
  19. Jon Snell

    Jon Snell Tele-Holic

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    No, PCB Trace material is tinned copper strip the width of the original trace and when cut to size, put in placed and heated with an aluminium bit, (to avoid solder sticcking and removing the adhesive) it sticks to a cleaned PCB. You then solder the good part of the old trace to the new trace, to make good the power line. I then paint the new trace with conformal coating and fit the replacement components.
    I was given a sample pack for SMD rework when I was a main service agent for Sanyo Electronics and it is still in use today.
    It comes as a sheet of varying thickness tinned copper in different sizes to suit different land widths/guages from 256pin SMD work to 3/16" wide heavy strips.
    I believe it is a Sanyo product as I have not seen it for sale anywhere else.

    24awg is ideal.

    Edit; spelling typo
     
    Last edited: Jun 2, 2021
  20. MrCoolGuy

    MrCoolGuy Tele-Holic Silver Supporter

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    Oh, gotcha... I've seen this stuff. I guess that would come in handy. I'll check it out. Thank you, I'll let us know how this goes.
     
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