Need some help troubleshooting a JCM800

Discussion in 'The Workbench' started by Turbo Johan, Nov 30, 2021.

  1. Turbo Johan

    Turbo Johan New Member

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    Hi,

    I have a JCM800 here from a friend to see if i can fix it.
    Never worked on amps, but work with other electronics.

    The amp is a JCM800 2204.
    Problem is it has really low output.
    It did fall from a cabinet during a gig, but still worked at that point.
    So i start at reading schematics and start measuring from the beginning.
    I'm in Europe so have 220V from the wall.
    1st transformer is a Drake 1202-324/1.
    Now i found a problem i think, i have 560VAC out of the transformer. (280VAC each side)
    And after the rectifiers i have 360VDC, now that seem to be really low.
    I measured different resitance on all diodes, so solderd in 4 new 1N4007's.
    Now i have 370VDC B+, but still way to low i think.

    Should B+ be 460VDC on these amps?
    Should this transformer should have 320VDC on each side?

    Do these transformer go bad?
    Can this cause my problem?

    Hope you guys can help me with some info on this issue.
    And hope i don't ask stupid questions ;-)


    Thanks in advance!

    Johan
     
  2. neikeel

    neikeel Well-Known Member

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    There were some late 70s JMP50 that had that low a voltage (Drake 1202-164) from 77 onwards (date stamped on end bell side). The 1202-324 is typically higher - in the range you describe.
    The fact that the HT is symmetrical each side of the centre tap suggests that the transformer is performing as designed. I presume that 370vdc after the rectifier is not under load?

    Bear in mind the first filter cap and the choke comprise an LC filter and the first filter cap needs to be in good condition as it can drop the voltage significantly.
     
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  3. South Park

    South Park Well-Known Member

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    Transformers don’t loose voltage . What does the bias voltage look like
     
  4. Pete Farrington

    Pete Farrington Well-Known Member

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    Have you actually measured it?
    What is the PT primary voltage set to?
    What does the 6.3VAC heater voltage actually measure, ie probe tips on pins 2&7 of an output valve socket?
     
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  5. Spanngitter

    Spanngitter Active Member

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    I anticipate this is a vertical input with the pots wired to the PCB? If yes, then this is one of these low B+ 2204 which where around in the early 80s. For me they sound a tad browner then the later versions which had horizontal inputs and the pots mounted on the PCB.
     
  6. pedecamp

    pedecamp Well-Known Member

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    Probably a bad preamp tube, take a known good tube and replace into each slot to find the bad tube. Do this with the power tubes if need be.
     
  7. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

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    Or simply dirty input jacks.

    Or the impedance selector switch isn't making good contact. I've had that happen before as well. Give it a jiggle.
     
    Last edited: Nov 30, 2021
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  8. Turbo Johan

    Turbo Johan New Member

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    Good one, will try this.
     
  9. Turbo Johan

    Turbo Johan New Member

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    I ordered all new tubes, so will test.
     
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  10. Turbo Johan

    Turbo Johan New Member

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    Pots are wired to PCB, not mounted on PCB, so this could be the case.
    This has a Drake 1202-324/1 transformator, could these be lower than the 324/2 and 324/3 which you see often in JCM800?
     
  11. Turbo Johan

    Turbo Johan New Member

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    Yes did measure it first to be sure rest of measurements are ok. 221V on master switch.
    Heater circuit is perfect 6.3V an all tubes.
     
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  12. Turbo Johan

    Turbo Johan New Member

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    370VDC is measure with amp on and all valves in place, but no input and no cabinet connected.
    First Cap can cause this if it leaks voltage, i will try if i can desolder it and check voltage again.
    All Caps do look ok optical, ends are flat and don't leak.
     
  13. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Don’t run that amp with no cabinet or load connected. Also the number after the slash indicates the year, probably 1981. My 1986 2204 has a 1202-324/6 power transformer.
     
  14. neikeel

    neikeel Well-Known Member

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    If it has tubes in the plate voltage will be affected by output tube current draw. So if output tubes are tired or biased very hot the voltage will be low.
    Did you try new preamp valves?
     
  15. Spanngitter

    Spanngitter Active Member

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    /1 denotes 1981 so this is the range I've seen multiple 2204 which do have this low B+ (they ranged from ~1980 -1982) and all had been vertical Input/wired Pots (and missing Screen Resistors)...
     
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  16. Pete Farrington

    Pete Farrington Well-Known Member

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    Valve amps have a high output impedance. Removing the load greatly increases their gain, which can cause them to oscillate. That will draw HT current and cause the HT voltage to sag. And also puts the output valve’s screen grids, and anode circuit insulation, under stress.

    The HT voltage at idle will be significantly affected by how much current the output valves are drawing, ie the bias setting.
    About 30 years ago, the very beneficial trend to fit a 1ohm resistor between the output valve cathode and its return to 0V common began to take off. That allows the output valve current to be easily and safey measured.
     
    Last edited: Dec 1, 2021
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  17. Turbo Johan

    Turbo Johan New Member

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    It has the big screen resistors, but could be put in afterwards.
     
  18. XTRXTR

    XTRXTR Well-Known Member

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    Just to further corroborate the 1202-324/n where n = integer for the year in the decade was mfr-ed e.g. /3 1983, /7 1987 etc. I have one each of these xfmrs in a couple amps.

    Wonder if set to 240VAC on mains selector? or selector switch wired improperly? Also, remove all tubes and then check VDC after rectifier. I recently measured (Below) on the 1202-324/7. I believe this PT is ~360-0-360 VAC on the HT secondary, These measurements may help you:

    76 ohms red to red HT 2ndary
    From blue common to:
    2.8 ohm primary 120V winding black
    8.4 ohm primary 220V winding yellow
    7.4 ohm primary 240V winding orange

    Wall Voltage: 122 Vac
    Open circuit no tubes HT Vac measured at standby switch open circuit terminals: 728.8 Vac
    Idle HT Vac with all tubes in place: 716 Vac

    using Dummy load

    Idle Tubes Russian 5881: plates 471Vdc, cathode 28mA, 13.2 watts 13.2/24=55%

    1kHz input

    With Max undistorted signal on output HT 703 Vac, 452 [email protected] fuse, 67mA cathode current

    Max clipped signal on output HT 676 Vac, [email protected] fuse, 196mA cathode current, 38.4 milliwatts through 2 watt 1 ohm resistor

    No Power OT CT: to red V4 pin3 43 ohms, Power Idle Vdrop 1.032Vdc, plate Current 24mA; No Power to white V5 pin3 42.5 ohms, Power Idle Vdrop 1.031, plate Current 24.2mA.
    Max sine V4 Vdrop 2.601 Vdc, plate Current 60.4mA, V5 Vdrop 2.521 Vdc, plate Current 59.3mA
    Max square V4 Vdrop 5.74Vdc, plate Current 133.5mA, V5 Vdrop 5.22Vdc, plate Current 122.8mA
     
    Last edited: Dec 1, 2021
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  19. Turbo Johan

    Turbo Johan New Member

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    I didn't had time to work on it again.
    But today i had some time and i found the problem.
    Bridge didn't close on input jack from clean channel.
    Stupid i didn't check this first, but there was no nut on the clean input jack and was never used..
     
  20. neikeel

    neikeel Well-Known Member

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    Kudos to @Gunner64 on this one!
     
    Last edited: Dec 31, 2021
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