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Thread: Marshall JCM900 Dual Rev 100w SLXq-06 Problems Bad OT ?

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    Marshall JCM900 Dual Rev 100w SLXq-06 Problems Bad OT ?

    Hello. I have my buddys' Marshall JCM 900 Dual Reverb 100 watt head. With 6L6GC output tubes. Issue SLXq-60. I think this the 4100 model? Came to me with main power fuse blown, and both output valve fuses (on back panel) blown. It also had a shorted output tube (6L6gc) which is V7 , the one closest to the power transformer.

    R8 and R9 (100k) looked like they have been cooked. So, I changed them. R28 (47k), and R29 (8.2k) were both out of spec, so I changed those too. Pulled C11 and C16 (10uf 100v) and tested out of circuit. Both tested fine. I then retensioned and cleaned tube sockets. Tested remaining tubes, put together a fairly close matched quad of 6L6gc tubes. Replaced fuses. Checked all diodes. I was able to power it up and adjusted bias to -49.0 mvdc.

    The amp sounds ok, but seems to be half volume ( output power switch set to high). I then noticed V6 & V7 red plating . I shut her down quick. Tried a NOS quad of Sovtek 6L6GC's. Same thing. Before that, I noticed the output transformer would rattle every time I played a note or chord. I think this amp has a faulty output transformer. I pulled the output tubes and powered it up, with standby on. I got -84, VDC on pin 5 of each output tube socket. How can I check for sure that the output tranny is bad? Thanks
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    Somebody please help?

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    Schematic?

    I don't think your OT is bad. But measure DCR on both sides of the primary wrt CT.

    Do all 4 6L6s show an idle current of 49mA?

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    Last edited by Helmholtz; 02-19-2020 at 08:35 PM.
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    no scope i imagine...
    best thing to do first is to disconnect all wires from the amp to test the OT.
    One thing you can do is feeding a low ac voltage to the secondary taps and check the primary voltage between the plate wires,it should be consistent and respect the different turn ratios,if you don't know how to calculate those i can explain better.
    You can also feed the primary with mains voltage,remember to put a fuse in series,and make the measurements on secondaries.
    In this case,if you have a wattmeter you can also check if there is abnormal consumption,in case of short after 30 minutes its easy the tranny becomes hot.
    Another method is the neon light and a battery,it should be on geofex.com,check out R G Keen

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    Thanks for the good advise guys. Iíve been battling a head cold this week. I plan on jumping back in this JCM900 next week.

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    Finally getting over this head cold. Feeling a lot better now, I decided to jump back into this beast.

    I unsoldered all of the output transformer wires. And started checking the OT resistance. While doing so, I noticed Iím getting fluctuating ohm readings. With my meter leads to the green and yellow solder tabs of the OT, the resistance goes all over the place. So, Iím assuming an intermittent winding? What you guys think?

    No O-scope. Previously, all output tubes had -49 mvDC bias. Iím not sure how to apply low voltage to the transformer. But I do have a Variac.
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    Supporting Member The Dude's Avatar
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    Are the two halves of your primary reasonably symmetrical DCR- brown-white and red-white? Do you measure any resistance from primary to secondary?

    Also, try this:

    https://www.premierguitar.com/articl...ormer-tester-1

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    I unsoldered all of the output transformer wires. And started checking the OT resistance. While doing so, I noticed I’m getting fluctuating ohm readings. With my meter leads to the green and yellow solder tabs of the OT, the resistance goes all over the place. So, I’m assuming an intermittent winding? What you guys think?
    For reliable readings short the secondary while measuring primary DCR. (If it works I'll try to explain why.)

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    Last edited by Helmholtz; 02-26-2020 at 01:41 PM.
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    OT DCR readings

    16.6 ohms between white and brown

    32.4 ohms between white and red

    with the secondary shorted together

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    Quote Originally Posted by CoopDaKill View Post
    OT DCR readings

    16.6 ohms between white and brown

    32.4 ohms between white and red

    with the secondary shorted together
    Values are perfect. Your OT won't cause the redplating.

    Please post schematic and measure screen resistors/voltages.

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    Last edited by Helmholtz; 02-26-2020 at 08:28 PM.
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    Here is the schematic I'm working from

    https://www.thetubestore.com/lib/the...-Schematic.pdf

    I'll recheck screen resistors ASAP
    Thanks

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    I'm not sure how you would end up with -84VDC on the power tube grids. Suggest you re-check pin 5 voltages with power tubes removed. Check both DC and AC.

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    I'm not sure how you would end up with -84VDC on the power tube grids.
    ^^^That!
    Especially considering the rather hot 49mA idle current.

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    And you need to check that pin5 voltage in 'play' mode, not standby.

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    If I remember correctly the bias voltage is very high and when the standby is ON it goes down to the precise adjustment gradually.

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    Ok guys. I soldered the OT back in. Removed all output tubes. Powered on, in play mode, I get -48 VDC on pin 5 of V4 and V7 sockets.

    Screen resistors (470 ohm 5 watt) are all around 483 VDC, with output tubes in, and amp in play mode.

    Before this, I was checking resistor values, and noticed the 1.5 kohm resistor going from pin 4 to pin 5 on V4 socket was reading 1.8kohm. It was a carbon comp, so I changed it out for a metal film 1.5 kohm. Could that carbon comp resistor been causing the red plate problem? I know they're known to drift in value with age, and heat.

    I don't have a bias socket, but I did just order one from China. From my understanding, I can get my bias voltage at the intersection of R26 & R27. Is this correct? And, how would I determine a proper bias voltage setting?

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    Could that carbon comp resistor been causing the red plate problem?
    No, to cause redplating resistance would need to be very high (several hundred k) or open.

    But you might have bad tube socket contacts. Or a leaking coupling cap C9/10.

    And, how would I determine a proper bias voltage setting?
    You need to measure tube idle currents.

    and adjusted bias to -49.0 mvdc.
    How did you measure this?

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    Last edited by Helmholtz; 02-28-2020 at 09:43 PM.
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    I checked the capacitance of C9 & C10 (The two 50uf + 50uf @ 500v can caps) while I had the OT unwired. All were more than 20% off tolerance. Like 76uf , and 69uf. So, I should change those, right? As far as a bad tube socket contact, thats a possibility. Even though I've retensioned them already.

    I set my bias voltage at the intersection of R26 and R27

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    No, those values are fine. Big caps like that are not close tolerance. besides, as filter caps, a little extra capacitance works even better.

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    At R26/R27 you are setting bias voltage, and I think it is -49V, not -49mV.
    You need to measure idle current, not bias voltage. You can do it by putting a current meter in place of the output valve fuse. It will give you a mA reading for a pair of tubes.

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