jcm800 2205 reverb troubleshooting

Discussion in 'The Workbench' started by tuna141, Jan 27, 2015.

  1. tuna141

    tuna141 New Member

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    I'm hoping someone can give me a hand troubleshooting the reverb on my JCM800 2205, which has no reverb at all.

    I'm pretty sure the tank is fine (the resistance is what it should be and I tried another tank in the amp and it did not work either). The switching circuit seems to be working okay too. The return circuit for the reverb seems fine as well, as I can rattle the tank and I can hear the noise of the springs and I can adjust the output level of the noise using the reverb pot.

    So, the input to the tank appears to be the problem, at least I believe it is.

    Can someone confirm what types of voltages I should be seeing at the reverb transformer? Here's what I measure at the transformer pins with the input to the reverb tank unplugged:
    Pin 1 = 450dc {I assume this is fine}
    Pin 2 = 435dc + guitar signal which is around 10VAC - the amplitude varies with how hard I play the guitar {I assume this is correct too}
    Pin 3 = no output {I believe this is the input to the tanks, so I think I should see some guitar signal (low AC voltage) here}
    Pin 4 = ground {okay}

    So, at this point, I'm guessing that my transformer may have died. What type of voltage should I be seeing at pin 3, which is the input to reverb tank?
    I ohmed the pins and none were shorted. Is there anything else I can check?
    Am I correct in my assumptions or am I completely off-base? Any help or tips would be appreciated. Thanks !
     
    Last edited: Jan 27, 2015
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    V3 driver tube could be bad
    C28 R37 could be bad
    Pin 3 of transformer should have a lot of audio signal, with the tank unplugged.
    You KNOW the return is working.

    Before you replace the transformer,
    replace or test V3, C28, and R37 first.

    It's likely V3 may have fried R37 when the tube went bad.
    The tube shorts, and R 37 fails...
    Pretty common problem in reverb driver circuit, burning the cathode resistor because of a bad driver tube. Happens all the time.
     
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  3. tuna141

    tuna141 New Member

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    When I began troubleshooting, the first thing I tried was replacing V3 & V4 with known good tubes - still no reverb, so I assume both tubes are good.

    I'll try replacing R37 & C28., and while I'm at it I'll put another tube in V3 just in case there is something funky with that tube.

    Thanks for your help. I'll let you know how it goes....probably won't get to it until the weekend.
     
  4. tuna141

    tuna141 New Member

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    I tried a new tube and it didn't help - still no reverb. R37 and C28 appear to be okay too.

    I then measured the resistance of the primary winding of the transformer in the reverb drive circuit (between transformer pins 3 - 4) and got 3.3kohms. I disconnected the reverb input cable from the reverb and then measured the resistance of the secondary side of the transformer (pins 1 -2) and it was infinite (no reading on meter).

    Each winding (primary and secondary) of the transformer should have some resistance, correct? No resistance would indicate an open circuit / break in the winding, right? Just looking for confirmation.

    I can't see a manufacturer's name or part number on this transformer.Anyone know made these, or better yet, the model number and/or specs for this transformer? Any idea where I can purchase a replacement?
     
    Last edited: Feb 1, 2015
  5. tuna141

    tuna141 New Member

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    I contacted Marshall and they informed me that I can buy the reverb transformer as a "special order" through tubesandmore.com. The p/n for the transformer is M-TXRE-00002. I figured I'd share this info in case anyone ever needs one.
     
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  6. mickeydg5

    mickeydg5 Well-Known Member

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    Yes that reverb transformer should have a low resistance at the secondary, definitely not infinite.

    That p/n does not show up on T&M's site. Did they have to order it or have it in stock?
     
  7. tuna141

    tuna141 New Member

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    It's a special order, and will take 8-10 weeks for delivery. The Marshall p/n is M-TXRE-00002 and I was quoted a price of $ 61.20 from tubesandmore.com
     
  8. zaiata

    zaiata New Member

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    My apologies for resurrecting this old thread. But just in case....:
    I am going through the same experience of troubleshooting the reverb on my JCM800 2205. Went through the steps: V3, C28, R37... In the end, it is the reverb transformer. Ordered a replacement TXRE-00002 from Europe, couldn't find one in the states. But... I have a problem. I thought the TXRE-00002 will be a drop-in with the same writing installation as the original. After installing the new one I loos most of the volume on the JCM800 2205 and it sounds weird and thin. I did get reverb working but at what price.... : Uninstalled the new transformer (TXRE-00002) and installed the old original one (Not brand or any indication) and got the normal sound back... as expected not reverb though.
    Question: Perhaps the four pins in the new TXRE-00002 transformer are configured differently and I need to connect the wire in a different configuration?

    Any suggestion, tip, recommendation, will be greatly appreciated.
     
  9. headcrash

    headcrash Well-Known Member

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    If the new tranny hadn't been shipped without any manual or the like, you shpuld contact the seller and ask for instructions. Sounds to me like a wrong wiring.
     
  10. zaiata

    zaiata New Member

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    Thank you headcrash. I did contact the seller who politely admitted that they only sell Marshall parts but have not idea on the technicalities. Yes, I also think it most be a wrong wiring. Will keep trying and also will check all the components related to the reverb circuit.
     
  11. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

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    There's a guy you can Contact, I think his name is Kevin Drury, he's somewhere in Illinois iirc, he's a factory Marshall parts/ service guy. He should be able to tell you if you have the correct part number.
     
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  12. Musicmaniac

    Musicmaniac Well-Known Member

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    Kevin's the man! [email protected]
     
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  13. South Park

    South Park Well-Known Member

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    This may not sound right if you can’t get a Marshall transformer you can use a fender transformer with the same specs
     
  14. zaiata

    zaiata New Member

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    Thank you so much Gunner64, Musicmaniac, South Park. Good suggestions. Really appreciate the support.
     
  15. zaiata

    zaiata New Member

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    SUCESS!!!
    With the support provided in this forum the amp is working beautifully.
    I trashed the information provided by the old transformer since it is broken. Followed DonP instructions: "There will be two wires with a high resistance (10K? maybe higher?), and two wires with a low resistance (20 ohms or less). The higher resistance winding goes to the tube" and wired the transformer accordingly. Them applied thetragichero advice: "might have to swap the leads on one end if it is out of phase"... Back in business!!!
    I have not words to express my gratitude. THANK YOU!
     
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