Can anyone help me diagnose a reverb issue on a modded 83' JCM800?

Discussion in 'Amp Tech Center' started by darkbrewery, Jan 13, 2019.

  1. darkbrewery

    darkbrewery TDPRI Member

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    So I have this Classic Marshall 2204 that someone has added an Accutronics 8EB2C1B to.

    All the tubes and caps were replaced about 10 years ago when it was purchased and the amp works flawlessly just no verb.

    You can hear a boing if you smack the tank. Swapping Input/Output RCA's doesn't make any noticeable difference. Moving around the preamp tubes does nothing either.

    Upon further inspection of the amp: The low impedance input jack was removed and replaced with a 1M pot for the reverb wet/dry. An extra 12ax7 has been added for the recovery circuit. Also some sort of small transformer is using part of an original 12ax7 for the drive circuit.

    Measuring the reverb tank:
    Resistance across input transducer: 57.7 Ohms
    Resistance across output transducer: 197 Ohms

    I've been searching for days to find a schematic for this mod but nothing yet.
    Closest I can find is on this page: spring-reverb-tanks-explained-and-compared

    Just hoping someone on here could guide me through a diagnosis.
    I'm leaning towards a faulty transformer, the one added for the mod.

    Images: https://imgur.com/a/3RYb54F

    [​IMG]
     
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  2. bftfender

    bftfender Friend of Leo's

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    Ahh..JCM800 on here ..what a pleasant viewing moment i just experienced !! That head on that cab prob got some great natural reverb in general. Hope you can get it all figured out !! Would love to play that rig !!
     
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  3. clintj

    clintj Friend of Leo's

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    What test gear do you have? What would be ideal is a scope or a simple audio tracer to check the Send side of the reverb circuit, but even a DMM that can measure AC voltage could work.

    Looks like a one tube reverb circuit was spliced into that, creatively.

    Sent from my Pixel 2 using Tapatalk
     
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  4. darkbrewery

    darkbrewery TDPRI Member

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    I just have a cheap multimeter, DC voltage between 1 and 6 on the drive tube is 220v.
     
  5. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Has the reverb ever worked since you owned the amp? If not, it's possible the mod never worked and it isn't even wired correct at this point.
     
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  6. clintj

    clintj Friend of Leo's

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    I keep thinking this over, and I'm not seeing a good way to walk through this, let alone guide you through a repair remotely. I'd have to have it on my bench to study for a little while and figure out the schematic as it's built. I think @keithb7 and a few other members are in BC (not a small province, I know) and might be willing to have a look and get hands on, or recommend a good repair guy to you.

    Sent from my Pixel 2 using Tapatalk
     
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  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    ^That. Since it's not a stock thing, there's no way for us to be like "well check the resistor on pin $' since we don't know what all was changed.
     
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  8. keithb7

    keithb7 Friend of Leo's

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    A "booiiingg" when hitting the tank tells me that likely the return section of the reverb circuit is working. It would appear that there is no guitar signal being sent to the reverb circuit. Tracing the guitar signal from the preamp section into the reverb send circuit is helpful. It could be something simple like a bad resistor. Not allowing the signal past it. Have you tried testing all the resistors and connections in the reverb circuit? Look for broken wires. Could be a transformer.

    I have no idea how much experience you have inside an amp. So I am reluctant to say more. There are lethal voltages in there that would need to be controlled or eliminated while you test components.
     
  9. darkbrewery

    darkbrewery TDPRI Member

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    Yeah it did work initially and has just stopped, not sure exactly when this happened.
     
  10. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Cool - so if it did work, tracing the signal into the reverb driver side (if you have the skills to do this) would be the best plan.
     
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  11. robrob

    robrob Poster Extraordinaire Ad Free Member

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    You can switch the tank's input and output cables and bang on the amp again. If you hear the spring crash (it will be quieter this time) you know the input transducer is working. After this test you will know the problem is upstream of the tank, which is the reverb driver circuit including the reverb transformer. Swap the tank cables back to normal.

    Before replacing the reverb transformer I would measure resistance (with the amp off) from both reverb driver (the tube connected to the reverb transformer) cathodes (pins 3 & 8) to ground to make sure it's reasonable, somewhere around 2.2k. The grid to ground (pins 2 & 7) should be around 1M. With the amp on verify you have heater voltage around 6.3 volts AC between pins 4 & 9 and 5 & 9. In a darkened room verify the tube glows, at least a little.

    If all that checks out then order a transformer.
     
  12. darkbrewery

    darkbrewery TDPRI Member

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    Awesome thankyou I will test all this tonight after work much appreciated!
     
  13. darkbrewery

    darkbrewery TDPRI Member

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    Pins 3 and 8 to chassis ground both measure 840 Ohms
    Pin 2 to chassis ground reads 1M Ohms
    Pin 7 to chassis ground reads 300k Ohms
    Pins 4 and 5 are bridged on all preamp tubes, the heaters read 5.4V AC
    All the tubes glow.
     
  14. Snfoilhat

    Snfoilhat Tele-Afflicted

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    I'm curious what the original owner/modder was thinking with respect to the load on the reverb driver. Most amps I have seen that use a transformer use a low impedance tank (8R) with an Accutronics label XABXXXX. But this person chose the XEBXXXX tank with an 800R input impedance, which typically is directly driven by an op-amp. So, should the OP buy a known good reverb transformer with something like a 22k:8R ratio (Fender) and re-mod his amp using the typical DIY 1-tube reverb type setup, spending an extra $20 for a XABXXXX tank, or should he keep the 800R tank and try to find some unknown single-ended transformer that will together with the old tank make some kind of workable primary impedance for a triode to drive? Because just popping over to whatever shop and buying a 'reverb transformer' will likely end up with a 22k:8R plugged into a 800R load on the secondary, for a primary impedance of 2200k! (2M2!)
     
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  15. robrob

    robrob Poster Extraordinaire Ad Free Member

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    ^Great point.
     
  16. darkbrewery

    darkbrewery TDPRI Member

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    Alrighty so I found some help diagnosing this over at MarshallForum, We determined the Primary winding on the reverb drive transformer is likely shot, so I'll replace that with a readily available fender 8 Ohm output version, along with a new Accutronics 8AB2D1A as per Snfoilhat's suggestion!

    Much appreciated!
     
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  17. robrob

    robrob Poster Extraordinaire Ad Free Member

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    What was the diagnosing technique used to test the reverb transformer?
     
  18. darkbrewery

    darkbrewery TDPRI Member

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    Tested resistance across the primary winding on the reverb driver, came out at 11.9k Ohms, apparently should be closer to 1k.
     
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