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Thread: Marshall JCM 25/50 Low Volume & Distorted

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    Supporting Member Aerostoon's Avatar
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    Marshall JCM 25/50 Low Volume & Distorted

    I have been working on a JCM 25/50 Silver Jubille and it has me a bit puzzled. The issue seems to be in the preamp. Some of my voltage reading on the preamp tubes look strange. Any thoughts?

    EDIT 08/03/2017: Chart of voltage readings in this post are incorrect. Please refer to PDF file in Post #2.
    The Janitor
    Attached Thumbnails Attached Thumbnails image.jpeg   image.jpeg  
    Last edited by Steve A.; 08-03-2017 at 10:30 PM. Reason: Requested by member

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    Supporting Member Aerostoon's Avatar
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    Here is a better more readable version.
    Attached Files Attached Files

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    Senior Member Enzo's Avatar
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    Well we have no idea which voltage readings you took and what makes them strange, so without that data how can we help?
    Education is what you're left with after you have forgotten what you have learned.

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    Supporting Member Aerostoon's Avatar
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    I attached a PDF with the readings in the 2nd post. Does it show up for you? Sorry if it doesn't.

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    Senior Member Enzo's Avatar
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    Sorry, I thought it was a more readable schematic, so I had not opened it.

    What do you object to in that chart?
    Education is what you're left with after you have forgotten what you have learned.

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    +1 for the voltage chart , but they look pretty normal to me, what kind of tube current? How low volume are we talking here? lots of failure modes are possible.
    my guess is output transformer, which is a wild guess at that.

  7. #7
    Supporting Member Aerostoon's Avatar
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    I guess I am more used to dealing with problems in the power tube area. I was expecting close voltages on the same pins for each preamp tube. With all of the volume and tone controls I guess that was a silly mistake.
    For the transformer I did remove the power tubes and put a voltage on the primary with my variac and I measured the secondary (speaker connection) and it seemed correct. I will redo that test and post the results.

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    you might try to put a cable in the effects loop and see if it is in the loop jacks.
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    Supporting Member Aerostoon's Avatar
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    I tried a jumper cable as well as metering through the effects loop jacks. They seem very good. It was the same reading as holding my leads together.

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    do you have another power amp you can plug the send jack into to see if the pre is sounding ok?
    one thing i see is v2b pin 7 says -32.8mv is a bit odd, have you tried different tubes, i missed that last night.
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    sorry second thought not so odd because lower plate goes grid goes up, or more negative.
    Last edited by dstrat; 07-17-2017 at 03:07 AM.
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    nope i felt like that statement was incorrect so i went to the bench and tried to duplicate those conditions,
    but could not so either bad tube , resistor , solder joint on v2b.

    apologies if i sent you on a goose chase.

    dave

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    g1
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    Which are the correct voltages, the ones in the pic or the ones in the pdf? The voltages are very different from one to the other.

    In any case, grid voltages can be hard to read, I would not worry about a touch negative reading there if the plate and cathode readings look ok.
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    g1 your right much different, the pdf has pretty much what i got on the bench. i obviously failed to even look at that file.

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    Supporting Member Aerostoon's Avatar
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    Thanks guys, the PDF is the correct chart. So you guys think the chart voltages are ok? I don't seem to be able to go in and delete the first chart.

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    yes I think so.

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    Supporting Member Aerostoon's Avatar
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    Distrat I tried your advice on connecting a cable from the send of the JCM 25/50 to the return of a different amp and it seems to be working just great. Lots of volume and a nice clean tone on the clean channel and a nice gritty tone on the lead channel. I will apply voltage to the output transformer secondary and measure what I get on the secondary.

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    Senior Member Enzo's Avatar
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    Am I overlooking something? You seem to be concerned your voltages may be wrong, but I don;t see a description of what you are trying to fix. I mean measuring voltages is great, and testing the OT for whatever is fine, but what is the goal?

    You report that patching across the FX loop with a cord results in great sound. Are we to assume then without the cord it does not? In which case the return jack needs service, not the transformers.
    Education is what you're left with after you have forgotten what you have learned.

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    Supporting Member Aerostoon's Avatar
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    I sure didn't introduce the problem with my amp very well. The amp volume is extremely low. You almost have to have all 3 volume controls up all the way to get any sound. The sound is extremely distorted sounding.

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    while testing a output transformer with voltage will tell you the ratio,
    you may have better results with a simple tester you can build. find it at
    New Page 1
    go to " Tube Guitar Amp Tech Pages "
    then " Transformer Short Tester - transformer busted? Find out! "
    will tell you if it has shorted turns, which is what i suspect is the problem.


    dave
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    Senior Member Enzo's Avatar
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    OK, all the more reason then...

    If putting a cord from FX send to FX return restores good strong sound, then you have the VERY common problem just about any amp with an FX loop can have, a bad return jack. The cutout contact in the jack is likely dirty.
    Education is what you're left with after you have forgotten what you have learned.

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    Supporting Member Aerostoon's Avatar
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    Hi Enzo, I used a cable from the send jack on the Marshall to the return jack on my Peavey Ultra 112. It had a lot of volume and nice tone. I tried using a jumper cable earlier on the effects loop jacks on the Marshall with no improvement in operation. I hope this makes sense.

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    Supporting Member Randall's Avatar
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    OK, that would seem to say that your preamp on the Marshall is functioning. So wire in reverse. Send from Peavey to return on the Marshall. What are the results of that?
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    It's weird, because it WAS working fine.....

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    Senior Member Enzo's Avatar
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    Sorry, that information didn't stick.
    Education is what you're left with after you have forgotten what you have learned.

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    Supporting Member Aerostoon's Avatar
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    Good idea. I will give this a try. I also have an Anatek Blue Ring Tester that should work to see if there is a shorted winding in the transformer.
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    Nice that should work fine, may be even better.

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    Just thought I'd ask... did you look for the obvious, like broken solder joints or other?
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    Supporting Member Aerostoon's Avatar
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    Quote Originally Posted by Familyortiz View Post
    Just thought I'd ask... did you look for the obvious, like broken solder joints or other?
    I did look the chassis over. I pulled up the circuit board and looked everything over with a 10X magnifying glass. I didn't get to test the transformer out last night. Yard work got in the way!

  29. #29
    Senior Member Enzo's Avatar
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    A good visual inspection is always good, but you cannot always see a failed solder connection. best to reflow with a bit of fresh.
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    Education is what you're left with after you have forgotten what you have learned.

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    Supporting Member Aerostoon's Avatar
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    I tried connecting the send effect loop on the Ultra 112 to the return loop on the Marshall. Still very low volume. The volume is as low as using the Marshall itself. I removed the power tubes, drained the filter caps and used the Anatek Blue Ring tester on the secondary windings. Moderate Q reading. I then tried the Anatek on the primary output transformer windings with a moderate Q reading. I'm going to try injecting a small AC voltage into the secondary windings tomorrow and see what the primary readings are. I found a test that Enzo recommended on a similar thread. Any other ideas?
    I did try a brand new pair of EL34's as well with no change.

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    Supporting Member Aerostoon's Avatar
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    Quote Originally Posted by Enzo View Post
    A good visual inspection is always good, but you cannot always see a failed solder connection. best to reflow with a bit of fresh.
    Thanks for the suggestion. I will give this a try after the OP Transformer tests.

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    Without the switch, it is clear that the issue lies past the effects return. I like the idea of testing the return jack and it can be done with a simple resistance measurement, testing both the signal and ground continuity, when the return jack is plugged in.
    Separately, I see nothing mentioned where you go tube by tube, with a known good tube, power or pre, to test the tubes.

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    Supporting Member Aerostoon's Avatar
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    Quote Originally Posted by Familyortiz View Post
    Without the switch, it is clear that the issue lies past the effects return. I like the idea of testing the return jack and it can be done with a simple resistance measurement, testing both the signal and ground continuity, when the return jack is plugged in.
    Separately, I see nothing mentioned where you go tube by tube, with a known good tube, power or pre, to test the tubes.
    Thanks for the reply. I have tested the effects loop jacks and they check for continuity. I also tried a jumper cable to bypass them. I have swapped out known preamp tubes and I know the issue isn't in the preamp section. In my previous post I indicated a new set of EL34's did not make any improvements. I am going after the Ohm selector switch and the output transformer now. My Q meter is giving me a moderate Q reading for the output transformer windings. I still have the resistance checks and the ac voltage checks to do.

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    Sounds good. Sorry for missing the power tube swap. I know I'm getting back to primitive but does the noise respond to tapping anywhere on the chassis?

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    Supporting Member Aerostoon's Avatar
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    The issue isn't related to noise. The amp has very low output which is distorted as well. The problem is definitely in the output stage. Hopefully it isn't the transformer. I am going to meter through the impedance selector and make sure continuity is good through it. I am going to induce a small ac voltage into the secondary windings (6VAC) and measure what I get out of the primary halves. Power tubes out of course.

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