Marshall JCM 25/50 Low Volume & Distorted

[dstrat]

yes I think so.

[Aerostoon]

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.

[Enzo]

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.

[Aerostoon]

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.

[dstrat]

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

[Enzo]

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.

[Aerostoon]

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.

[Randall]

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?

[Enzo]

Sorry, that information didn't stick.

[Aerostoon]

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.

[dstrat]

Nice that should work fine, may be even better.

[Familyortiz]

Just thought I'd ask... did you look for the obvious, like broken solder joints or other?

[Aerostoon]

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!

[Enzo]

A good visual inspection is always good, but you cannot always see a failed solder connection. best to reflow with a bit of fresh.

[Aerostoon]

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.