Marshall JCM 25/50 Low Volume & Distorted

[Aerostoon]

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.

[Familyortiz]

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.

[Aerostoon]

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.

[Familyortiz]

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?

[Aerostoon]

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.

[cjenrick]

measure across that 470 ohm resistor on the phase inverter, that pin 7 reading is skewed due to the hi Z nature of the grid acting against your volt meter.

the first hand written voltage chart is fine and you stated the problem well.
here is what i do, screen capture the schematic and save as jpg file. record dc voltages and add as text in say, the color blue.

then inject a 25 mv ac voltage and turns controls up to 50% and record ac voltages in the color red.

then post the whole enchillada and we can solve it in a heart beat.

right now try injecting a signal and measuring ac voltage on the phase inverter plates,

[Familyortiz]

My earlier comment was to isolate any mechanical cause of your level/distortion. Another simple check to isolate any diode failure in the power section, is a quick diode reading with a multimeter in diode mode. Until variables are isolated, everything is a guess.

[g1]

the first hand written voltage chart is fine

He stated (in post #15) that the first one is incorrect and the corrected version is in post #2 (pdf). The V2 voltages between the 2 posted charts are very different.

you stated the problem well. sometimes people put down others to feel superior.

I really must have missed something here? Who was putting anyone down?

[Aerostoon]

Well I did some tests on the transformer. The secondary winding with the selector at 8 ohms reads 1.7 ohms. On the primary outer windings I get 95.4 ohms. From the primary outer windings to Center tap I get 48.6 and 47.4 ohms.

Next I put 10 VAC into the secondary windings and measured 185VAC on the outer primary windings. Then 91.4VAC and 91.3VAC for each half of the primary to Center tap. This seems to confirm the output transformer is good. Do you guys agree?
I will try some of the good ideas you guys have posted. Thanks so much for everyone's help so far

[Enzo]

Looks like it is working.

The resistance readings don't tell us much other than each winding is not open.

[dstrat]

my math says primary imp is a little low, 185 / 10 = 18.5 * 18.5 = 342.25 * 8 = 2738 about 660 ohms low assuming 3400 is usual?
enough to make a big difference i do not know, maybe someone will verify my math
but it does look to me like some shorted turns.

aero can you verify you have a 784-139 transformer? and have you taken any current readings on the el34s?

[mac dillard]

Keep on checking. Your are penning down the problem. FWIW, I have much of the same problem with an AB165 Bassman. Low output and distortion. Put the scope on it and the signal started getting bad at the PI. Problem turned out to be bad OT..Someone on here explained how the OT could cause the readings we were getting.

[Aerostoon]

The output transformer is a Drake 783-303/7. I haven't taken any readings on the EL34's after I made the PDF voltage chart. I measured R36, R38, R50 and measured C28 with my cap meter. All measure very close to spec. All in circuit tests so I might not be able to trust them fully.
I will do some of the tests suggested prior next after I reflow the solder connections like Enzo suggested.

[dstrat]

i am not familiar with that transformer so that may be perfectly normal i don't know.

i was just curious about the tube's current. the voltage chart is good as far as voltages go,

someone else said there was something suspicious about v3 but it looks about right to me but i am no pro amp expert.

[Aerostoon]

Good idea, I will measure and post the plate dissipation. I have a eurotube bias probe so it's fairly easy to check.