Marshall JCM 2000 60W HELP!!

[diydidi]

Need some help!!
I have this amp for repair. It is a combo amp and the complaint is that it is soft and distorting.
I measured the output into an 8 ohm load, and it only reaches ±10W RMS at clipping. Inputting a signal into the FX return yields the same result.
Shouldn't it it give me around 60Watts?
Scoping the plates of the output valves gives me the same clipping.
I have replaced the phase splitter and the 2 EL34's. I measured the plates on the El34's at around 445Volts.
No dry joints found, amp biases properly to 40mA per valve.
What else can I check?

[drewl]

Which speaker jack you using?
The 16ohm jack is the ground for the other two so if it is bad you won't get much output.
Also check that rear pc board for bad/broken PC board connections for the jacks etc.

[diydidi]

I have checked with both output jacks.

[g1]

How is your screen voltage?

[J M Fahey]

1) you might have a shorted OT which would explain all symptoms so far.

2) that said, let's pray it's not and run all checks neccessary to prove it's not

3) I'll *assume* a lot of things, and am not solving he connect the dots puzzle Marshall wants us to, so I may make wrong assumptions, please take what I say with a spoonful of salt and doublecheck yourself.
If you see something wrong, say it here.

4) I assume black rectangles to the right of the OT called W6/5/3/4 (they don't even follow a sequence ) are taps: 0/4/8/16
Is it a connector?
Unplug it and either tack solder below to pads or kludge a connector on top, and connect 0/8 ohms or whatever matches to the speaker.
You'll lose NFB, at this moment I couldn't care less about such a minor point.

5) the direct input to the power amp PCB seems to be Con8(to Con 1) .
Unplug it also, check that you didn't lose ground connection to main PSU ground, if you did run a plain wire restoring it, and feed up to 2 or 2.5V RMS 1kHz to the hot pin, audio should go to C25.

Plan B: or if a standard through hole part with leads, lift left end of C25 (instead of unplugging the connector) and feed signal there.
If SMD, working on the connector is safer/faster

You *should* be able to drive power tubes to saturation.
Never saw real 60W RMS there at the onset of clipping, but would be happy with 45/50W RMS . Even clean unclipped 40W, go figure.

If still 10W or so, OT is shorted.

Reconfirm with RG Keen's shorted transformer neon lamp test.

Post results.

ONLY if OT is confirmed good, then go on having fun following lesser problems such as poor jack contact, etc.

[diydidi]

Screen voltages on Pin 4's :435Volts

How is your screen voltage?

[g1]

Another incomplete Marshall schematic .
I don't see the FX loop jacks anywhere, so I don't know if there are any related mute Fet's etc.
So see what the signal level is at the power tube grids when it clips.
That's the only other thing I can think of, aside from the OT checks outlined by JM.

[diydidi]

JM, I did all the tests you described. RG's transformer tester shows no short. Neon bulb lights on most all tap combinations.
Tried disconnecting secondaries and connecting with 8ohm load, still the same result.
The signal clips top and bottom exactly the same time. I'm dumbfounded...

[diydidi]

Another incomplete Marshall schematic .
I don't see the FX loop jacks anywhere, so I don't know if there are any related mute Fet's etc.
So see what the signal level is at the power tube grids when it clips.
That's the only other thing I can think of, aside from the OT checks outlined by JM.

With a sine 1Khz input to FX Return, I get
Each Grid shows 70Vp-p just before output clips.
Each Plate shows 350Vp-p just before output clips.
Output with 8ohm load shows 35Vp-p (±19W RMS)
Don't think this is normal?

[J M Fahey]

With a sine 1Khz input to FX Return, I get
Each Grid shows 70Vp-p just before output clips.
Each Plate shows 350Vp-p just before output clips.
Output with 8ohm load shows 35Vp-p (±19W RMS)
Don't think this is normal?

1) Halfway between normal which woukd be >40W RMS at clipping onset and the 10W RMS you quoted earlier.
Not there yet but getting closer.

2) what is the bias voltage?
Measured at the junction of R12/R13

3) what is the idle current?
mV measured across R3

4) indulge me, I'm going for Power, not Tone: bias tubes colder to 20mA each, or 40mA total, so 40mV across R3.
Just vto keep data updated, post the new bias voltage.

5) now repeat clipping tests, but 3 times:
a) 8 ohms resistive load across OT W6/W5 (nominal 4 ohms)
b) 8 ohms resistive load across OT W6/W3 (nominal 8 ohms)
c) 8 ohms resistive load across OT W6/W4 (nominal 16 ohms)

That transformer is not shorted (or you wouldn't get 20W RMS) but tubes may be poor/worn and simply not be able to pass enough current to drive the load in full .
There's also another possible reason, even with good tubes, which would be low screen voltage, but you already checked that (what G1 asked )
Just so as not to leave any stone unturned: please recheck that screen resistor values are fine.
Unlikely, but, hey, it takes 1 minute.

6) what I see from your data is that tubes get proper drive voltage, peak should be ~Vbias and you are about there, but plates do not swing enough , you should have from 600Vpp to almost 700Vpp and you only have ~350Vpp, too low, and consistent with halved power output.
I guess that some other output tap will give you more power or at least the same, showing a *current* problem there. That's the particular problem I'm chasing.

7) almost forgot: reading Vpp at a scope screen is a quick check in my book, but please read VAC at speaker/load terminals with a multimeter.
Most modern multimeters are flatter than rated, but just in case, use 400Hz for the test, where calibration is guaranteed.

8) as an extra check, plug another pair power tubes in there (rebiasing of course) and repeat power test, we might be chasing ghosts here looking for *circuit* problems while we might just have weak tubes.

That said, my idea well has dried, hope some of these tests finds the culprit.

Good luck

[TimmyP1955]

On the 100W models, there's a cap from Pin 4 to Pin 3 on one of the power tubes. When that cap shorts, the symptoms are clipping at a much reduced output level.

On the schematic you provided, there's such a cap on both power tubes (C26 and C27). I'll bet that one or both of those is kaput.

[diydidi]

TimmyP1955. I just managed to get the amp going, and then I saw your post. You are correct!! It was these 2 little basterds!! I just cut them out completely. Amp seems stable without them in.
Well took me almost a week to find. FML. Anyways thank you for everyone's help.