Problem with JCM800, fraps out over 2 on the master

[BluePat]

By "fraps out" it sounds fine until I dial anything over 2 on the master, then the signal cuts in and out in rapid succession, like a bad cord or something. I have swapped out all tubes, and plate voltage are where they should be. I'm suspecting the pot itself... but any thoughts or opinions are welcome.

Pat

[demundo]

input jack or power supply problem. which model is it?

[BluePat]

It's a 2203, vertical input.

Now it's worse, much worse... No output whatsoever, no hiss, dick all... :FM

Checked the output jack, not that, so I'm gonna havta go stage by stage I guess...

Any pointers or guidelines would be mucho appreciated!

Pat

[phsyconoodler]

Change the first preamp tube first.Then check the pot.It obviously had something happening before 2 on the dial.
You will need an ohmmeter to check the pot.
If worse comes to worse,book it in at L&M for repair.I work there.I could find the problem easily.

[jay42]

no hissThat's a bad sign. I was going to ask if it happened with both the high gain and low gain inputs. Pull all the tubes and start with the power supply...make sure the PT isn't getting hot.

[BluePat]

No signal/hiss on either hi or lo input...
Swapped several tubes in v1, cleaned the contacts, etc...
Removed and cleaned pot, sweeps ok as per ohmmeter...
Bias voltage ok...
PT or OT never got hot or smoked... but my nervous feeling is the OT...

[phsyconoodler]

Are you confident enough to do a voltage check?If so,list the voltages on all tubes.
That will tell us a lot.Also.is the HT fuse blown?It may be blown and the tubes will light up,but there will be no sound.

[BluePat]

*Update*

OK, it's back to where it was: a lead had broken loose from pin 1 on the inverter tube, so that would explain ceasing all activity to the output stage, but it still does that "frapout" over 2 on the master... It sounds like it's laboring to pass signal (gets compressed), then begins to cut in and out rapidly (like a sick tremolo), does it with different tubes, etc, so dunno?

[phsyconoodler]

You still need to check voltages.Voltages will tell us what's happening with the amp.
All the pins including the AC heaters.Everything else is DC.
The bias supply can crap out on the JCM800 if it hasn't been serviced for 20 years.That can be diagnosed with voltage readings as well.

[BluePat]

OK, took the time this morn to measure all voltages across all tubes:

Any pins not listed are 0 volts measured

V1
pin 1-260
pin 3-2.83
pin 6-220
pin 8-1.869

V2
pin 1-171
pin 3-1.05
pin 6-301
pin 7-171
pin 8-172

V3
pin 1-242
pin 2-23.6
pin 3-36.4
pin 6-223
pin 7-24.7
pin 8-36.4

Output tubes, from left to right, (pretty consistent measurements)

#1
pin 2-473
pin 3-468
pin 4- neg 43.1
pin 5-470

#2
pin 2-470
pin 3-465
pin 4-neg 42.9
pin 5-469

#3
pin 2-470
pin 3-465
pin 4-neg 42.9
pin 5-469

#4
pin 2-469
pin 3-465
pin 4-neg 42.9
pin 5-467

After cleaning the tube sockets with spray, it no longer fraps out above 2, but it doesn't have nearly the gain/volume it should either...

Pat

[rockon1]

Wht not change out all the pre's(if you havent already)especially the PI since you already tried the V1. Bob

[phsyconoodler]

Your readings are not correct on the tubes,but they look ok at a glance.(wrong way around)
If there was a change by cleaning the tube socket,there is likely still an isue there.Bad contct can still be the issue.You may try retentioning the pins and if all else fails,change the sockets.
You need to be able to check the current draw on each tube.

[svesta]

I had a similar issue and it was the effects loop. Try running a straight cable between the send and return jacks.

[rockon1]

I had a similar issue and it was the effects loop. Try running a straight cable between the send and return jacks.

Might be-but only if its a reissue. Originals didnt have an FX loop. Bob

[BluePat]

Update:

OK, all is good! I swapped out the old remaining LCR caps (2 were done, 4 were original) and she roared back to life, but that socket on V2 has to be replaced as it is intermittent in it's temperament, aside from that the amp is fine!

Thanks for all the replies guys,
Pat

[BluePat]

OK, it's back, but it seems it's intermittent...

I swapped out the MV pot, just in case, no change...

Could it be the cap at C10?..

[EVHindenberg]

Did you change the socket at V2? I had the same problem on an '83 2204 and a socket change proved to be the fix.