JCM 2OOO tsl 602 "won't switch channels"

[oc disorder]

V4 is the last tube before the output tubes.
Its the phase inverter / final driver sometimes also called phase splitter.
This should be alright as the clean channel works ok .
This valve is constant and not switched or altered in any way by the footswitches.
The reverb uses solid state MC1458 IC.
I suspect V3 may have an open resistor R 111 a 100K or R113 a 4k7.
See this previous post where I posted some voltage readings.
http://music-electronics-forum.com/t8548/

I'll put v3 here for convenience.R111 and R113 are lower left in the photo.
Also a section of the schematic ..note V3b doesn't have a load resistor
on the anode but is in the cathode.

Wonder if plexibreath ever redrew the schematics into a more readable
version ?

[brydon2]

So, V3b is utilized by both the od and lead channels and the "break" at the cathode is shutting down both channels?

Would the best approach be too replace R111 and R113, I always check R'S when building beforehand but not sure how to when their in circuit?

Wow!, I hope this all comes together, Thanks much

[Enzo]

And please don't tell us "no reading." The meter did something. You got zero volts, or you got an open indication, something...

[oc disorder]

"So, V3b is utilized by both the od and lead channels and the "break" at the cathode is shutting down both channels?"
Well I cant spend too long trying to figure it out but it looks to me that
with the jfet (F11) at the input the two tube segments cascaded (pin 1 to pin 7) and what looks like multiple relay contacts on the output this would insert heaps of grunt/gain into the circuit so I'll stick my neck out and say
V3b is utilized by both the od and lead channels !
With the amp off you should be able to read 100k (90 to 110) and 4k7
(ie 100,000 and 4700 ohms) across the resistors.

It still could be a faulty relay or fet and associated circuitry but based on your readings I'm guessing the 2nd half of the 12ax7 is out of action.

I would also replace that tube ( if it is an open resistor) after its been repaired.

BTW my comment re plexibreath relates to this thread
http://music-electronics-forum.com/t13068

[brydon2]

new cathode readings

Enzo, I just rechecked with the meter on the 200vdc range and its showing

58.5 on both V4 cathodes.I used the 20vdc range prior I believe, noob error I

suppose, my apologies.It's been a while since I worked on anything.

Curious as to why the V3b plate voltage is higher than the rest?

Thanks for the response!!

[brydon2]

I'll check the resistors in circuit and see!, Is there any way to determine which

relay is the problem? there is two of them.

R111= 98.1K AND R113= 4.7K (on 200k meter range)

Does this mean its almost surely one of the relays??

[oc disorder]

"Does this mean its almost surely one of the relays??"
Umm not necessarily .Could be a fet with permanent mute syndrome
Can you re take the readings on v3 ?

V3 plate=214 cath=1.3 PLATE=345 cath=BEEEP (not sure what that means

As per the schematic the plate of V3b is connected to the supply rail without a dropping resistor. The grid of V3b is connected to the plate of V3a.
There should be approx 190v on V3b cathode.

Can you try another tube in V3 ?

If your older guy Fender Tech has an oscilloscope we're back to tracing the signal through on the channel that doesn't work.

Like Enzo said "Trace the signal and see where it stops. Try different preamp tubes in the sockets. Look for missing B+ at the plates. etc."

Well you've tried the last one first !

[brydon2]

V3b cathode reads at around 200vdc w/ two different tubes and I thought that was a run-away current situation and the problem was close to be solved.

I don't know that I could "trace" this based on the schematic, how the hell do you do that?

I'll keep trying, who knows! Thanks for the help so far!

[oc disorder]

"I don't know that I could "trace" this based on the schematic, how the hell do you do that?"
Well using an oscilloscope set to AC you apply a signal (usually an oscillator but a synth with a note stuck on or even an attenuated mp3/cd player would suffice) to the input and you place the 'scope probe on the places one expects to find audio. ie grids and anodes.
But this can be deadly ..so you have to be familiar with this sorta work.
Regarding the relays you cant be sure its a faulty relay perhaps the 12 volts for it is not getting there .You could measure across the damping diode across the relay coil eg D19 and D14 and see as you press the footswitch if the 12 volts goes on and off.
Likewise using a meter you could check that the changeover contacts are ...well changing over but this would require some detective work to find available points to measure, from the schematic.
Also the two Fets F14 and F11 act like a switch shorting the signal to ground or not .Just noticed that the FX send comes from V2b so if you have another external amp there always should be a signal coming out of here regardless of the footwitch.
Again the quickest route is to follow the signal from that point which I presume is always on.

[brydon2]

Well I guess I know why the amp hasn't been fixed yet!, so I would need a signal generator and a scope to continue?

I have to find away to isolate the components used by ch 2 and 3 and not by ch 1 (working channel). Sounds simple enough, right??....LOL, this is insane!

I mean the power-supply is fine and the tubes are fine.Thats the two most common problems with tube gear!!

We'll see what happens, it doesn't look good!, Thanks for all the input

[Enzo]

The cathode there has 200v because that stage is a cathode follower. That is normal.

The relays could be at fault, but they are way down my own list of suspects. And if anything is wrong with them, it wouldn;t be the coil, it woulod be a bad contact inside.

Your problem is missing sound in these channels, not distorted sound or anything. So all you need is go or no-go tests. You don;t need a signal generator, you just need a signal. You could play the guitar into it, but that doesn;t leave your hands free. SO as oc mentioned, connect a synth to the amp input and sticka key down with something for a steady tone. Of use a CD player or mp3 player. As long as there is SOME sort of sound at the input, we are ready.

A scope is ideal, but if you lack one, set your voltmeter to AC volts and use that.

If I apply a half a volt of some signal to the input jack, I should see a goodly number of volts of signal at the plate of the preamp stages. (Except that one cathode follower stage) SO my AC voltmeter will detect that. If you are not sure that is what you are measuring, turn your test signal off and on and see if your meter reading goes off and on with it.

Then follow the signal path through the amp looking for that signal, and when you find the point after which there is no more signal, you are very near the problem.

Missing voltages and swapping tubes are fast simple things to check. But when the amp has an actual problem, instead of a bad tube, then we have to troubleshoot. The essence of troubleshooting is to isolate the problem.

yes, you couild have a bad relay contact, but you could also have an open coupling cap or a broken solder joint in the pathway.

[oc disorder]

Beats Rubic's cube.... Look for a start as you have the amp in front of you
find out the volume controls and labelthem on the schematic.
Eg to me the relays select different master volume controls eg top right
connector 13 goes to con 4 (bottom right) which is VR1 a 1 meg pot.
Seems Relay 2 does this.
Relay 1 seems to bring in con 15 which goes to con 9 (top left 2nd page)
which is another audio taper 1 meg pot VR3.
Notice R1 also switches in a 10 meg (very high) resistor making VR10 redundant .VR10 is also a audio taper 1 meg pot.
Looking on the circuit board you should be able to see VR1, VR3 and VR10
printed so I would label them on a copy of the schematic what they actually are. Master - xyz gain or whatever.
My guess they select different masters.
Also see where relay R2 grounds the signal from the junction of R110 and R112. My quick guess is that bypasses the eq circuit to the left of it.
Not sure what VR 14,15 and 16 are.However the mysterious M 5201 switches either pin 2+3 to pin 5 or 7+6 to pin 5 see attached.
Yes this is an awkward schematic to work from hence my musings whether
plexibreath ever redrew the schematics into a more readable version ?
Good luck !
ah just noticed that Enzo replied while I was typing this. Hopefully by labeling some of the controls you will have a better idea where the signal goes.
Also you can remove the two power tubes while you do this if its making too much noise while
you trace.

And yes bad solder joints could be all it is.

[brydon2]

How many components can the od channel use that the clean doesn't??

The main problem is I have no clue what direction the signal path goes in!

My meter has a 200 and a 600 setting on AC, and I cant "see" anything with

that I'm checking pins 1,2 and 6,7 of V1.

OC D I'll have to re-examine your last post! thanks

[brydon2]

Well, I guess I'm gonna need a AC voltmeter to continue.

Thanks alot to all those helping out with the problem, hopefully it will be fixed soon!

I wonder if it would be worth replacing the top 5 small components as it doesn't seem to involve anything major and these "bits" are usually quite inexpensive? Just a thought though it may be pure lunacy!!

[Enzo]

Put your meter on the lowest AC scale, set the amp up for clean so you know it is working. take some sample measurements. Your meter should be able to find 20v of signal, and the amp ought to have at least that much running around a large part of itself.

Top five small components? DOn;t know what that means, and who said it is a bad component we are looking for?