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

[brydon2]

Enzo, My meter has AC 200 & 600 settings.I'll check again in the A.M., I was getting some 425v readings on the plates of the preamp tube V1.

I thought I should be looking at pins 2 and 7?

I'll measure both V1 and V2 and see if anything makes any sense...Thanks

[Enzo]

Signal comes in the grid - 2 and 7 - and out the plate - 1 and 6. Except that one cathode follower stage.

[brydon2]

Update !!

After a bit of a hiatus, I have determined that the amps effects loop works for all channels which I suspect is VERY GOOD NEWS.

So if anyone wants to re-open the case I would certaintly appreciate it!!

jcm 2000 tsl 602 - Clean channel is all good

crunch/lead channel = no speaker output but the effects loop send DOES

WORK

I assume this would make the the fix a bit easier and hope that is the case!!

[oc disorder]

Hi again brydon2
I've attached some simplified drawings showing how VR1 , VR2 and VR10
are switched in. Excuse the rough signal path lines !
If it is a faulty relay it has got to be Relay 2.
However any breaks in the path when VR2 and 10 are selected
would also stop the signal getting through.
Working in reverse from the power amp input putting your meter probe
on pin 2 connector 8 (input to the power amp which we know works)
should create some hum.
Likewise with VR1 turned up putting the probe on the junction of R110 and 112 should get a hum as its connected to the power amp via relay 2 contact C.

With relay 2 changed pin 2 connector 15 should hum providing VR 3 is turned up and obviously the wiper of that pot/volume control should hum as its connected to pin 2.
The wiper is the centre terminal of the pot or volume control.
It "wipes" up and down the carbon track as you turn it.

With relay 1 & 2 changed VR 10 is in and the wiper of that should hum ( it has no connector ).

If c75 was faulty or the track to it was broken or faulty solder joint the signal couldn't get through.

Have a think along those lines the fault has to be in the diagrammed area.

I'm also attaching pictures of the fets.. they act like a switch when some change happens on the gate.
EG F11 switches that 100k to ground or not.
It would reduce the signal at the point those 2 470k's join.
This is probably part of the difference between the two gainy channels
or between the clean and the two gainy channels.

Also finding points to measure the relay contacts from the diagram could be useful.eg pin 1 connector 13 should connect to the junction of R110-112 when relay 2 is in one position and connect to the junction of R78-79
when the relay is in the other position.
This test could be done with the HT off (standby).

On all the diagrams I'm assuming that there is audio at the junction of R101 and C 65 as the FX send is working in all cases there must be audio there.

[brydon2]

I will start checking these ocd, thanks very much for the reply.

I can further add that the crunch and lead signal sound identical when listening to the effects send through another amp, they are very low volume and not distorted at all, I add this just in case it it helps.

Again, your help is appreciated.

[brydon2]

signal trace

ocd, It seems that the signal break is between V1 and V2.At R100/101 I can only "see" the clean signal.

I cannot determine the signal path from V1 TO V2 so far but I do see all 3 signals at R87.

If I knew the signal path we may be real close to finding the culprit,Thanks

[oc disorder]

Hmm a bit confusing this end you said initially that the efx send was working
on all channels.From the diagram this efx send signal comes from the junction of R100 & R101 just after V2 and FXSEND is written just below that.
At R87 the signal has only gone through the first half of V1 (V1A) and so at
that point will always be there for all channels.
You could guess that by looking at the diagram.
Did you try this? (use a continuity tester (beeper) or low ohms scale)

Also finding points to measure the relay contacts from the diagram could be useful.eg pin 1 connector 13 should connect to the junction of R110-112 when relay 2 is in one position and connect to the junction of R78-79
when the relay is in the other position.
This test could be done with the HT off (standby).

Likewise the top set of changeover contacts (B) could be checked from ground to the two juunctions mentioned above.
So when pin 1 con 13 is connected to the junction of R110 and R112 with the bottom set of contacts (C), the junction of R78 & R79 should be grounded by the top set of contacts (B).
If that checks out ok need to do the same for relay 1.
I suspect relay 2 switches between clean and crunch/lead and relay 1 switches between crunch and lead.
Relay 1 can be checked by measuring across R78-79-80-81.
Relay 1's B contacts short out either R80 or R81.
Likewise relay 1's C contacts short out either R78 or R79.
So on a low ohms setting should get 000.0 ohms (practically depending on the meter and the resistance of the leads and how well they are connected to the device being tested, you could get 000.3 or similar).
Probably the meter won't read 10 megohms it will read like you have nothing connected to the probes wheras the short will read like you have both probes touching together.
Have you removed the board and checked that area underneath for faulty
solder joints?
Maybe you can do that while replacing a faulty component you hope to find.
Next step is to go through the FET switching.
It could be faulty "interconnect crimpted cables" faulty fet or the circuitry
that converts the footpedal switching into usable triggers to activate (or not)
the FETs used for switching.
This diagram is on the second page and should find 15 volt and 24 volt rails.
They are marked on it.
The relays seem to switch in the master controls and the fets appear to switch in the gain controls.
Did you get any hum from touching the wipers of the master pots when switched in ?

[brydon2]

LEAD Volume VR10

Just a quick note before I start poking around, I cant hear the wiper on vr10 at all, with the volume up I should be able to hear the wiper but cannot.

The clean channel reads at R100/101, but no crunch lead signal that my meter will detect.

The output through the effects send for the crunch is very slight and not clipped/distorted at all.


VR10 seems to be dead, all other pots are fine.

[oc disorder]

VR10 is the lead channel master from what I can work out (maybe you can verify this) so the lead channel should be engaged (F/s or push button)
before it will be active turning it up full and touching it with your finger should get something..
(edit) ah just noticed your "LEAD Volume VR10"

[brydon2]

[QUOTE=Enzo;115580]In many amps, the high gain and ultra high gain channels are the same circuit, just a few things switch in and out. SO if there is a fault in that circuit, both the high gain "channels" will go dead.

I notice when playing the clean channel that I can "hear" the crunch volume knob through the spkr when moving it back and forth quickly but I can't hear the lead volume knob, does that sound like its worth investigating??

Maybe a better question is does the guitar signal travel through both volumes (crunch and lead) according to the schematic (I can't really tell) maybe the Lead volume knob is bad??