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

[brydon2]

I had my tech check this but he's an old Fender guy.

The amp plays fine in the green channel, when I switch using the front panel

the lights turn red but there is NO SOUND. I was hoping this was common to these amps and someone knows what to check first.

Its a shame these are so tempermental !

[Enzo]

So one channel is dead. Trace the signal and see where it stops. Try different preamp tubes in the sockets. Look for missing B+ at the plates. etc.

Its a shame these are so tempermental

Not sure what you mean. Yours has a problem. I don't know that any more of them have your problem. These are no less reliable than any other Marshall.

[bluesfreak]

Does it work when you use the supplied footswitch? If yes the Front panel switch is busted otherwise there is something wrong with the switching circuit (some FETs, ICs and a Relay). Check if 12Vdc on Relay1 and +/- 15Vdc on IC2 are present in your amp.

[brydon2]

The footswitch doesn't work as well, and I changed all the tubes.I really think

its in the switching power supply.I will check voltages and see if I can hear the

relays "pick" when pressing the front panel buttons.My troubleshooting

is mediocre so we'll see. Thanks much for the help for now!!

Also, because the clean channel is working that would eliminate the f/x loop as

a trouble spot, right?............Thanks again!

[Enzo]

Your switching is working, it is switching from a working channel to a non-working channel. If your relays were not turning off and on, then NOTHING would change when you switch channels. That is why I suggested tracing a signal through the dead channel.

[brydon2]

That would mean I have one working channel and two non-working channels?

this makes things a bit more challenging I think. I will have to wait till Monday

to get back home. I can study the schematic a bit is all. This channel

switching stuff may be beyond my ability to follow, we'll see.....Thanks!!!!

ENZO, Are you saying just do normal "pop" testing of the cirrcuit in thr dead channels?

[tubeswell]

ENZO, Are you saying just do normal "pop" testing of the cirrcuit in thr dead channels?

That would be one way of starting to look for the location of a fault in the signal path, but it only gets you so far. Then you need to check whether you have got the right operating plate and cathode voltages in each respective stage to work out where the problem might be coming from.

FWIW Jack Darr's book is full of good advice on troubleshooting

Jack Darr Book, chapters 1 - 8

[brydon2]

I will start by checking plate and cathode voltages in the non-working channels

I have the Darr book and will re-read some before I get started.Thanks for the

help and reference!!

[Alex R]

That would mean I have one working channel and two non-working channels?

this makes things a bit more challenging I think.

Well not necessarily. It might well mean that the fault is confined to the areas shared by the two gain 'channels'.

[brydon2]

That was my thinking too, I assume the amp adds gain stages and or cascades

the signal coming off the input, maybe find out what component is responsible

for the routing of the signal.If you ever looked at the schematic, it looks like a

small city.We'll I'll just take it slow and see what happens, its been quite a while

(almost 2 yrs) since the amp broke down............Thanks


I was hoping this was kinda "common" with dsl's and tsl's and someone could say "I know what it is, happens all the time"

[Alex R]

No it's not a 'standard fault'.

[Enzo]

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.

Imagine a restaurant that serves hamburgers and cheeseburgers. if they run out of meat patties, then BOTH hamburgers and cheeseburgers won;t be available. it won;t be two separate problems, it will be two aspects of the same one.

Everyone expects their problem to be on some list. it doesn;t work that way. Even if we find a certain resistor is often bad, it doen't mean that is what is always wrong. That is why it is far more valuable to know how to troubleshoot than it is to have a list.

[KingCameron]

JCM800 Head Amp

I have a similar issue in a JCM800 2210 Split Chan Reverb
I do not know how to troubleshoot the CA3045 transistor array...and am not sure I need to.
I know that V1P3 drops to 0VDC when I switch to the boost channel and thusly I get no sound.
However, I get ~260VDC on V1P1 and get a great Marshall tone.
(recently retubed pre-amp) Tubes = Good
Amp is electrcally good beyond V1

Any Ideas on what I should be looking for?

[Alex R]

Imagine a restaurant that serves hamburgers and cheeseburgers. if they run out of meat patties, then BOTH hamburgers and cheeseburgers won;t be available. it won;t be two separate problems, it will be two aspects of the same one.

Poetry .

Everyone expects their problem to be on some list. it doesn;t work that way. Even if we find a certain resistor is often bad, it doen't mean that is what is always wrong. That is why it is far more valuable to know how to troubleshoot than it is to have a list.

How true. Often and often I get an amp for repair with notes attached about how it's probably this cap or that resistor, info gleaned by the owner after much research in places like this. Generally of course, it's something else entirely. You learn to beware jumping to conclusions. Avoid the Skippy Syndrome.

In this case I'd want to make sure the tubes are working right, like brydon is doing, check plate and cathode voltages. Switching circuits fail but not so often as tube-related circuits do. JMHO.

[brydon2]

Initial "inspection"

I got a chance to pull the chassis and get some readings the best I could

In doing so realized how long its been and how little I actually know about these things. Clearly assembly and troubleshooting are two different animals.

V1 plate=222 cath=1.7 plate=205 cath=1.7
V2 plate=217 cath=1.8 plate=231 cath=1.8
V3 plate=214 cath=1.3 PLATE=345 cath=BEEEP (not sure what that means
v4 plate=247 cath=no reading plate259 cath= no reading


I assume V4 is used for reverb but I'm not 100% on that.

Not sure whats going on at V3B? Anyone have any ideas? Thanks!!