Marshall JCM2000 TSL 100

[ChucksDreamShattered]

Guys - please don’t yell at me, I did read the sticky’s. I just picked up a JCM2000(‘05 I think). No cabling, but I had a power cord. Was sold as “parts only”. Powers up(red illuminated switch operates correctly), though I’m hesitant as I don’t have a cab to connect and have read of power being applied without a cab, and it causing issues(explanation?).
I’m aware of the “cold solder joint” issue. I believe I can at least diagnose the problems(with the communities help), as I’m a somewhat capable 20 year electrician. Would you guys point me to the best thread to get started for disassembly and basin initial point-to-point testing?
Thanks for any advice, help, or constructive criticism.
Chuck

[nsubulysses]

The head cannot be turned on without a cab. Especially that amp which probably more than any other amp will have a failed OT if you turn it on without a cab (someone feel free to speak up if I'm wrong, but it seems like Marshalls, especially JCM2000 are among the worst amps at handling the no load scenario)

you need to get a cab and see how bad it doesn't work. If you can turn it on with a variac or current limiter that would be good, so you can turn it on safely and see if basic power supply voltages are there (B+, screen supply, and bias supply on all power tube sockets, pins 3, 4 and 5) and also to confirm there are not shorts making the amp draw excessive current. Because if there are and you just flip the switch and apply power from the wall it may be 4th of july early

If you are an electrician you must have a variac ? I dont know. Just guessing. Give the amp 20VAC from the variac and check power supply voltages against voltages on the schematic. Schematics for this amp are everywhere if you google. The PS voltages will obviously be lower since you are not applying full 120V but at least it will be telling in most regards to see if the essential voltages are there and if it is safe to keep applying more power

[ChucksDreamShattered]

The head cannot be turned on without a cab. Especially that amp which probably more than any other amp will have a failed OT if you turn it on without a cab (someone feel free to speak up if I'm wrong, but it seems like Marshalls, especially JCM2000 are among the worst amps at handling the no load scenario)

you need to get a cab and see how bad it doesn't work. If you can turn it on with a variac or current limiter that would be good, so you can turn it on safely and see if basic power supply voltages are there (B+, screen supply, and bias supply on all power tube sockets, pins 3, 4 and 5) and also to confirm there are not shorts making the amp draw excessive current. Because if there are and you just flip the switch and apply power from the wall it may be 4th of july early

If you are an electrician you must have a variac ? I dont know. Just guessing. Give the amp 20VAC from the variac and check power supply voltages against voltages on the schematic. Schematics for this amp are everywhere if you google. The PS voltages will obviously be lower since you are not applying full 120V but at least it will be telling in most regards to see if the essential voltages are there and if it is safe to keep applying more power

You’re the greatest. Just needed a push in the right direction. Thank you!
Will post updates. Likely take or week or two to secure a cab.
Thanks again!!!

[Enzo]

Addendum: If you remove the power tubes, then you can fire it up without a cab. Problems arise when you bang out a chord and there is no speaker to receive it. No power tubes means no output.

[nsubulysses]

Perhaps it is not a problem with this amp in particular, I don't know. But if you remove the power tubes and fire up the amp with the power supply unloaded, DC voltages may exceed max rating of power supply capacitors so you should check with a meter just in case

Also I'm not the greatest but I appreciate the sentiment. Almost everyone on the board is more great than me in regards to music electronics knowledge and experience

[ChucksDreamShattered]

Addendum: If you remove the power tubes, then you can fire it up without a cab. Problems arise when you bang out a chord and there is no speaker to receive it. No power tubes means no output.

Thank you Sir! EVERY bit of information you can share is GREATLY appreciated. I know NOTHING. I want to
resuscitate amp if at all possible. This is the first tube amp I’ve ever owned or worked in. I got it for a song, and last night found “Amplified Parts” as a parts supplier. They have a replacement PCB board @ approx. $230.00 - should I go that route off the rip, to address the original issues with the board(bias drift?)?
Thanks - you guys are the true Gods of Rock!

[ChucksDreamShattered]

Perhaps it is not a problem with this amp in particular, I don't know. But if you remove the power tubes and fire up the amp with the power supply unloaded, DC voltages may exceed max rating of power supply capacitors so you should check with a meter just in case

Also I'm not the greatest but I appreciate the sentiment. Almost everyone on the board is more great than me in regards to music electronics knowledge and experience

Will do! I’m a complete noob when it comes to valve amps - so ANY bits of help, knowledge, etc. are beyond appreciated!!
Thank you!

[Enzo]

Consider why it was sold as "parts only". Chances are something expensive is wrong with it. Before investing in a new circuit board, you need to check out both transformers.

Over at RG Keen's Geofex web site is a tech tip on transformer testing. He shows a very simple transformer tester you can make cheap.

It concerns me greatly that you are already proposing buying a new circuit board before you even know what is wrong with the amp. You seem to assume that all problems will be on the circuit board, and that is not a valid assumption.

[drewl]

With power tubes removed you could run the line out or FX loop out into another amp to test the preamp.

do you have another amp like a combo you could use the speaker from just to check if the power amp works?
You could even use a 4 to 16 ohm resistor to check the power amp if you keep the output below the resistors power rating.

Just to see if it powers up okay, the monitor the bias draw to see if it has the older bad PC board.

[J M Fahey]

I´d check first something even more basic: that power actually reaches the Power Transformer and that it outputs voltage from its windings.
Sorry but:

Powers up(red illuminated switch operates correctly),

by itself means little to nothing: switch has an internal neon bulb which will light when getting mains voltage, even if said voltage goes nowhere else.
A described as "parts only" amp might be missing 90% or more of its guts and still qualify by the label promise so first bcheck that PT.

[nsubulysses]

Juan may be on to something.

in my experience parts only usually means cannot be fixed, too expensive to be fixed or failed repair, or a DIYer tried to do something with it, or a shop pulled parts from it and is now selling the rest

Does not work or unknown condition means it may work as is if you're lucky or it broke and was never attempted to be repaired and probably sat collecting dust for a while

hmmmm, hope you get lucky. let us know

[drewl]

Hey, did you give or sell the amp to someone in Pa?

Just kidding, but I got one of these that the owner trash picked or something claiming it's missing stuff!

It works but needs alot of work.
Bad output tube blew HV fuse and it's got the early bad/conductive PC board.

But for a few hundred bucks it could be okay.