Marshall JCM 2000 Super Lead - won't turn on

[feckn_eejit]

Folks,

Please forgive my newb-ishness... I'm a bassist with a solid state rig and am pretty useless with tube guitar stuff...

One of my bands shares a rehearsal space with a few other bands whom are normally respectful of the gear we leave there. However one person allowed another band temporary access to the space... tonight was our first practice after this event and our lead guitarist's head won't even power up in standby mode The red light doesn't come on.

Everything was basically cranked and the thing had obviously been run hard and ridiculously loud.

We're not sure how old it is or about its previous history but we have gigged it plenty and it has certainly been lugged around. Its current broken state could be coincidence or just bad luck, who knows...

I checked the two fuses (main and HT) visually and for continuity with a multimeter... both seemed fine. Tried other basics such as known good mains socket and power cord... no love.

Is there anything else I can check? I'm comfortable going in and doing some cursory poking around...

Thanks very much for any insight!

Cheers,
--jeff

[Enzo]

If you pulled the mains fuse and measured it with an ohm meter, and it had about zero ohms resistance, then it is good. If the amp won;t even light up, then it is not likely getting the mains power for some reason.

Have you tried more than one power cord?

Pull the chassis from the cab, turn it over to access the open side of the chassis. Look at the back side of the mains cord connector. Set your meter to AC volts. Is there 120VAC, more or less, between the two main terminals? I forget, but if the power connector is on a small pc board with the fuse holder, then we may need to demount that board from the chassis to look on the far side of it.

What we would do is find the primary wires to the power transformer and see of ther is 120VAC there. Failing that we would look closely at the little board to see if the solder connctions are solid to the mains connector, the wires over to the power switch and back, the fuse holder, any inrush limiter, etc. We want to check the power switch itself. Umplig from the wall, and measure resisyance across the power switch terminals. As you flip the switch, do you get open and closed readings betwen the wires coming in and the wires leaving?

If we have 120v at the transformer primary all that stuff must be OK, but if you don;t the problem is there.

A different way to check the primary is to unplug from the wall, turn the amp power switch to ON, and measure resistance between the two main prongs of the power cord plug. An open reading means there is a break somewhere in the path up through the cord, through all the switches and fuse and stuff and back down the cord. If you get a resistance of somewhere between 10 ohms and maybe 200 ohms, it is probably OK.

If 120v gets to the primary and the primary is not open, then we have to decide why the power switch doesn;t light up. (Does it?) Why the tube heaters stay dark. ANd why the high voltage is not present. The power switch and tube heaters light up visually, so that is easy to check. But the ONLY way to check for high voltage is with a volt meter and the amp running.

The voltages the amp runs on inside can KILL YOU, so do be careful.

[feckn_eejit]

Thanks very much for your post Enzo.

I will rip it apart today and have a peek at what you have suggested.

I have done stuff like this preliminary digging around in solid state stereo equipment, never tube... but I understand the HT supply needs to be treated with major respect. The amp has been unplugged overnight so that seems to me a good safe opportunity to take it apart and start. I'll decide if I'm comfortable proceeding and can identify everything you're asking me to check before I apply power.

Thanks again,
--jeff

[Don Moose]

Does this model - like others I've seen - have another mains fuse inside the chassis?

[bluesfreak]

Not only one but a few more fuses for the heaters, etc...

[Enzo]

The power doesn;t come on. Does it really make any difference whether it is a tube or solid state amp when it comes to finding out where the power went?

[feckn_eejit]

The findings:



On the main board one of the spade connectors to which the leads from the main transfo hooks up was charred, as was the plug on the end of the lead.

Actually after having jacked around with that connector and putting it back together to take to the repair shop, it powered up...... scary!

What could cause such a thing?

[Enzo]

The solder cracks to the post which then arcs and gets hot, burning the connector. Or the connector itself gets resistoive which causes heating, which makes things worse, and so on.

[feckn_eejit]

So it's just a bad solder joint finally rearing it's head, not likely symptomatic of something else?

Thanks

[bluesfreak]

Did you replace the plug and resolder the spade connector on the PCB? If not, do so and your amp is most likely fixed...

[Don Moose]

I'd actually suggest hard-wiring the thing. Those cheap-*ssed connectors corrode and, whenever they're asked to pass significant current, this kinda thing is going to happen.

RoHS legislation (no Pb) + beancounters unwilling to pay for even the cheapest selective gold flash on the mating surfaces are at fault here.

Hope this helps!