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JCM800 2203 does it need a new OT?

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3/13/2004 12:02 AM

JCM800 2203 does it need a new OT?

My guitar player just got his JCM800 from ebay the other day. When he got to practice he promptly pulled the JCM900 of the cab and put the 800 in it's place and fired her up. No sound. I promptly yelled at him to turn it off, NO LOAD!

When we plugged in the cab and turned it back on there was some LOUD hum. Then no sound. I swapped the tubes with those in the 900 same story. Now all the 800 does is remain really quiet unless you hit the strings really hard. Then it sounds all buzzy and nasty.

I suspect that I need to put in a new OT. Antique Electronic Supply has 'em for $75. Should I pick up some other supplies for it?

Thanks in advance,

3/13/2004 7:43 AM
Gil Ayan	I Don't know the 2203 circuit by heart, but don't those amps have a fuse in the OT? I doubt ANY OT could be fried by what you described -- running the amp with no load for a few moments. Gil

3/13/2004 7:19 PM
nic	Yeah, I have heard Marshalls are particularly touchy about no load. I think it was running for a maybe a minute... I know I have accidentally run my Super Reverb chassis without it's speakers when doing repairs. The SR always held up. nic

3/13/2004 8:31 PM

I suppose the recipie of one minute + band volume + Marshall may be not so good.

Speaking of this, someone in a book (Weber?) made what I think is a good point: if you realize your speaker in unlugged after you turn the amp from standby to on, rather than putting the amp in standby again or shutting the amp off, just plug in the speaker. That will save the OT from dealing with the "thump" (current surge) twice with no load.

Gil

3/13/2004 8:45 PM
nic	Good call! I never thought about it this way. PLUG IN THE SPEAKER! I am starting to look at what appears to be an aftermarket effects loop "hack job" (bad solder joints) attached directly to the master volume pot on the circuit board. There were no additional coupling caps added either which seems odd. nic

3/13/2004 9:08 PM
nic	Now there is sound, but bad. So I reversed the "effects loop" reconnecting the Master wiper to the trace on the PCB. With all the knobs to 10 I am getting a consistent and sizzly distortion sound that dies out quickly as the string decays. The amp is NOT loud, quieter than a champ. And the distortion is ugly. I am starting to wonder if it is the transformer and maybe something in the preamp.

3/14/2004 12:49 AM

Plug into the second input jack and see what you get. If the sound is clean & relatively strong look back toward the first stage. The second jack (low gain) has a switch that connects to the output of the first stage. If that switch loses contact you will lose most of the signal at the Hi Gain jack. (you will probably still get some bleed-thru signal) If the amp isn't blowing fuses I would guess the problem is NOT a transformer. The fact that the amp has been modified is a warning sign that things may not have been right to begin with.
I would run a signal generator into the amps input jack and check each stage with an oscilloscope to find where the signal is getting lost. The O.T. should the last thing to suspect, not the first. Check the power tube sockets between pins 2-3 and the power tube bases for any signs of arcing. That's usually what will happen if the o.t. kicks back from a no-load condition. You must thoroughly clean or replace the sockets/tubes if they show any sign of arcing.

RE

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