JCM 800 2203 transformer diagnosis

[313guitars]

Hey guys,

I know this isn't a metro kit but since it's virtually the same I thought I'd ask here. So I recently traded another amp and got this one. It's a 1983 JCM 800 2003. It was in pretty decent shape until I took it to the studio and on the way it fell off the dolly and now no longer works. On power up after the fall I had one tube red plate really badly on me with no audio output. I noticed the impedance was had shifter after the fall and so I fired it up a second time. No problem until I flipped the standby. Then immediately I saw a spark (I think in the faulty tube maybe in the OT) and shut down. Still no audio.

I just finished testing the amp without power tubes and with and all the measurements are good. When I turn it up now I can hear an audible crackling from the OT that coincides with the chords I'm playing. Still nothing out of the speakers. I'm fairly certain the OT is toasted but I thought I'd try here first in case it might be anything else. The guy had told it had been rewound. I'm not sure if that matters.

Thanks,

J-C

[flemingmras]

Hey guys,

I know this isn't a metro kit but since it's virtually the same I thought I'd ask here. So I recently traded another amp and got this one. It's a 1983 JCM 800 2003. It was in pretty decent shape until I took it to the studio and on the way it fell off the dolly and now no longer works. On power up after the fall I had one tube red plate really badly on me with no audio output. I noticed the impedance was had shifter after the fall and so I fired it up a second time. No problem until I flipped the standby. Then immediately I saw a spark (I think in the faulty tube maybe in the OT) and shut down. Still no audio.

I just finished testing the amp without power tubes and with and all the measurements are good. When I turn it up now I can hear an audible crackling from the OT that coincides with the chords I'm playing. Still nothing out of the speakers. I'm fairly certain the OT is toasted but I thought I'd try here first in case it might be anything else. The guy had told it had been rewound. I'm not sure if that matters.

Thanks,

J-C

My first concern is the redplating. I would check into the bias supply circuit and make sure everything is good to go there...worst case scenario you end up having to replace all the bias supply components which you can probably get for no more than $5-$10 easy. Could also be broken solder joints in the bias circuit...worst case scenario would be that you could also have a cracked PCB right in the bias supply area and if that's the case you'll have to jumper some tracings to eliminate any bad connections in the bias circuit. Also you may wanna consider replacing the tube sockets as pin 5 could be mucked up from the fall, causing a faulty connection and as such intermittent bias voltage at pin 5.

As far as why you have no sound I would check into either the speaker jacks or the impedance selector itself. The fact that you hear the "OT ring" when you play tells me that there should be signal at the OT secondary, but it's not making it outside of the speaker jack. The jacks themselves and the impedance selector are the only two things in between so check those things before proceeding any further.

[313guitars]

Well I found the problem. It was the impedance switch. I do have a question about this amp though. The OT has a black and green tap and no yellow. I'm assuming black is 4, green is 16. With that in mind, the labels on the actual switch only provide 4 and 8. The negative feedback is hooked up to the green. The reason I'm asking about this is that I've got a beautiful '65 tv cab that is 16ohm that I would like to use with this. Should I set it to 4 on the switch to be safe?

Also I'm wondering about biasing technique without having the bias resistors going to ground like in the kits because this original doesn't have them? Is there a way I can do it with just a meter and without installing the resistors?

Thanks a lot for your help.

J-C

[flemingmras]

Well I found the problem. It was the impedance switch. I do have a question about this amp though. The OT has a black and green tap and no yellow. I'm assuming black is 4, green is 16. With that in mind, the labels on the actual switch only provide 4 and 8. The negative feedback is hooked up to the green. The reason I'm asking about this is that I've got a beautiful '65 tv cab that is 16ohm that I would like to use with this. Should I set it to 4 on the switch to be safe?

Also I'm wondering about biasing technique without having the bias resistors going to ground like in the kits because this original doesn't have them? Is there a way I can do it with just a meter and without installing the resistors?

Thanks a lot for your help.

J-C

OK first question two questions for you is...

Are you in Canada?

Whether you are or not...is this a Canadian Marshall?

[313guitars]

Yes I believe it's the canadian one. It has the regular toggles not the fat switches. And the two inputs are vertical not horizontal.

[bubb_tubbs]

Yes I believe it's the canadian one. It has the regular toggles not the fat switches. And the two inputs are vertical not horizontal.

If it's a Canadian model the taps are 4 and 8 - we didn't have a 16 ohm for some reason.