Marshall JCM 800 Model 2203 VINTAGE - Blowing Fuses, Sound Problems, just purchased

[fanaticbychoice]

Hey, seems like a really cool forum. I'm going through a lot of stress over this amp right now. Basically, here is the story in a nutshell:

Purchased a custom modified Marshall JCM 800 2203 with a line-6 cab. The customizations and specs will be at the end of this post.

Played it for about an hour, everything was fine. Seemed a little quiet for a marshall, but I was listening to the tone and thrilled. Started doing some more rowdy playing at the upper neck near full volume. Amp shorts out, goes dead. As this is the first day I've owned it (bought it off craigslist), it's pretty disturbing. Check some forums, seems like it's just the fuse. Talk to the original owner, he says to make sure I'm running it into the 4ohms mono jack on the cab and not the stereo. My ignorance there, I was running it into stereo (8 ohms) when it was set for four. Feeling idiotic, whatever. Replace the fuse, amp is able to turn back on, hear feedback humming, tubes warming up, no longer worried. Plug guitar in, no sound. No matter how much I mess with the amp's settings, there is no feedback from the cable or guitar. Did I blow the tubes? Did I blow some transistors/diodes? I am fairly ignorant about a true vintage amp like this. I bought this amp instead of a car though, so I'm very serious about learning how to make it sound as good as possible.

So... how deep of a hole am I in? Has my ignorance caused me to seriously damage the amp by toying with it/not having it set up right? Has the fuse protected the amp when it blew? I would think if my amp is no longer responding to a guitar there is a pretty big problem, but it turns on and hums like it should...

thanks, sorry for so many questions.

here is the original ad for the amp:


Marshall JCM 800 Model 2203 VINTAGE Amp -- MODIFIED BY LEE JACKSON! - $1200 (CASTLE ROCK, CO)

Date: 2012-03-10, 10:21PM MST

Marshall 2203 Super Lead Mark II Master 100 Watt head. NOT A REISSUE. . ..ORIGINAL VINTAGE!

This amp was manufactured in the 80's and was modified by the legendary Lee Jackson. He gave it some great features you won't find on a vintage amp: It has an active effects loop, a frequency adjustment switch to change the overall tone of the amp, enhanced low end response and half power switch to get great tone at low volume levels.
- An active effects loop is a feature that you won't find on many vintage amps. Flexibility to shape your sound.
- This amp can give you that classic rock sound, a nice vintage blues sound or it can SCREAM with overdrive.
- The half power switch is great for getting very overdriven sounds without losing your hearing or for just playing at a lower volume with that great vintage overdriven tone.
- The frequency adjustment can give you 6 different midrange tones. Great for dialing in some different sounds.
- Lee enhanced the low end response of this amp meaning you can dial in more precise lows than with most Marshalls.
- Classic Marshall sound controlled with Master Volume or Modern Overdriven sound (Brown sound) controllable with a second volume knob like a high gain volume.

The Line 6 cabinet in the picture is sold separately ($300). If you purchase both together I will take $1,400. This is an AMAZING amp and the only reason I'm selling it is because I need the money.

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[Chuck H]

Considering the modifications and vague description of the problem it's impossible to tell you how far you are in the hole. That you hear sound from the amp is a very good thing.

The first thing to try is plugging in to the effects loop return. It won't sound right of course but it may help isolate the problem. Do you have sound now? If yes try plugging in to the input and put a jumper in the effects loop. Anything?

Also, after the amp warms up, do you hear any change in the his/hum from the speaker when changing any of the controls?

[Gtr_tech]

2203/4 didn't have a loop....

[Gtr_tech]

Given the impedance mismatch its likely he wasted either the output tubes, socket(s), or the transformer.

[big_teee]

2203/4 didn't have a loop....

The loop was probably one of the many mods.
T

[J M Fahey]

was modified by the legendary Lee Jackson. He gave it some great features you won't find on a vintage amp: It has an active effects loop,

So plug your guitar in the Loop return: any sound?

amp is able to turn back on, hear feedback humming, tubes warming up, no longer worried. Plug guitar in, no sound

So amp is nor "dead", just deaf.
Test loop as suggested above.
An 8 ohm load into a 4 ohm tap won't kill that amp.
Using just 2 speakers full tilt when 4 were expected may kill *them*.

[fanaticbychoice]

So plug your guitar in the Loop return: any sound?

So amp is nor "dead", just deaf.
Test loop as suggested above.
An 8 ohm load into a 4 ohm tap won't kill that amp.
Using just 2 speakers full tilt when 4 were expected may kill *them*.

No sound from plugging in to the effects return. I take it this guarantees it's an output, not input problem?

"Also, after the amp warms up, do you hear any change in the his/hum from the speaker when changing any of the controls?"

No, just monotone humming that doesn't change with tweaking. That seems bad. But like you said, this is all very vague and we are dealing with a modified amp which doesn't help anything.

I called one of the local amp techs here in denver, they said that it was a very common problem with the 2203 head and it probably isn't anything major. This kind of makes me want to just fix it myself, though, and not have to wait two weeks for them. Any more suggestions on isolating the problem? You guys are great, thanks.

[Chuck H]

OK. The next thing would be to check the effects send. Is there a signal from the preamp there? You'll need some kind of power amp to check this of course. Obviously there is something wrong ahead of the effects return. But it would be good to know now if there is anything wrong with the preamp as well.

[J M Fahey]

Big problem here is that by being an unknown mod, we can't suggest you much more than we did.
You *might* write the man himself and ask fo a schematic, although I very much doubt he'll answer.
You lose nothing by asking, though.

[big_teee]

Some GutShot Pictures might help.
If it has the Output section of a 2203.
Pull tubes and read basic B+ Voltages.
Test your grid stoppers.
Outside of your fx loop, I would think most of the mods would be in the Preamp.
Good Luck,
Terry