JCM800 Goes Boom:

syscokid said:
What about it? I have no idea what part of the circuit that is. It's such a mess in there... it makes my poor little head hurt... :BH:.

Good job on finding out the source of the no-volume problem... :cheers:
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I was trying to figure out what he was trying to do with the box capacitor.
 
Robert Herndon said:
I was trying to figure out what he was trying to do with the box capacitor.
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First let's do the voltage readings if you want.

Take power tubes out for ALL tests.

Turn power on, turn standby to "operate."
Set meter to DC volts.
Caution high voltage.
Connect black lead to chassis.
Connect red probe to pin 3 of output tube socket.
Then connect red probe to pin 4 of output tube socket.
Write down down readings of all output sockets pins 3 and 4.

Turn standby to "warm up."
High voltage is now OFF.
Set meter for AC volts.
Red probe to pin 2 of output tube socket.
Black probe to pin 7 of output tube socket.
What is the AC volt reading for all output tube sockets?
Write this down also for all output tube sockets.

Turn power off, disconnect all power from amp.
Set meter for ohms.
Connect black probe to chassis.
Connect red probe to pin 8 of output tube socket.
Read all output tube sockets pin 8.
write down all ohm readings for all output tube sockets pin 8.

Connect red probe to pin 4 of output tube socket.
Connect black probe to pin 6 of output tube socket.
Read all all output tube sockets.
You are measuring the screen grid resistors.
Write down readings for all output tube sockets pin 4 and 6.

Now report all those wonderful readings from above tests.

Do not install output tubes.
Do not operate amp.
 
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Robert Herndon said:
I was trying to figure out what he was trying to do with the box capacitor.
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67plexi said:
What the white ceramic capacitor stock would be 470pf if it's going to the treble pot.
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I think the white coax cable is coming from the center of the master volume pot. On a stock 2203 the master volume connects to a .022 cap. Then the cap connects to the first grid of the phase inverter and the first 1meg resistor that leads to the tail of the PI. That .022 cap might have been modded to a different value. Probably the 1meg resistors were changed too, which would explain why all the resistors in the PI circuit have been poorly covered up.
 
And another thing regarding that little white-boxed cap: I don't think that cap should be laying down like that. Whomever modded the Morris mods, bent that cap by accident and just left it like that.
 
Robert Herndon said:
This amp has never been touched by anyone other than Morin. keep in mind that I am finding out this is typical of his work.
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Well it could just be a bad preamp tube, so swap out the tubes one at a time with a new one and retest each time.
You don't need a whole set of 12AX7 tubes, just 1 good one.
If not, you will usually see one of those voltages above giving you an indication of the problem.

I would not start tearing the whole thing down. (usually)
I would fix the one problem and get it working first. (above)

Then if you want, start fixing things one at a time, and re-test each time you fix a small section.
Then if something goes wrong, you know the area that's causing a bug.

That's good advice for a person who is not 100% familiar.
Or a person who is just learning how.

Where people get into problems is they do too much at one time.
 
Robert Herndon said:
This amp has never been touched by anyone other than Morin. keep in mind that I am finding out this is typical of his work.
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Thanks for the clarification.

I wonder when Mr Morin did these rough looking mods on this amp. In the last ten or 15 years, I've seen on the internet many incredibly immaculate and OCD infused wiring techniques for amp builds and amp mods from established pros to even semi-amateurs. The bar has been raised and a lot of amp builders and modders have stepped up their game.
 
syscokid said:
Thanks for the clarification.

I wonder when Mr Morin did these rough looking mods on this amp. In the last ten or 15 years, I've seen on the internet many incredibly immaculate and OCD infused wiring techniques for amp builds and amp mods from established pros to even semi-amateurs. The bar has been raised and a lot of amp builders and modders have stepped up their game.
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Good question. I will see what i can find out...it could be very old.
 
Robert Herndon said:
Good question. I will see what i can find out...it could be very old.
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My advice is
Just make it work again, and sell it as a "morin modded marshall."
There are people who think he is some kind of genius, let them worry about the slop insides.
Personally I think it sounds like :poo:.
To me it sounds like fuzzy mud cakes.
I think the stock amp sounds considerably better.

Tearing this thing down and fixing that abortion, is a waste of your time and effort.
I mean just think of all the time you would need to make that Holy again...it's ludicrous. It's absurd.

The fact that any person would do this to a Marshall is.....................heresy.

I have listened to and modified many amplifiers, comparing them carefully, playing guitar for many many hours.
I mean let's get real. It doesn't sound that great at all. Really?

You can make an amp sound way better, just by moving a couple parts on the circuit board.
Secret: It's he layout that is the key.
It's like night and day.
You do not need to do this to make an amp scream. Seriously.
 
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At the end of the day..we can talk this smack or that..BUT the proof is in the MUSIC we make & not what an amp looks like,, maybe i shuld take my amps out the shell..hire a tech & my sound would be better...nah...Robert your one of hell of a musician,,the Proof is in the Music ya make,,no hiding..up front..making music..actually posting it..would be honored to play in band with you !!
 
BFT Gibson said:
At the end of the day..we can talk this smack or that..BUT the proof is in the MUSIC we make & not what an amp looks like,, maybe i shuld take my amps out the shell..hire a tech & my sound would be better...nah...Robert your one of hell of a musician,,the Proof is in the Music ya make,,no hiding..up front..making music..actually posting it..would be honored to play in band with you !!
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Thank you for the kind words, my friend. I tell people I am more persistent than I am consistent. Now, if I could write more original stuff, like you do, then things would be great!!!!
 
chilipeppermaniac said:
Local to me Mr Jeff Bober of Budda Amps Now doing East Amplification.
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Yeah Jeff is a cool guy. I used to have a Superdrive 18 and talked to him about knocking some volume out of it. It was an amazing sounding amp but good god had to be shockingly loud before it really started giving up the goods (who even knew an 18-watt amp could get that loud?). I tried a couple of things to tame it but nothing really worked: It was too loud to use with the band, so I sold it, but man it sounded sooo good.
 
Yes Gary, Jeff sure seemed nice in our personal conversations to me too.

When I first got my Superlead, I rang him up to ask, if I got stuck on anything in trying to make it 100%, if he would look it over for me and see if anything stood out to him as being "off " and needing attention. He said sure, bring it by if I need to. 2 years later after more poring over amps, and learning from our clan here, I feel pretty confident in getting my Used heads into proper spec to be in good health. Since I am not counting on these amps for making a living, or for recording or perfection of tone etc, I can be take my time and be patient as I use the conditions they are in to teach me so many good lessons about amp diagnosing and fixing. Oh yeah, and I strive for as close to factory neat and tidy wiring, soldering etc instead of half assed messes whenever I remove and replace any parts of my amps.
 
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