#1
Ok, so I have a jcm 900 4100, and I foundthis mod. I followed it accordingly, and I get no sound. The funny thing is though I couldn't find the two diodes (ZD1 and ZD2) on the pcb as shown in the diagram. So what could be the problem?

here's the original diagram for the preamp of the 4100:
http://www.drtube.com/schematics/marshall/jmp52-02-iss11.pdf
Last edited by HELL_AWA1TS at Aug 14, 2008,
#3
update: I resoldered the resistors and capacitors back to stock, and I put together a bridge rectifier consisting of white LEDs. The result is a much warmer, defined tone, but alot of gain was lost. I heard theres not a noticeable gain loss using red LEDs, can anybody vouch for that?
#4
Yeah UGer's usually dont help AT ALL when it comes to amp modding or maintenance. I have one question for you though about reading schematics. How do you read the tube sockets and then actually translate that onto the pins on the sockets? Mostly on the pre amp tubes. I have a jcm 800 4210 that was made in 1982 and as far as I can tell none of the schematics I have found have been for that year. all where for 83 or later.
#8
Yes mine has the long blue ones that come down out of the amp. Also is there a kit I can order for a whole recap job?
Thanks,
Alex
#9
No kits...

Just get the right value and voltage caps and dont buy cheap ones either. Buy American made caps.

Sounds like you might wanna go to an amp tech. Caps store lethal voltages.
#10
Yeah I know they do I know how to discharge the amp. I wish I could but there isn't an amp tech within 100 miles of me.
#11
Quote by kurtlives91
^you must be stuck under a hole then, genius.

Are you sure those Zeners weren't there? Are you sure that mod is for your model?


I'm positive. and if weren't made for my amp model it wouldn't of given the result it was supposed to (warmer gain)
#12
I ran a fender ultimate chorus through the send of the fx loop, and it actually worked. this confirms the preamp is at fault. So my guess is I should replace the bridge rec. But, this is what confuses me. I looked up the specs for the bridge rec i pulled out. (Model W005F)

Its rated at 1.5 amps at 50-1000 volts. Now I know the voltage rating in the preamp cant be 50 volts, and 1.5 amps sounds kind of funny. Does anyone know the actual voltage and currency that is going across W005F in the Marshall preamp? I think that;s what caused the LEDs to burn out so quickly.
#13
Kinda bump but....

SOMEBODY CALL INVADER JIM!!!!!
Then there's this band called Slice The Cake...

Bunch of faggots putting random riffs together and calling it "progressive" deathcore.
Stupid name.
Probably picked "for teh lulz"

Mod in UG's Official Gain Whores
#15
Quote by HELL_AWA1TS
I ran a fender ultimate chorus through the send of the fx loop, and it actually worked. this confirms the preamp is at fault. So my guess is I should replace the bridge rec. But, this is what confuses me. I looked up the specs for the bridge rec i pulled out. (Model W005F)

Its rated at 1.5 amps at 50-1000 volts. Now I know the voltage rating in the preamp cant be 50 volts, and 1.5 amps sounds kind of funny. Does anyone know the actual voltage and currency that is going across W005F in the Marshall preamp? I think that;s what caused the LEDs to burn out so quickly.

WHAT? hahaha

How did you come to that conclusion.
#16
Because putting my axe through the input resulted in the shit sound, but when I ran through my ultimate chorus sent through the fx loop of the marshall and the sound was fine. something in the preamp circuit is blown.
Last edited by HELL_AWA1TS at Nov 17, 2008,
#18
Ok that's nice and all but how do you figure the rectifier is blown. If the rectifier didnt work the whole amp would be up in smoke or nothing would be happening at all.

From what you said those things don't seem to be occurring.
#19
Quote by forsaknazrael
Well, to test if the preamp is the problem, I'd be using a distortion pedal into the RETURN of the effects loop, using the pedal as a preamp.
Don't know what you're doing.


That's basically the same thing. The only difference is I used an amp instead of a pedal.

Ok that's nice and all but how do you figure the rectifier is blown. If the rectifier didnt work the whole amp would be up in smoke or nothing would be happening at all. From what you said those things don't seem to be occurring.


I meant the SS bridge diode rectifier in the preamp, I didn't make it so clear, excuse me.
#20
Quote by HELL_AWA1TS
That's basically the same thing. The only difference is I used an amp instead of a pedal.

I don't recommend using another amp ever again, unless you're bringing it to line level using a dummy load, or you're going from AMP B's preamp out to AMP A's power amp in.

And no, it's not the same. You said you went into the SEND. This is the 'preamp ou't of your effects loop.

I said I would go into the RETURN. This is the 'power amp in' of your effects loop.
#21
Quote by forsaknazrael
I don't recommend using another amp ever again, unless you're bringing it to line level using a dummy load, or you're going from AMP B's preamp out to AMP A's power amp in.

And no, it's not the same. You said you went into the SEND. This is the 'preamp ou't of your effects loop.

I said I would go into the RETURN. This is the 'power amp in' of your effects loop.

I said it was "sent through the fx loop", I didn't mean to the send. and it was at a low enough wattage, I'm aware of that.