Marshall 4500

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Gustav

Well-known member
Joined
Jun 4, 2004
Messages
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Location
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I got hold of a pretty beat up Jcm900 model 4500, and I wanna try switching the pots. Getting inside the damn thing was a bit harder than i thought at first, so if possible, I would like to order the pots before I take it apart, so it wont have to be out of business too long..

I found a schematic which gave me these pot values

http://www.marshallschematics.com/schematics/4500.zip

VR1= B220K

VR2 = A1M

VR3 = B220K )

VR4 = B22K

VR5 = A1M

VR6 = B10K

VR7 = B220K

VR8 = A1M

VR9 = B220K

VR10 A1M

Question 1. Does that look correct?

Question 2. Do I find some "marshall pots", or will any pot do, I guess what I mean is, are they mounted to anything, are only some of them mounted, which size etc.

If its no big difference in price, Ill get the Marshall replacements if that makes it easier, but if they are wired and it doesnt matter at all, Ill just get whats convenient.

Probably dumb questions, but rather ask than open it up and have it on the bench for weeks while waiting for parts.

Am I possibly overlooking something when it comes to disasembly? Might be easier than I think ?!

Gustav
 
dont know the answers but the folks at this site will know for sure and i've found they are happy to answer whatever questions you may have.(phone or e-mail):
http://www.torresengineering.com/index.html

or ask here:
http://music-electronics-forum.com/forumdisplay.php?f=10
 
torres? i wouldnt bother. enough said!

the pots are small, pcb-mount. unless you feel like jerking around with catalogs and dimension drawings- get the exact replacements from marshall to be assured of a correct fit.

may i ask- why do you think you need new pots? i would replace any that are noisy or damaged but wouldnt do it just for the hell of it.

while youre in there, replace the crappy plastic 1/4 inch jacks especially input and f/x loop. they always go bad.
 
try torres and judge for yourself. they will give you whatever info you need (you dont have to order from them)and they have full size good quality pots(i've used them) as well as cheaper stuff. i've never seen mini pcb mount at the site. maybe these come with their cheaper kits?
 
[quote author="solder_city"]torres? i wouldnt bother. enough said!

the pots are small, pcb-mount. unless you feel like jerking around with catalogs and dimension drawings- get the exact replacements from marshall to be assured of a correct fit.

may i ask- why do you think you need new pots? i would replace any that are noisy or damaged but wouldnt do it just for the hell of it.

while youre in there, replace the crappy plastic 1/4 inch jacks especially input and f/x loop. they always go bad.[/quote]

Good advice. I just serviced one of these, and I would think the pots are not likely your problem. In addition to the 1/4 inch jacks, you might want to replaced the filter caps if the amp has physical damage or is over 7 years old. F&T's are good for main external caps, and Vishay Spargue for the internal caps. If you plan on servicing this amp, I suggest adding a 1 Meg Ohm (2 watt) bleeder resistor over one of the filter caps.

A fresh set of JJ tubes will do wonders for the sound as well.
 
[quote author="solder_city"]

may i ask- why do you think you need new pots? i would replace any that are noisy or damaged but wouldnt do it just for the hell of it..[/quote]

They are noisy and scratchy, The preamp gains are the worst, but they are all a little scratchy maybe except one or two.

Thanks guys.

Gustav
 
[quote author="louder"]Hi Gustav
i think tube-town.de carries those pots.
best regards
pedro[/quote]

Awesome Pedro. That made things a lot easier for me.

Its a bit more than I thought, but what the hell. the amp didnt cost a whole lot because of this problem.

Gustav
 
check the plate resistors. my friend doug hoffman retro fitted some 2 watt plate resistors for the 1/4 watters in there and it made pretty big difference.
 
Check to see if some DC is leaking onto them.......may not be the pots at all....

1/4 watt plate resistors....Oo bay goom..... :roll:
 

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