Marshall JCM900 100W Dual-Reverb Full Restoration Questions

[GunalORSEL]

Of course I will not remove anything just to check if its OK or not. I was measuring them while they are all onboard, but won't keep measuring neither. Instead of desoldering and mesuring and resoldering it, one could measure new resistors and install new ones more easily.

Only the electrolytic caps will be replaced, together with the pots and valves.

[hamfist]

Only the electrolytic caps will be replaced, together with the pots and valves.

Good plan !!

[Metro]

Hi all, Im reading this thread with interest as it is clear that the advice provided is of great quality. I am also reasonably new to the amp and electronics field, having come from being a sparky.
I might suggest that Carbon Composition (not component) resistors are unnecessary on this amp as I believe they are generally chosen to alter the characteristics of the tone circuit which it sounds as if you like as it is.
I believe the film type resistors are more reliable, cheaper and can be of higher tolerance. The reason you would generally use carbon comp is to keep an older amp stock. Am I correct? M

[GunalORSEL]

Thank you very much M, I will definately follow your input.

Has anyone any idea on what type of components did early vintage marshall's used to use, specifically JCM800s? Are there any methods, and/or any past experience(s) of anyone here, considering you all are professionals and are dealing with so many amps, to modify an amp to get closer to a vintage tone?

[GunalORSEL]

JCM900 Alive

Dear all,
Just wanted to update: all resistors have been replaced with 1/2W metal films, and the caps with mallory and sozos, all soldered w/ silver.

Now the amp, is different. Totally different. It's got the famous marshall growl and bass response, and it's a JCM900. It's unbelievably beautiful. Thank you very much for encouraging me on modifying the marshall.

[Lee W]

Dear all,
Just wanted to update: all resistors have been replaced with 1/2W metal films, and the caps with mallory and sozos, all soldered w/ silver.

Now the amp, is different. Totally different. It's got the famous marshall growl and bass response, and it's a JCM900. It's unbelievably beautiful. Thank you very much for encouraging me on modifying the marshall.

That's great! I just wanted to know about how much all those components cost, and how much time is involved for swapping everything? Must have been tedious making up that list...

[GunalORSEL]

Replacing the components, after building the "bill of materials" and getting them, did not required much time: one evening (19:00 hrs - 22:00 hrs) for the poweramp stage (power amp pcb) and another evening for the preamp stage.

The actual time consuming job is removing the factory bad solders out of the components... Once I have put replacement components, and soldering them the correct way, it's easier to replace them in the future.

All the components, including a full set of valves, full set of pots, power switches, resistors and caps costed around US$200, after it did well worthed it, best bucks spent and best DIY project ever, compared to the outcome which is splendid.

I have also performed, then, some mods I have found over the web, which consists of removing some components and re-routing a resistor.

Poweramp stage, it almost became a soldano slo-100. The circuit is almost the same.

I can send you an excel file, for the materials.

[theztovski]

i have the same amp and want to do the same mod as you could you sent me that exel file

thx

kriZtov

[AmpFix]

Please!!! don't replace all resistors and other components... and those measurements you are making, unless you are taking each resistor out of the PCB to measure them, your measurements are worthless.
In terms of replacing resistors with better tolerances, you're also wasting your time and run the risk of damaging the amp or introducing new problems. Fix only what you need to fix.

[theztovski]

well I repalced all elco's and the bias trimpot.... just as new now