1987x Acquired - Would love your comments on the mods.

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splitslim
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1987x Acquired - Would love your comments on the mods.

Post by splitslim » Tue Feb 06, 2018 9:38 am

So I passed on the JCM800 2203 which was heavily modd'ed and grabbed this 1987x instead.

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Master Volume installed - pre-phase inverter at the effects loop (before or after?). Green lead connects to board at BN4 and the blue lead goes back to the effects loop card ;

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Cap installed on across R11, 330uf if I'm not mistaken;

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Hum balance VR removed and replaced with 2 resistors and what looks like 2 caps beneath (at R31, R32 and VR2)? Would love more information on this one. Why the caps under the resistors and what benefit does this mod bring?

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I'm considering having the master volume re-done to a post-phase inverter Trainwreck style (professionally). What are your thoughts on the pros/cons of making that change from what is currently installed here? Any comments on the proximity of the master volume control to the power and standby switches? Am I asking for trouble leaving it there, introducting AC noise into the signal path?

Thanks!

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neikeel
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Re: 1987x Acquired - Would love your comments on the mods.

Post by neikeel » Sun Feb 11, 2018 11:00 am

There has been some input to this on Marshall amps forum.
The heater anti hum pot has been replaced by the 100R resistors, a sensible mod. No idea why the caps are there, I can think of no reason why you might have capacitors bypassing the resistors in that circuit.
I quite like the pre-pi Mv in a typical cascaded v1 (2203/2204 style). I have no experience with the crosswire mv. Although the Fischer mv that Mark and Larry made popular works very well particularly when you use a high quality pot (like a PEC). I personally use an attenuator to manage overall volume even on an Mv amp.
Regarding the 330uF on v2a cathode resistor. Guys chasing the brown sound like it but I find you lose a lot of definition in the preamp. I like the oem 0.68uF or even 0.47uF both give a nice upper mid gain boost without all the crap the 330uF brings with it.
Neil

jarrodthebobo
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Re: 1987x Acquired - Would love your comments on the mods.

Post by jarrodthebobo » Sun Feb 11, 2018 1:31 pm

I've found that a 10uf cap over v2a is pretty much a full gain boost without a big deal of mush the 330uf cap brings. I'm not too sure on the science behind it and what frequencies are technically being boosted here, but that's just from experimentation.

I do however have a switch on my current project that switches between the 10uf and a .47uf cap over the v2a stage. Offers either full boost or a mid/treble boost. They tend to 'sound' like they have the same amount of gain, but the feeling is definatly different and so is the bass response.
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