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Need Some Help With Marshall 100v Head


TenderSurrender
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Hey once again :D

As some of you may know, i recently became the proud owner of a Marshall Valvestate 100V (8100) from 1992.

However with the age comes a couple of problems and id apperciate it if someone could give me some pointers B)

1) The master pot and sometimes the treble pot on the "Boost channel" doesnt work.

- For example when turning the master pot to say "4"... it either produces none/very little noise OR the actual volume meant to be produced at that level, Theory says dodgy pot but wouldnt know where to start?

2) How do i get inside this head? The guy i got it from wasnt aware it had a tube preamp ( :D ) and so never replaced the tube, I want to get inside and replace it but dont have a clue how to get in!

Im presuming the 6 screws on the top of the headunit secure the preamp/poweramp in, anyone care to shed some light onto that one?

3) Are there any common mods for these amps? as many people know, i like to tinker so any ideas are always welcome!

Cheers guys!

~~ TS ~~

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1) The pots are most likely wore in the spots you describe at position 4. Cleaning them with an electronics cleaner may help but you will need to disassemble it.

2) Yes removing those screws on top should allow you to slide out the amp chassis for inspection or repair. Be careful when sliding out the chassis, sometimes the tolex or outisde covering may be stuck to the metal work and can tear.

3) Modifying the distortion section and/or updating the opamps with higher quality pieces are the common mods we make on any amps.

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1) The pots are most likely wore in the spots you describe at position 4. Cleaning them with an electronics cleaner may help but you will need to disassemble it.

2) Yes removing those screws on top should allow you to slide out the amp chassis for inspection or repair. Be careful when sliding out the chassis, sometimes the tolex or outisde covering may be stuck to the metal work and can tear.

3) Modifying the distortion section and/or updating the opamps with higher quality pieces are the common mods we make on any amps.

Cheers :D

I Opened up the amp and things seemed to be in good shape HOWEVER...

There was a strange brown luquid inside near the circuit board, it was sticky(ish) but no idea what it is?

The caps all seemed ok so i dont think it was a leaking cap?

Also with the master pot, they are very small on the inside so im not so sure how easy this is going to be :D

~~ TS ~~

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It could be glue, especially if its between or around caps. Or it could be left over from something that was spilled on it years ago.

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The sticky brown goo is probably just some flux left over from some rework or repair at the factory. Modern production electronics are wave soldered and don't require the use of large amounts of flux for production. However, if the circuit was ever repaired or reworked then someone globbed some flux on there and didn't bother to clean it up. Unless it's gathering large amounts of dust, I'd leave it alone. Otherwise, a little rubbing alcohol will clean it up.

Also, you probably don't need to replace the preamp tube unless you're getting a lot of transient distortion on the clean channel. Preamp tubes tend to last a LONG time compared to power tubes, so I'd just leave it alone. Hybrid designs tend to be more fickle about hot-rodding than pure tube designs.

Contact Marshall about getting replacements for the master and boost volume pots. Those are the most used pots in any amp so it's not surprising they may need to be replaced.

Edited by crafty
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every one i have worked on had the old jrc chips in it. which read the whole tube screamer theory [better yet breadboard it and hear the difference] if you change them out with a better chip you will need to track down the caps that are in the feedback loop and increase this range. trust me on this one, every time i change them out to say the tlc2262 or such i get osc at high gain. however on the clean channel it does wonders for it

you can play with the voltage for the preamp add in a microboard and a socket for the dual opamp in the preamp and change it out for a cd40107 its not a direct drop in, but if you look at the datasheet i think you can see a cool little trick to make and adaptor on it.

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