trouble with replacing jcm800 master volume pot

[foocankill]

the volume on my jcm800 has been jumping up and down so i took it into a repair place and he told me it might be a bad master volume pot. because he didnt have the pot on hand he had to special order it, saying it would take almost 2 weeks. i didnt want to wait that long so i figured, having some soldering experience, id replace it myself. i found the master volume pot to be a 1m ohm audio taper, so that is what i picked up at my local electronics warehouse. everything went well in replacing it, but when i turned my amp on and slowly turned the volume up i started to hear a massive amount of some type of interference or buzzing. i played my guitar on it and it kind of worked, i got sound out of the notes but it wasnt as loud as it should be, the buzzing sound was louder then the notes.


my question is, is there some sort of special 1m ohm audio taper potentiometer that i should be using, or are they all the same? or if not the wrong pot, then what could have happened to my amp?

thanks so much for your time!

[Enzo]

1 meg is 1 meg. If the taper is wrong, the only thing that will happen is that some of the settings will be bunched up near one end of the dial and the rest will be spread out. But it will still work and sound no different. Clearly when you disassembled it, you knocked out some connection or damaged a component.

Any time you work on something and it has a new problem after reassembly, it is going to be the result of something you did.

[dai h.]

is the ground connection to the pot connected? Which amp is this exactly?

[foocankill]

i connected everything exactly the way it was, soldered perfectly, which one is the ground? its a marshall jcm800, thanks for the replies guys. this is really stressin me out because im scared if i messed something up. really though im almost completely sure i didnt knock anything off or disconnect anything on accident. ive repaired and soldered a million different types of electronic equipment before and this is the first time i messed something up. does anyone have any ideas of what i could have messed up, or any way i can narrow down the problem? thanks

[Enzo]

We were kinda wondering what model the amp was, since JCM800 series includes:
1959,2203,2210,1987,2204,2205,3203, and those are just the heads. SOme of those amps also come as combos.

I don't really think it is the ground leg of the pot. If that were the case, it wouldn't introduce noise, it would just leave thing all the way up, and the control wouldn't do much.

What I said last time is just a rule of thumb in an electronics shop. I been doing this a long long time, and I still screw it up now and then. If the amp was working more or less OK except for the funny pot vefore, and now after working on it, it has a new problem, really, it has to be something that happened during the repair. Wires get flexed when boards are handled and can break, as an example. Doesn't mean you weren't careful, stuff happens.

Whatever the trouble is, if you can turn down the master volume and the noise turns down with it, it tells us that the control is working, and it tells us the source of the noise is before the control.

Any control that affects the noise is after its source. In other words, if the EQ controls affect the tone of the noise, then the noise comes in before those controls. If they have no effect on it, then they are before its source.

That is one way to isolate the problem to a certain stage of the amp.

Does it make this hum and noise with NOTHING plugged into the input jack?

[tonezoneonline]

Other than agreeing with what Enzo has to say I will add that a 1M pot is the most common pot used in tube amplifiers and if I took it to a shop and the place had to order one I would reconsider the shop.The two week order time is also suspect in my book.

[Amp Kat]

If it's a combo try swapping the reverb wires around. I also agree that isolating the problem is cutting your work in half. Try using the send out to another amp and see if the noise is there also.

[Enzo]

Two week order time? Most shops won't order ONE pot from somewhere. I can't see ordering one $2 pot and paying $9 shipping on it. Most places will want to wait to put together a parts order from a particular supplier. Especially if they are dealing with someone like CEDist or MAgic parts who have like a $40-50 minimum order. DOn't know what is happening in that shop, but I;d guess something similar.