Identifying and trouble shooting a modded Marshall JMP 2203

[nikolas7333]

Hi All,

I recently bought a modded JMP 2203, and I'd very much appreciate some help from the amp experts here with the following issues:

1. Identifying what the mods are and how they are affecting the signal,for interest's sake. A previous owner has added 'high gain', 'fat' and 'full' switches on the front, as well as an effect loop around the back.

2. There's a high pitched whistling sound coming from the amp, which is worse when using a single coil. Based on some internet research, I've tried swapping preamp valves, but to no avail.

To give a bit more detail:

- There is no noise when the volume on the guitar is all the way down.

- When the high gain switch is off, the whistling doesn't occur

- The whistling is there when the gain is set high, and I'm on the single coil.

- The whistling starts to happen when the preamp gain is set past 5.

- When I'm close to it, it's less prone to whistle, and moving around the room causes the problem to disappear and re-appear again.

- When using a humbucker, there is no whistling, but at really high gain settings, some weird sort of clipping occurs whereby when I pluck the string, the impact momentarily causes the same pitched whistle to occur.

Here's a link to a folder containing some photos:

http://s330.photobucket.com/user/nikola ... t=3&page=1

I can take more if they aren't good enough.


Thanks in advance,

Nik

[bulldozer1984]

Hi Nik,

Just wondering if you rectified this problem or not ? I am happy to help (6 months later)