Hi,

I have a DIY fuzz pedal, which is way too bright. I thought it'd make sense to build a fixed low pass filter into it, so that it will sound ok with my regular setup, without having to change the rest of the setup just for this fuzz. So, if you think this is a sound idea:

I know something about LPF's in theory (how to calculate Xc etc.), but I'm not sure where I should place it in the schematic (input or output side), and what other factors I should take into account.

Here's the schematic:

http://www.deviever.com/fx/wp-content/uploads/2012/09/hyperion.jpg

Where should I place the LFP there..?

Hi,

I've had a Marshall Mosfet Lead 100 for some years, it's needed repair twice, and now it's the third time there's something wrong with the sound. I don't feel like handing out another 100€ for a repair, so I thought I'll try to do it myself (rather than give up the whole thing). I'm familiar with electronics in general, I have built a dozen guitar effects and some synthesizer stuff, but I'm not particularly familiar with amplifiers. I have a rudimentary ability to read schematics, know e.g. the basics about voltage divers etc., but I can't really read complex schematics like for amps without someone holding my hand.

So I do have intermediate skills in electronics, in a way that I believe allows me to understand most advice I will get here, but I won't manage without help.

Here's a schematic for the amp:
http://www.drtube.com/schematics/marshall/3210-iss7.gif

The problem: When I plug in, I can hear the guitar only faintly (the faint signal's kind of clean). When I strum harder, the volume jumps up a lot, and the sound is very distorted, almost in an "8-bit" way or something. When I switch the amp off, there's a loud swoosh, plus some guitar sound for about a second, if the strings are ringing at the time of switch-off.

When I plug into the return jack, it's basically the same, only without the faint clean sound, so I suspect it makes sense to start by checking the stuff between the return jack and output. Please correct me if this doesn't sound like a good assessment.

If I remember correctly (and I'm pretty sure I do), last time the amp was in a repair shop, IC5 was replaced. At any rate it's not the chip specified in the schematic: according to the schematic, IC4 and IC5 are MC1458, but IC5 is in fact BA4558.

So I could just desolder IC5, put a DIP8 socket in it and replace the chip with some suitable op-amp, and see if it starts working, but I'd like to complicate things a bit and ask for your help to improve my amp repair skills as well, if you dont mind :-)  Now I know that many op-amps are kind of interchangable, but I'm not sure which ones are compatible, and what are the crucial characteristics to look for in a datasheet.

What I have available in the local electronics shop are at least RC4558 (0,50€) and MC1458P. The pinout is the same, but do you think it matters which one I choose? Here are the datasheet for them, in case it's useful:

RC4558: http://pdf.datasheetcatalog.com/datasheet/texasinstruments/rc4558.pdf
MC1458P: http://pdf.datasheetcatalog.com/datasheet2/3/07j9c8li1155f0estliki76oc2cy.pdf

In case it's relevant, I've used the amp with a Boss GT-10 multifx so that the mfx connects to directly to the return jack. My impression is that this is ok, but if you think it's problematic, please explain. If it is problematic, it could explain why a chip behind the return jack gets broken repeatedly – if indeed that is the broken thing here. Anyway it's been 6-9 months since it was in repair, and the amp has been used every second week for two hours at a time.

If you are able and have the time to help, but need to know something further, please ask! I dont' have an audio probe, but I can build one if that's necessary for your troubleshooting tips.