Boss SD-1 Mod problem [please help]

[kingmobmaskreplica]

I am having a problem with a mod I have attempted for fun on a Boss SD1 pedal. I've built a couple of pedals and wanted to try modding one.

The distortion oscillates then chokes out nastily. I have checked the connections as best I can but am unsure as to what the cause is.

Soundclip here - https://clyp.it/igqqdlzm

I have a cheap multimeter but not sure what I should be testing first.

I have replaced three cap/resistor filters changing the values (R6/C3 1k .22) (R7/C4 1k .22) (R8/C5 220 .22), two diodes with LEDs but retained the third for asymmetry. The chip has been socketed and replaced with a BB. I also replaced R5 (10k) as supposedly this helps to clean up more with low gain. These were found on this forum? and in various articles online so they should be ok changes (but I maybe haven't done them right I guess).

Edit: I forgot to say that I burnt the pad out between C5 and R6 so I have soldered their legs together. I am pretty sure this isn't the issue though as the pedal did this noise before I burnt the pad out.

Schematic here: https://www.hobby-hour.com/electronics/s/sd1-super-overdrive.php

[Rob Strand]

The mods have the following effect:

1) increasing the clipping levels
2) increasing the gain to compensate for the higher clip levels
3) reducing the amount of HF cut on the tone control (and changing the frequency)

What I think is happening is the higher clip levels are now causing the opamps to clip instead of the diodes.
If the tone control stage is clipping then there's nothing to removed the evil sounding highs after it.

As a first step I'd replace R7 with 1.8k then add a 2.2k across C4.   That shouldn't change the character of the mods but it will reduce the level going into the tone control stage by a factor of two.   If that removes the junk then you know the tone control stage is clipping.

[kingmobmaskreplica]

Excellent, I wondered if I'd just upped the internal gain too much or something and clipped the op-amp? When I switch back to the silicon diodes the sound seems to go away (not quite sure it isn't still there slightly). I will try your suggestions and get back to you.

Ps When you say 'across' do you mean place a 2.2k in parallel to C4, or do you mean after the connection to C4 but in series with R7 going to pin 3 of the op-amp, sorry I'm quite new.

[Rob Strand]

When I switch back to the silicon diodes the sound seems to go away (not quite sure it isn't still there slightly). I

That's a good test also.

place a 2.2k in parallel to C4,

Yes in parallel.

Keep in mind this mod only helps stop the second opamp clipping.   If the second opamp clips there's no filter after it and it will sound harsh.  If the first opamp is clipping and contributing to any undesirable sounds then you are best trying some different LEDs.

Anyway see how you go.