DS-1 Mod problem. Please help, I'm new to this!

[mr.adambeck]

In an attempt to get an overdrive for the bassist in my band I did some quick and easy mods to a cheap DS-1 pedal.  We like the tone a lot (it's got a nice mid-range growl and it's keeping a lot more low-end than most pedals), but now it's getting a bunch of radio interference when it's plugged in.  I's pretty noticeable and bad.  Does anyone know what I could have done?  Did I maybe somehow mess up the ground or something?  Here are the few changes I made:

R13 - 2.2k
C4 - 150 pf
R16 - 1k
R14 - 4.7k

Please Help!  This is only my third attempt at any form of electronics, so I'm still experimenting, and don't really know much about what I'm actually doing..

[5thumbs]

...  Here are the few changes I made:

R13 - 2.2k
C4 - 150 pf
R16 - 1k
R14 - 4.7k

Being unsure of the methodology used when making the mods (i.e., following someone's mod plans, testing after each component change and noting difference, etc.), I can only offer some guesses that might help you out.

For starters, if you put all the parts back to stock, does the "radio frequency interference" go away?  If so, then repeat each change above one at a time, noting the change in the tone.  When you find the change that causes the RFI to go away, skip that mod and continue until you've repeated all the mods minus the ones that cause the alleged RFI.  If the sound is still what you want, then you've solved your problem.  If you remove all the mods and still have the problem, then you may have indeed "messed something up" in the way of creating a solder bridge/lifting a trace/cracking a solder joint/stressing a wire and creating a break in the wire/etc.  In that event, you've got a lot of continuity tracing and troubleshooting to undertake to find the break.

That suggestion aside, the first change that jumps out at me is the replacement of C4.  From its position in parallel with R7 between the base and collector of transistor Q2, it appears that cap is there to help reduce high frequencies, which may be the "radio frequency interference" you're referring to.  C4 is 250pF in every version of the DS-1 I've ever seen, so I'd suggest you change it back to 250pF first to see if resolves the "radio frequency interference" problem.

Next, R16 is a part of a low-pass filter (with C12) in the Tone control sub-circuit.  That LP filter has a stock corner frequency of 234Hz.  By reducing R16 to 1K, you've increased the corner-frequency of the low-pass filter to 1592Hz, which is allowing more mids into your signal, with the low-pass filter/high-pass filter balance being controlled by the Tone pot VR3.  On paper, this change is likely the source of the altered "mid-range growl" you mentioned.

I'm not sure why you changed R13 from 4.7K to 2.2K.  That will effectively increase the bass cut-off frequency coming out of the Dist portion of the circuit (by raising the corner-frequency of the low-pass filter from 72Hz to 154Hz.)  If your goal was to increase the Dist sub-circuit gain and reduce bass frequencies, then never mind.  However, if you are after more gain AND keeping the bass frequencies the same, you need to change C8 as well.  Along with reducing R13 to 2.2K, you should increase C8 to 1uF.  By changing R13 and C8 in this manner, you will keep the corner-frequency for that low-pass filter around 72Hz (which is what the stock R13/C8 configuration cuts off at) and increase the gain coming out of the Dist sub-circuit.

The R14 change don't raise any red flags to me, but that does not mean it don't have a part in the problem.  My gut feeling doesn't implicate it as a prime suspect at the start, but if the aforementioned suggestions don't resolve your issue, putting the pedal back to stock condition and repeating the mods one at a time is probably your best bet for finding the source of the problem.

[mr.adambeck]

Wow!  Thanks a bunch. Very In depth.  I no longer have all the parts I took out (yes, I've learned my lesson), but I will mess around with some of the changes you mentioned and see what happens.  Thanks SO Much!

[5thumbs]

Wow!  Thanks a bunch. Very In depth.  I no longer have all the parts I took out (yes, I've learned my lesson), but I will mess around with some of the changes you mentioned and see what happens.  Thanks SO Much!

You're welcome.

If it were me, I'd start by changing C4 back to 250pF and change C8 to 1uF.  Unless you have 1uF aluminum electrolytic capacitors lying around, go ahead and replace C8 with a 1uF metal film capacitor.  You might have to order the 250pF ceramic disc capacitor from smallbearelec.com/mouser.com/digi-key.com/futurlec.com, as the Radio Shack near my place doesn't carry 250pF ceramic disc caps.

The other changes are probably beneficial to the sound you're going for.  Of course, while you're buying the caps for C4 and C8, buy all the original value resistors and caps as well, just in case you need to revert to the original config.  I keep a baggie filled with caps/resistors/diodes/etc pulled from modded pedals, so you should start your own baggie next time.  You'd be surprised how often you can make use of some of those old bits (especially when you forget to order 1 part for a build...that grab bag might allow you to complete the pedal without having to wait several days.)

Good luck!