Modding the Digitech PDS-1550 (ElectricDruid article inspiration)

[Strategy]

My bandmate asked me to modify his Digitech PDS-1550 and pointed me towards ElectricDruid's great article here https://electricdruid.net/digitech-pds-1550-distortion-pedal-analysis/
which I see also grew out of an analysis thread here on the forum.

I subsequently found another couple PDS-1550 units on local Craigslist for $50 each and am going to have a go at modifying all of them now. I have modded my PDS series delay pedals to accept voltage control and gate inputs (to interface the pedal's controls with my modular synth) and I see even more potential on the PDS-1550 which i think is one of the funnest things Digitech ever made.

My initial plan was pretty basic:
1) CV input to SVF cutoff frequency using a home-rolled LED/LDR 'vactrol'
2) Auxiliary LPF out to its own jack

My main question is bringing the DIP elements to be externally controlled as toggle switches. Initially I thought I would just do the delay in/out control -- but the more time I spend with ElectricDruid's article, the more I think that all of the DIP controls are useful on the front panel (my band does stuff on the weirder end of noise/experimental/ambient, so...it's not really excessive tweakability from our standpoint)

I've never swapped out a DIP switch for other switch types.
Would you a) completely desolder the DIP switch and start from there?
or-- b) is it possible to just wire my toggle switches to the DIP pads in parallel, without removing the DIP switch at all? I assume this would override the DIP settings and it could get confusing for someone trying use both internal and external switches, but if I can get away with less invasive, desoldering intensive approach I think that is beneficial.

Curious to hear your reactions thanks much!
Strategy

[ElectricDruid]

Glad you've found the article inspiring.

I don't agree that *all* the switches are useful. The two "delay range" ones just add parallel resistance across the delay pot (1M preset) which limit it to half or a quarter of it's full value. That might make for easier adjustments with a tiny trimmer, but I think rather than bring the switches out, I'd just take the trimmer out and put a 1M delay pot on the panel. If you want to get experimental, you could also add a feedback pot so the delays can recirculate a bit, but that's not entirely simple. Maybe a 10K pot from the switch end of C52, with a 100K from the wiper to the junction of C17/R26 in the pre-delay filter. The 100K parallels R5/R25 bringing the input in, and then the feedback and the input are mixed in the filter. I hope! - untested idea.

(Schematics are here if anyone is following along: https://electricdruid.net/digitech-pds-1550-distortion-pedal-analysis/)

The other switches control the delay on/off, and switch in/out various tone control networks, so those might be good to play with.

Since the switches are all simple SPSTs, yes, you could simply leave the DIP switches on the board open and wire up the panel switches in parallel. Closing either is then "On", but if a DIP switch is on, the associated panel switch won't do anything (It's a wired-OR connection, technically, I suppose!).

Finally, since the SVF is actually based on a LM13600 OTA, it's pretty much a voltage controlled filter already. In this case, they've just hard-wired the control voltage to +V and then adjust the current into the OTA's control pin by changing the resistance (the 500K Frequency pot). But you could check out a few LM13700/600 filter designs to see how they've done the CV input and steal a bit of circui to add in there. At the most basic, another resistor patched to the +end of C47 would work. The two 10Ks R69 and R72 will provide enough current limiting to keep the filter chip safe (2mA max, iirc). Basically, you swap a fixed voltage with a variable resistor for a variable voltage with a fixed resistor, if that makes sense.

HTH, good luck!

[Strategy]

Hey ElectricDruid,
I see what you mean about the delay controls. I'll need a pretty small pot to fit given the tightness but that is where the battery compartment comes in handy - I never use batteries on my PDS units as they get drained so quickly, so as far as I have done on my PDS delay mods, that is space that is easy to sacrifice and offers room for mods that need ample real estate. I do think the most important Delay DIP switch to bring to the front panel is the on/off - on this unit I just don't always want the delay on, and its silly to have to go under the hood each time to change that.
I'll post my results in this thread!
With pretty neatly constructed boards, cheap used prices and nearly every part being fairly replaceable on these units I feel there's really no harm in hot rodding these things.
- Strategy