Tuesday 14 November 2017

Digitech PDS 1000 & 2000 Repairs

I am fond of the Dod/Digitech PDS series delays. On paper there's nothing too amazing - mid 1980s 8-bit delays with 1, 2 or 8 (if you can afford the PDS 8000) second maximum delay times. There are two features that win me over.

The first is the "Infinite Repeat" footswitch that locks the delay into a repeating loop. The loop can be pitch-shifted by changing the delay time, and when Infinite Repeats is turned off the delayed signal trails out like normal again. This is like playing with the shittiest looper and is a lot of fun, and surprisingly a lot of modern delays & loopers don't (or can't) do this. Some will repeat indefinitely without oscillating if the feedback is maxed (Boss DD series) but it's nice to have it on a dedicated switch, with feedback control available and ready for switching back to "normal" use. A surprising amount of 90s and early 00s delays can't smoothly pitch-shift recorded audio when playing with the delay times, and glitch or "jump" somewhere through the pot travel.

The second win is the internal design. The expected parts for a simple delay are all there - an ADC, a DAC, some memory and some logic gluing things together - but there are no custom ASICs, microcontrollers or programmable devices at all. Everything is done with standard CMOS logic. These delays could be built from scratch with new parts today, and they are very repairable. This is a little bit of a backwards approach for a 1985 era product - at this point Boss had already released the DD2 & DD3a delay pedals and they used the same ASIC from the Roland rackmount digital delays. This could have been a cultural difference, as Japanese manufacturers seemed to prefer going for custom ICs and offsetting the cost against using them in lots of products (Yamaha have also done this). Digitech (or DOD at the time) may have to bring the series to market quickly, which could have forced them to use off-the-shelf parts. Whatever the reason, nearly ever part can be sourced pretty easily, unlike finding a replacement Boss controller IC from 1985 (good luck).

I repaired 4 of these delays recently. Here is a braindump:


PDS 2000 #1



All of the pedals in this series came with snap-in plastic battery doors that are not held captive by the enclosure (i.e. they are removable). This means they get lost, and I have never actually seen one.

This PDS 2000 (mine) was fairly easy to deal with. It's a 2 second delay that adds sampling modes that can trigger the delay sample by a footswitch or an external trigger signal. It had some broken wires in the harness between the PCB and jacks/switches. I think this is due to using the pedal with a battery but without a battery door - the weight of the battery pulls on the wiring if it's free to swing around on the end of the battery clip. This just needed some soldering to bring it back.

The footswitches were also unreliable, so I replaced the microswitches. More on that below.




In an unusual move, DOD actually sprang for a silkscreen print on this PCB. Schematics are available online but can be hard to match them with a PCB when there is no silkscreen and no part designators.

PDS 1000 #1

 

 

This one was partially working when I got it. The output jack wouldn't hold a cable in, the footswitches rarely worked and the delay length maxed out about 75% of the way through the pot's travel - but it did work as a delay. I replaced the footswitches and bent the retaining lug on the output jack (I may go back and replace this) so that it was a usable effect.



I have never seen a PDS 1000 schematic, but the PDS 1002 2 second version looks like it's very similar. The delay time control circuit is shared across the earlier series. Delay time is varied by changing the frequency of the main clock signal that shifts digital samples into and out of DRAM memory. Faster clock frequencies shift data faster and give shorter delays. Delay time is adjusted by 3(!) potentiometers, the delay time pot on the front of the enclosure, a trimpot that adjusts range of the of the main delay time pot, and a final trimpot that globally adjusts clock frequency by small amounts. You don't really get a very wide range of adjustment so trying to get longer delays by tweaking pots might not work too well.

Tweaking these delay pots brought the delay back to 1 second and adjustable through the full range of the delay pot.

PDS 2000 #2






This is the only pedal of the 4 that didn 't power up at all. None of the logic chips had any sensible voltage at the VCC pins, but the opamps were getting 9V. There is a 78L05 to drop the 9-10V input down to power all the logic chips, and this has a JFET soft-start circuit that ramps up the 5V power supply, presumably to protect the digital chips from a loose or intermittent power cable. I replaced the timing cap with no luck, then swapped the FET for a new J201 and it powered up.


This a soft-start for the 9V supply on the PDS1002. The 2000 has something similar.




Everything seemed to work, except that I couldn't change delay ranges, it was stuck on the longest range (2 seconds). The sampling modes also weren't quite right, trying to trigger samples would switch the pedal into bypass mode or sometimes do nothing at all. I traced signals back from the delay range switch to a 74HC04 hex inverter, which looked dead. I swapped this chip over from the other 2000 and it worked. I ordered a new 74HC04 and got two working pedals.

This one also was not quite a 2 second delay so I did some pot tweaking. Unfortunately I managed to slip with an oscilloscope probe and shorted two pins on the DRAM chip, and killed an input pin. I replaced with a TMS4256 DRAM from eBay - works fine. With the new chip the delay rates could be carefully dialled in to 2 seconds pretty quickly.

PDS 1000 #2

 


This was in the worst condition. I got this in a box of failed repairs from a music store in the US some years ago and never did a lot with it until now. Someone had already taken a crack at fixing it, it was missing knobs and a back panel. Two of the pots were broken, the bodies would wobble freely against the legs as if they had detached internally. The 1Meg trimmer for clock frequency was also broken off.


I replaced the trimmer and set it to the middle of the it's range, based on how the other 1000 was set. The PCB mount pots were a little harder, DOD use Alpha pots marked "W" which I don't think are actually W taper (W taper is logarithmic for half the travel, then reverse log for the other half). I replaced with long-leg alpha pots. The replacements are a little taller than the originals, so I cut the legs a little shorter and soldered some bus bar to them, then soldered the bus wire into the PCB. This actually worked pretty well, but replacing these pots and running wires to the PCB is probably a better long-term solution.


After replacing pots I found that delay time wasn't working because of broken trace, I ran some Kynar wire to restore it. The mix knob didn't work and it was stuck at 100% wet - this turned out to be another bad JFET.

At this point things appeared to work again, except the delay was very distorted and noisy. I messed around with compounder trimpot as I thought that may be distorting - no improvement. Eventually I realised that bad memory chips would give corrupted repeats which would probably sound like distortion, so I swapped the DRAM from the other PDS 1000 and it worked. I ordered some replacement DRAM (I used MK4564) and they worked just fine.

Adjusting delay time

There may be an "official" method for adjusting maximum delay length using a testpoint on the PCB but without a service manual I've come up with my own.

The capacity of DRAM can be looked up from the P/N and the nominal maximum delay in seconds is known. My reasoning is that DRAM should be completely filled in this time, so I probe the DRAM with an oscilloscope and adjust delay length until the frequency of data in and out matches the DRAM size divided by nominal delay length in seconds.

For the 2000 the DRAM is 262144 memory locations and max delay is 2 seconds, so I want data to be going in and out of memory at around 262144/2 times per second or 131072 Hz when the delay pot is all the way up. This can be measured from the DRAM Write Enable pin, and probably from the Data in and Data out pins as well. Most of the tweaking is done on the lower global clock frequency trim, usually after making sure the delay pot is working throughout it's whole range. Sometimes I had to iterate and go back and forth between the two sets of pots. Adjustment on trimpots is fairly coarse, so this can't be dialled with really great precision but I find it much easier than trying to listen to the delay and sync to a stopwatch or something similar.

Modding for more delay time

There are online discussions about modifying these pedals for longer delay times that usually involve tweaking the delay pots away from nominal positions to get longer delays. I think this will only work well for very lo-fi sounds, as the sample rate will get much lower and aliasing effects will get worse.

The delay loop in the PDS series is basically some 4040 ripple counters counting through memory addresses and resetting when the end of memory is reached. A better approach to extending delay time is replacing DRAM with a larger IC (or multiple ICs) and adjusting the counter reset logic to address the larger memory. I have some spare DRAM for both of these pedals so I would like to try this.

I haven't seen a schematic for the PDS 8000 (8 second delay). I would like to see how this was implemented, as it should have 4x the memory of the PDS 2000. Extending the 2000 to 8 seconds may be possible depending on how extensive the differences are.

The best picture I can find online shows a single 18 pin (not 16 pin) IC, but the label isn't legible. Presumably it's a 1Mbit x 1 DRAM.

Please let me know if you have a PDS 8000 schematic or high-resolution board shots.

There is a PDS 20/20 delay schematic available (http://www.experimentalistsanonymous.com/diy/Schematics/Delay%20Echo%20and%20Samplers/Digitech%20PDS2020.pdf) which uses 2 4464 DRAMs, which is twice the memory of the PDS 2000 but still only does a 2 second delay. America's Pedal has a catalog for the PDS series which explains why - the 20/20 has a delay signal bandwidth of 16 kHz, the rest of the series only has 7 kHz except for the PDS 1700. I am guessing the higher sample rate is for the chorus/flanger modes where really short delays are needed.

Footswitches

Dod used a fairly cheap mechanical design for actuating footswitch buttons on a small PCB. The plastic foot panels are cantilevered and are returned to their neutral position by a spring at the "fixed" end, not the free end which would make more sense. There is also no real end-stop to limit the force on the footswitch - if you stomp hard you will crush the button. This is problematic as most players will stomp harder if the switch doesn't work, so once they start to fail they deteriorate quickly.




There is advice online on adjusting spring tension to get a pedal to switch more reliably. Don't bother. If a DOD pedal does not switch every time, just replace the 10mm button on the PCB. They are very cheap and should last another decade of use. Play with the mounting hardware only if the levers aren't rotating correctly.

33 comments:

  1. Please send me your email to maneco gmail dot com and i can send you the pds 8000 schematic

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    1. Fantastic, thank you. My email is falseelectronics@gmail.com

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    2. Please send the PDS 8000 schematic to adamdada1987@gmail.com. thanks!

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    3. PDS-8000 Schematics
      mlehr077@gmail.com

      Thank You

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    4. If someone could send the PDS-8000 schematic to domdecosa@gmail.com I'd greatly appreciate.

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  2. Hey i got sone issues with my pds 1002. Ja zehre a Chance that i could send oh to you for repair? Or do you have an idea where i could Let oh repair?
    Thanks

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  3. Hi, This 2 second delay is a ton of fun! Unfortunately my "Mix" Pot snapped off a couple a months ago and I lost it. Do you know where I can reorder it and which exactly to get? I am pretty noob at this but I know I can resolder it... Thanks!

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  4. Hi, I'm writing to you from Argentina, you know I have the pds 2000 and the delay does not work, do you know what the problem may be? saludos!

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    1. Do you have access to a multimeter/voltmeter? I would start by checking the 5V power supply.

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  5. Hi, Help... i've recapped my PDS2000 it wasn't working at all, now its going, the manual settings seem good but the sampler part seems to only get less than 1/2 seconds of sample can i get any info on the adjustments of this?

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  6. Hi, no sound from my PSA1002. The two lights work when the pedals are pressed. I’ve taken it apart and cleaned it up, but no luck. All wires seem to be OK. I’ve even tried different guitars. This happens every time I have it repaired and then put it in long term storage. Then I try using it and no sound. Do you know a good repair outfit I could use. Thanks.

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  7. Hi krivx
    Thanks for this post! It aided in my troubleshooting of a PDS8000. While I was able to get it working intermittently, I think it needs new pots and switches. I do not want to put the time into it so I am looking to sell it if anyone is interested - dvandre@optonline.net

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  8. Thanks for all the info! I've got a dead 1002 on my bench. I've restored power and all chips are powered correctly and bypass is working. switches, connections, pots, diodes, resistors - tested. There is an audible single delay when active but no ability to change delay times and repeat hold does nothing nor does it light up the LED for it. Aside from starting to trace back from the output to see where the delay signal is dying, is there any chance you have the schematic or at least the block diagram for the 1002 available and could send to me at luke.awtry@gmail.com? Much appreciated in advance!

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  9. I've recently decided to do some repairs to my original PDS 20/20 which I bought in the early 90s, but died around 2000. I had watched eBay for a long time to just buy a functioning one, but they were often well over $200.

    So I finally recently bought instead a cheaper PDS 1000, which died as well not long after getting it. I might have sent in the wrong polarity using a power supply, I'm not sure.

    Either way, now I have two units to repair. The PDS 20/20 is what I really care about, so I began with the schematics provided here (THANK YOU) and began following traces and mapping out what component is what on a image of the board in Photoshop.

    I've also created a spreadsheet listing every component as well as the part or replacement that is available and where.

    The only thing I'm having issues with is the resister network. I'd imagine you would be the perfect person to ask about it. Some folks on Reddit pointed me to R/2R ladders, but there are different types of resistor networks.

    Basically, do you know what would be a good replacement for the LA10-502 that is in these units?

    I was thinking it would be a fun project to breadboard a complete new unit based on the schematics and the actual board.

    There are some differences also between the schematics, for example R95 is 10 Ohms on the December 4th 1989 schematic, but on the Sept 29th 1988 has 51 Ohms. My board has the 51 Ohms. Also, a lot of the rails are shown as 5V on one, but 9V on the other -- one is wrong, I believe the correct value is 5V.

    And lastly, the transistors include NP100, NP100A, and NP200, which are all obsolete. In trying to find replacements, I couldn't detect anything substantially different between the NP100 and NP100A. I've got both the spec sheets, but couldn't suss out what to look for what would be different.

    I've presently selected 2N4401BU as a replacement for the PN100 and PN100A, and 2N4403BU for the PN200 (which is the same as PN100, except PNP instead of NPN).

    Any insights?

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    1. Sorry, I missed this comment. 2n4401/3 sounds fine, just make sure the pinouts are right.

      For the LA10-502 you could replace with individual resistors. I don't know if there is a modern direct replacement.

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    2. 2n5087 and 2n5088 are suitable replacements and DOD used them later themselves.

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  10. I'd appreciate a copy of the PDS8000 schematic if you can send it to FPIP@outlook.com

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    1. It's linked in the recent PDS 8000 article.

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  11. "A better approach to extending delay time is replacing DRAM with a larger IC (or multiple ICs) and adjusting the counter reset logic to address the larger memory. I have some spare DRAM for both of these pedals so I would like to try this.

    I haven't seen a schematic for the PDS 8000 (8 second delay). I would like to see how this was implemented, as it should have 4x the memory of the PDS 2000. Extending the 2000 to 8 seconds may be possible depending on how extensive the differences are.

    The best picture I can find online shows a single 18 pin (not 16 pin) IC, but the label isn't legible. Presumably it's a 1Mbit x 1 DRAM."

    Did you ever try this? Wondering as I have a PDS2000 coming in the post and whilst I'm checking it over internally - it would be possible to upgrade it to PDS8000 lengths (for fun really) if that was possible.

    Looking at your gut shots in the PDS8000 post, it definitely is an 18 pin 1mb DRAM chip so there is no chance we could use that on the PDS2000 board as it's 16 pin - could RAM stacking be an option? I realise that that may mean you may not be able to close the bottom! Was just wondering if you ever got round to trying it.

    Thanks!

    p.s. big fan of the blog, always seem to find myself back here.

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    1. *if it was possible to upgrade it to PDS8000 lengths (for fun really) with a bit of tweaking then that would be cool.

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    2. I've tried RAM stacking, and it sorta works. It does indeed double the memory length, but the loop function gets kind of wonky - when switching the loop on with stacked RAM installed (and everything else left stock) the resulting loops will always consist of only what's stored in the second 4464 or 4124, since even though both DRAM chips are being counted up in parallel, the output comes directly from only the second chip in the series.

      To get a 41024 to work in place of a 4464 or 4124, you'd need be able to address the new memory locations so you'd need to find another output from the counter to generate the new highest address bit(s) and then run a line from it to the new address pin(s), and on the output side you'd need to cascade another shift and store register to catch those new serial bits and then a longer resistor network to A-ify those new D's. Not impossible, though not ideal. Good luck.

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    3. This is the third time I'm typing this, tried to post and twice the site has returned errors. Exhausting.

      The PDS1002/2000 and PDS 8000 all use the same five chips in their counter circuits, just configured differently. Going from a 256kbit DRAM to a 1mbit DRAM means there are now 2 x 2^1 or 4 times as many address slots to control. There will be quite a few cut traces and flying leads involved, but it is theoretically possible to reconfigure the counter circuit to gain the additional address bit necessary to address the new total number of additional locations, increasing the total storage from (2^9)^2 bits to (2^10)^2 bits. It's actually pretty plainly laid-out in the schematics, all the info needed to try this is there, one must only follow the lines to figure out which connections to break and which to make, then decide how to mount the longer 1mbit DRAM physically (I propose a slightly-offset daughter card with a header to connect it to the existing DIP-16 socket, with the pinout suitably arranged)

      Monumental, but doable.

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  12. I have pds 2020, no sound out when fx on, but lights is on,can you help me?

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  13. I also have a non-functioning pds 1000. The music shop I took it to get fixed told me it would be quite the project to fix it, and that they didn't have the time or resources... Go figure, lol. I want to repair it myself but my knowledge of digital electronics is limited. Anyone have any tips for fixing one of these pds 1000's? My unit, in particular, lights up, but does not process any input from the guitar.

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  14. Help! Can you upload a few pictures of the wiring... it seems i having trouble with a ground lost issue, betwen input, output, and dry.

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  15. Can you recommend what pots to buy for a pds 2020?
    Thanks for your help.

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  16. Do you happen to have a picture of the other side of the board for the PDS1000?

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  17. Thanks for this info. If you or anyone on here has a PDS 8000 they'd like to sell, please get in touch.

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  18. I have a pile of these that all have the same problem: the trigger/infinite repeat switch doesn't work/light up (and the pedal itself will never latch into this mode). mechanically they're all sound, no bad switches, wires, etc. Ive checked the voltages on all of the ICs and they're all 5v and 9v where they should be

    something apparently fails on these units after 30+ years and I haven't figured out what. I've done a full IC replacement with all of the cmos chips, to no avail. I will attempt a full re-cap. if anyone has any insight into what dies on these units, it would be very appreciated!

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  19. Any luck figuring out the problem with the repeat switch? I have the same issue plus the opposite with the bypass (it always stays lit).

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  20. I have a PDS100. LAST TIME i tried it came on. Lights seemed to work. Sound output was very staticy and distant. Seemed to be functioning but it was hard to tell for sure, and definitely unusable. Would love to get it working. Has been in a drawer for a couple years

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