Thursday, 1 March 2018

Voodoo Labs Amp Selector

 This is a 2 input, 4 output amp selector and A/B switch. One guitar can drive 4 isolated outputs, all with their own volume controls and ground lifts. There is some preset save/recall functionality, so I am expecting a microcontroller in here somewhere. This is exactly the sort of thing I would have loved to play with when I was renting a practice room and had access to lots of amps, but it will probably just be sold on nowadays.


Main PCB
No surprises on the insides - there is an P87LPC764 MCU reading switches and turning JFET switches on and off. A MAX1044 provides a -9V supply. The transformers are 6 pin devices with any labels or markings removed. There is one OPA2134 dual opamp, and a 24C01 serial EEPROM for presets.

Hacked in parts at the power input.
The back of the PCB has some factory bodges/mods at the power input - there is a series resistor and a capacitor added to the MAX1044 input.

The digital side seemed to work, everything lit up and it did switch signals, but there was lots of bleed between outputs and the volume was quite weak.

I know from experience that a MAX1044 will only tolerate ~10V input before dying, so I looked there first. The inverted voltage was measuring less than -1V. I desoldered the MAX1044 and replaced it - no change. There is a zener diode at the input pin which should limit the input voltage, I lifted this part out and now I had -8V and a working pedal.

Replaced MAX1044 (pictured here sitting crooked in a socket) and lifted diode D10
This diode was a 1n5239b (9.1V 0.5W zener) so I replaced it with a 1W part I had at hand. Now the negative voltage was pulled low again! It turned out that the cap across the zener that was added to the packside was leaking. It still measured 2.2uF but had ESR >5 ohm - this looks like a dipped tantulum part. I replaced with a similar electrolytic. It's not even technically needed, the cap is just stabilising the 9.1V zener clamp when it's conducting.

TC Electronics Nova Drive NDR-1

Programmable overdrive & distortion from TC Electronics. All overdrive/distortion parameters are controlled digitally, so presets can be saved and recalled, the order of the two circuits can be changed, they can be run in series or in parallel. All of this can be triggered by MIDI.


 TC did a Nova series of pedals and multi-effects units, but I don't think they were all built around a common digital design (like the Line 6 4x4 series). TC stuff is interesting as they have a history of high quality and targeting high-end markets and they tend to use fairly modern designs and parts. Unfortunately they don't seem to release much information or contract out any repair services, so I have not come across any modern TC Electronics schematics, either official or leaked.
TC were sold to Behringer in 2015 and their latest range of pedals look to be more standard stuff, a bit watered-down compared to the Nova series. I'll reserve judgement until I see one.
This series used a 12V power supply for some reason (why not 9V, like everything else?)
This Nova Drive started up but only passed a signal in bypass mode. The LED display and indicators did react to the knobs and to button presses, which at least meant that the digital section is working and the problem was likely in the analog end.

Top PCB (digital)
The build quality is really nice, they use a folded and tapped sheet metal chassis instead of cast box. There are two PCBs inside, the first is a "mezzanine" board with the encoders, buttons and displays. There is an Atmel ATmega168 and some '595 and '165 shift registers for digital IO. I'm pretty sure the ATmega is reading out pots and buttons and controlling something on another PCB to vary the effect parameters, as well as driving the LED displays and responding to MIDI commands. I would guess that this top PCB is probably very similar across all the Nova pedals. I am assuming the ATmega has some code-protection so I haven't tried desoldering and dumping it.

Lower PCB (analog)

The bottom PCB fills the whole enclosure - it has a bunch of opamps (presumably the analog overdrive and distortion circuits), DG441D quad analog switch ICs, switching power supplies, the MIDI optoisolator, a relay and a Cirrus CS3308. The CS3308 is a cool part, it's an 8-channel digitally programmable volume control. Presumably each channel is mapped to one encoder (4 knobs for overdrive and 4 for distortion) and the ATmega sets the levels over SPI or I2C. TC have used the +-5V version with 123 dB of dynamic range, which is kind of ludicrous for a distortion pedal.

Lower PCB, backside

U3 generates -12V from 12V input

All parts on this side are glued in placed before soldering

The bottom PCB is very parts-dense, but helpfully there are test points for all supply voltages. I found that -12V was reading very low and so all of the negative supplies that are derived from -12V were also missing. -12V appeared to come from a switching supply controlled by a CS51411 on the underside of the PCB, the circuit looks liked a close match to the inverting converter in the datasheet (Fig 25). The inductor was getting burning hot, so the switching IC was a pretty likely culprit. TC used a double sided load for this PCB, and parts on the underside are wave-soldered instead of reflowed, which means they are all held in place with a dot of red epoxy. Desoldering the IC took a lot more heat and force than I'm used to but it did eventually lift off. It still didn't work with a new chip, solid 12V DC across the inductor with no switching happening, which explains the heating. The SYNC pin did have a ~340 kHz square wave - I don't know if this was also present on the original chip. The huge number of test points on the back of this larger PCB suggests TC use some kind of bed-of-nails jig for testing during manufacturing.

Around this time I held my hand over the board when powered up and found another hot spot. One of the two DG441D switch ICs was also running hot, so I desoldered it. This part does run off +12V and -12V rails, so it may have killed the -12V supply when it failed.

There is another switching power supply using a L5970D controller - this one is generating 3.3V for the digital parts and was working correctly.

As I knew there may have been a short from -12V to ground or some other supply, I wanted to see if I could completely isolate the CS51411 from the -12V supply and use an external power supply to provide -12V. I removed the output capacitor and the inductor. Using an bench supply wired to the board and providing -12V, the pedal worked. Surprisingly, it pulled over 100 mA which seemed very high for some opamps and switch ICs so there may have been some other damaged part on that -12V rail.

The series/parallel switching did not work, only series mode passed a wet signal. This confirms the DG441Ds are routing the signals into series or parallel combinations, one of them was still missing. A new IC here restored all modes.

The CS51411's inductor measured 33uH out of circuit which seemed correct. I decided to try swapping it with this part, which fixed all the power supply issues. I am guessing that the old inductor got so hot that it reached it's curie temperature and it's magnetic properties changed (??) The new part runs cool. Looking again at the CS3308 datasheet shows it pulls 36 - 50 mA on both positive and negative analog supplies (!), which explains the high power consumption seen earlier, and why the pedal originally shipped with a 12V supply rated for 400 mA. This chip does run noticeably warm, but the power consumption matches the datasheet figures. I can only guess that power consumption is targeted  high to help lower noise and increase dynamic range. For a distortion pedal a low-power version with poorer specs would probably be a much better fit. The pedal will actually work with 9V input, and will generate a -9V supply instead, but will need a beefier supply than is usually seen on pedalboards.

All good again.
I broke the ribbon cable connecting the two boards when I was working on this. I just replaced it with individual wires - I would really like a tool that strips ribbon cable so I could just buy a small reel, if anyone knows of one please let me know.

Sunday, 18 February 2018

Electro-Harmonix Pitchfork

I don't know if the PitchFork exists alongside the POG series for market segmentation reasons or as a direct competitor to the Whammy pedal. Whatever the reason, EHX have another polyphonic pitch-shifter/harmonizer with a slightly different feature set, and strangely, a lower price. This one will add a harmonized voice at a selectable interval, at a higher or lower pitch or both. You also get a clean blend. There is an expression pedal control and the footswitch can be set to work in a momentary mode, which suggests Whammy style punch-ins, but it does POG style octaves as well.



The insides are very similar to the newer POG pedals, with an Analog Device Blackfin DSP (ADSP-BF592) and a AKM AK4552 24-bit/92 kHz ADC/DAC. There is also a 25L1005 serial flash with the program code - I did end up desoldering and dumping this in case I come across another and need it.

Picture taken after repair - clean.


Taken before repair - PCB appears cloudy

This one would not show any signs of life, and turned out to be shorting out my power supply. The reverse polarity diode (D2) on the back of the DC jack measured as a short circuit, so I removed it and the pedal worked! For about 10 or 15 minutes. Something else between 9V and ground was shorting.

11-detent "Mode" pot showing some stains
After a little while spent probing around and occasionally getting a short burst of life followed by nothing, I noticed that one pot had some corrosion on the back sides. I desoldered it and wire-brushed it until it was clean but couldn't figure out how this would be the cause of the problem. The corrosion was probably from some liquid spilled into the pedal.

I could see some kind of dirt at the power jack, so I removed it and things were then pretty obvious, some liquid had been trapped between the jack and PCB and had corroded the board, causing intermittent shorts.

Original DC jack

..and underneath the jack
Some scrubbing with PCB cleaner and a new DC jack later and it is rock-solid. The rest of the board was cleaned as well, what looked like cloudy flux residue was probably stains from dried liquid.

This is a pretty clear case, something was spilled and the pedal was never taken apart and cleaned afterwards. If it had been looked after at the time then no parts would have needed replacement. Underneath DC jacks and instrument jacks are probably the worse places for this, as they can trap liquid easily.

Thursday, 1 February 2018

EBS ValveDrive

This is another 2017 leftover, I want to write it down before I forget it.

EBS's ValveDrive is a valve overdrive/preamp that uses a single 12AX7 dual triode. This is the original version, there is a newer model that adds a balanced XLR output and has an internal switching power supply.
 The "vintage" circuit is similar to various Fender Baseman preamps, the "modern" setting adds some diode clipping - the original schematic is available at Freestompboxes.org. It uses a 12 VAC power supply and has an internal 12 VAC transformer wired "backwards" to supply the high voltage for 12AX7 plate. This used to be a very popular pedal with bass players, especially as the high voltage supply means it can get really loud and will happily drive a standalone power amp. Maybe it is not as fashionable right now, I don't see them as often as ~5 years ago.

Like nearly everything that comes across my bench, it doesn't work.


After verifying that my 12 VAC supply was working, I removed the top shield and looked at the valve. No wonder it does nothing, the vacuum has escaped from this one.

I put in a Jet City RetroValve that I keep around for testing, and still nothing. After pulling the valve and probing the socket I measured 0V at the plates - something wrong with the high voltage supply. The heaters are getting 12VDC, so low voltage power should be OK.

A leak has developed somewhere...

The build quality is very good, a nice folded steel enclosure and multiple PCBs connected together with ribbon cable (switching signals) and runs of coax for the audio signals. This was probably expensive to manufacture but it's hard to kill and convenient to work on.


The high voltage transformer is on the lower left of the topside of the PCB. The surface mount fuse on the backside is not blown, but the transformer looks weird, as if it had been re-soldered or replaced. The output winding only measured around 0.5 VAC with the power supply connected.

Transformer pins are not coming through the plated holes...

Additional holes drilled in PCB...
Once it was desoldered, it was obvious why things looked strange. The transformer pins were not going through the plated through-holes, instead someone had drilled holes in the PCB (!) and bridged over to the pads with some bus-bar to make the transformer fit. The low voltage measured at the transformer made sense now, scaling 12 VAC to 230 VAC is a roughly 19.16 ratio. If the transformer was scaling 12VAC down by the same ratio I should get 0.6 V AC, which is pretty much what I had measured.

I reinstalled the transformer the other way round and now measured 230V AC, and 400+V DC at the valve plate with no valve installed. With a new valve installed, this dropped to around 350V, and everything worked and sounded great. The original filter capacitors were only rated to 400V, so I replaced this with 450V rated Nichicons in case this was ever left running with a dead or missing valve.

The part number on this transformer is slightly different from the schematic's part list, and obviously the footprint is different as well. My guess is that someone attempted to replace the transformer, installed it backwards, and then gave up and sold the pedal on. The slightly higher plate voltage(~350V vs 300V) is probably due to small differences between transformers.

Wednesday, 31 January 2018

Tech 21 XXL

I picked up this 90s Tech 21 XXL as I had read that it was a "sleeper", a nice distortion but not particularly well known. I knew that the design was based on opamp clipping with no diode clipping. I have never built or (to my knowledge) even played anything like that, so I was curious and also pretty confident that this would be simple to repair. This one would power up and would pass a bypass signal but the effected signal was very quiet - volume and gain had to be cranked all the way to hear anything. The tone knob sounded like it was working correctly. A bad opamp or switching FET maybe.


The insides were a bit of a surprise. All the jacks and pots are board-mounted, but it looks like it was assembled using the least amount of solder possible. A lot of the through holes are not completely filled. Re-soldering all these joints didn't help.

Dull joints, very little solder

This construction style is very annoying to work with. The input/output jacks do not butt up flush against the sides of the enclosure like with many other common designs, instead they extend through the sides. This means that the whole pedal has to be basically assembled inside the enclosure, the jacks and pots are installed and the PCB is aligned with all of their pins and then soldered in place. This seems very labour-intensive and makes any kind of repair a real pain.

WTF

After desoldering the 1/4" jacks and DC power connector I managed to get the PCB out and found another surprise on the other side. Tech 21 have used a SMD design (fine) but have covered it with a block of potting compound, presumably to prevent clones of the distortion circuit. Unfortunately this makes repair much more difficult as well.

The switching FETs actually seemed to be working correctly, so I decided to see if I could remove the potting (there are methods for this written up online, often described as "de-gooping"). I guessed that potting material for surface mount parts are probably softer than old-fashioned epoxies to prevent them from cracking solder joints as the compound cures. I hoped that heating with hot air and picking with a tweezers might be hood enough.

Potting removed, plus some parts

De-gooping went fairly well, I started at 100C and the compound cracked and separated from the board in large pieces. I increased the temperature as I went but eventually went too far (I think above 250C) and re-flowed some solder joints - this meant an entire chunk pulled away from the board, holding two opamps and some passives with it. The resistors and caps were easy enough to separate and re-populate, the opamps were replaced with new parts based on a schematic I found.

Restored jumper. Cut trace is visible.

Another surprise, my pedal is a different revision from the one photographed on Diystompboxes. It has a cut trace and a jumper wire embedded inside the epoxy block. Is this a genuine correction, or something designed to make reverse-engineering more difficult?

The pedal still didn't work with new opamps, but now that I could probe them I could see one was not getting any bias voltage on one of the inputs. The Warp control which sets the DC bias of the first opamp was not connected to +9V, I found a trace to the pot that was open circuit, possible from a scratch from a nearby electrolytic cap. A jumper wire fixed this.

Restoring +9V to Warp pot.

Re-assembling was another pain in the ass, I broke the DC jack and had to order a replacement with PCB pins instead of solder lugs. Getting the board into the enclosure and aligned with the jacks so they can be soldered is fiddly, in retrospect I should have tried to replace the jacks with the more common units that have a parts that threads from the outside of the enclosure, like modern EHX stuff.

The XXL sounds quite good - less compressed (and less sustain) than what I would usually expect from a diode-clipping distortion like a RAT. The Warp control doesn't seem to do much throughout a lot of it's range. It also does very little with a weak input signal (especially single coil pickups), probably because the first opamp stage isn't driven into clipping. With a loud signal generator I could different hear distortion flavours, presumably the clipping gets more or less symmetrical through the pot travel. Online reviews and clips confirm this, so I'm confident this is working as expected.

I like this pedal, but I'm a bit soured by the weird construction and the complete lack of giving-a-shit towards repairs. Uncovering the circuit took less than an hour, so it's not much of a deterrent to anyone who wants to make a clone, but it's definitely a hurdle for finding what has wrong. I'm not sure if I think it's worth looking at more Tech 21 stuff.

Sunday, 19 November 2017

Eurotec Black Box Faze Module

Eurotec was a brand run by Sola/Colorsound in the late 70s. They released a line of pedals called the "Black Box" series which seemed to be re-issues of their earlier products as well as some clones of contemporary designs. The pedals could be used individually with battery power, or plugged into a "Black Box Module Energiser" base station that could power and route signals to four modules. I don't know if this modular system was successful but it's interesting to see an earlier commercial stab at a pedalboard concept.

Image from https://www.tonehome.de/eurotec/


I have the phaser of the series, and it doesn't pass an effected signal. There are no schematics for this series that I could find online, but 4 JFETs and 4 opamps nearby give the game away - this is most likely a Phase 90 clone. The "React" knob seems to replace the mixing resistors for the clean and wet signals on the original so the phasing effect can be adjusted in intensity.

Build quality is a little cheap, but not unusual for the period - a single sided board that looks like a phenolic substrate. Traces will probably lift if overheated. The footswitch is unlike any I've seen before, it has a threaded black plastic cover to anchor itself to the enclosure. The enclosure is more like a sheet metal can than the usual diecast aluminium.


I went through this expecting to find some dead capacitors, but everything measured OK out of circuit. I did find one capacitor lead that had either broken loose from a cold solder joint or was never soldered in at the factory.


Feeding a sine wave into the pedal and probing the opamps showed that all had phase-shifted signals on the outputs except one - replacing this opamp restored the effected signal.


It really sounds great - the "react" control will dial the phasing all the way to a very resonant "thumping" sound - it can get much more extreme than the MXR originals.

Adding external power:

Despite having an LED, there is no indication that the pedal is active when pushing the footswitch.

I found it interesting that the output jack on these boards are stereo, and the ring contact is tied to a diode that switches out the battery clip. Pictures of the original base station seem to show stereo TRS jacks, so it's possible that they supplied power through the additional conductor.

Full modular system - image from https://www.tonehome.de/eurotec/

I wired up a a mono 1/8" jack to a stereo 1/8" connector and added a 9V DC connector to the ring. When plugged into the output jack, the output signal is passed through and 9V power is used instead of the battery. As a bonus, the LED works as well.



This seems like a worthwhile cable if you don't want to modify the enclosure.

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.