Electro-Harmonix Doctor Q

I have realised that since I started this blog I have managed to post every month, except for December. I want to keep going at this, so here's a quick cheater before the finish line.

I got this vintage EHX Doctor Q for fairly cheap, missing a battery door and "not working". The seller described some torn wiring inside. I thought this would be simple.

Graphics are actually quite clean for a pedal of this age.

I tried it out with a 1/8" power adapter in the DC jack and it worked just fine. It sounds pretty good too, and worked with weak single coils and humbuckers without having a sensitivity control. Range controls the width of the filer sweep, it has a fairly sensible & usable amount of variation.

Single sided PCB.

Opening it up, and the battery snap has broken off the power jack. Quick soldering job to restore it, and it works with a battery as well. It looks like EHX were not using the extra contact on the jack to switch the battery. The pot code is a CTS with a 1978 year of manufacture, so the pedal is probably that old as well.

Unfortunately it's hard to get the board out to take pictures as I don't have tools that can easily remove the round knurled nuts that EHX were using in this period. In the interest of not scratching things up I've left them along. The circuit is fairly simple, schematic is below.

It's pretty clear what happened to this one. With a missing battery door the 9V battery can just dangle out the hole in the chassis. The battery snap probably broke off when moving the pedal around. Sometimes I'm lucky.

I have a fairly complicated envelope filter build (maybe more on that in the future) so this one is surplus to requirements. Get in touch if you are interested in buying.

Electro-Harmonix Graphic Equalizer

Another EHX big box pedal, a mains powered 10 band graphic eq. I'm guessing this one is a late 70s/early 80s vintage. Google finds at least two versions, this one and a more modern looking black-and-yellow version with a power switch and LED/lamp. Oddly enough, none have a knob in the lower left corner instead of a stomp switch, so this may be a less common variant or someone's science project. It doesn't work - moving the sliders just brings in some noise, the knob in the corners seems to attenuate this.

A few things are apparent once the lid is off. The "master volume" is not stock, it's someone's mod in place of a bypass switch. The jacks are probably replacements, as EHX didn't use those isolated plastic jacks. There is a third jack that has been added, probably for a remote footswitch for bypass. The PCB is also broken! It's held in place to stand-offs with screws and washers and one corner has broken off. Some of the plastic sliders of the potentiometers are sheared off, it's clear that someone stamped over the sliders hard and managed to break the PCB as well. There is a pair of washers and a screw holding the board in that corner, and it actually seems stable mechanically.

A schematic was easy enough to find, the model name is the same as on the PCB art and the circuit seems to match. It's 9 notch filters (using single opamp gyrators) in parallel with different centre frequencies plus a passive 16 kHz filter. This seems to be a common design for guitar EQ. Power supplies are approximately +-13.5V using a centre tapped transformer and zener regulators.

Schematic found online

I took the board out of the box to take a lot at the pots. Date codes are from 1979. Some black foam was attached at some point, probably to keep dirt out, and over the decades it has perished and turned to sticky fluffs of black shite that are now all over the PCB and inside the pots. I cleaned these with contact cleaner and cotton buds until the pots slid smoothly. Also washed the enclosure while I had it gutted, removed the plastic jacks, installed some new metal open-frame 1/4" jacks and pulled out the volume knob and wiring.

Some of the pots have broken sliders, so it's possible that they have broken tracks or the wipers have disconnected from the tracks. I tried to measure the resistance of the left-most pot from the backside and it didn't seem to reading correctly. All the pots extremes should be wired together in parallel, and it turned out that left hand one had no continuity to it's neighbour. I tried to desolder it to measure out of circuit, and the pad and track lifted right away. The pad on the next pot also started to lift as soon as it was heated. Usually I don't see any lifted traces or pads when desoldering. It turned out that these tracks were physically broken, when the PCB corner was broken the pots must have been pulled away from the board. The left-side sliders were completely disconnected, the ones on the far right were fine and the middle were a bit flaky. I ran a piece of bus wire across the top pads to connect them, much like how the pot cases are grounded together.

Cracked pads

Pots jumpered with bus bar

At this point I clipped a test signal to the board and fired it up. Power supplies were OK. The signal didn't get any further the first opamp stage, so after checking the feedback network looked good I replaced it with a new 4558 opamp. Everything works afterward, all bands boost and cut and none have any noise or crackling.

I added a new switch and wired for true-bypass, and then managed to confuse myself when I saw a huge 50 Hz hum on the output. I thought the power supplies had gone bad and replaced the filter caps with new Nichicons. It turned out that although the enclosure is grounded, and the PCB is bolted to the enclosure, the PCB is not grounded through these bolts. Running a wire from the input jack fixed this up. At least I know this won't need new filter caps any time soon.

The extra hole in the enclosure is a decent fit for a large LED bezel and some spare hardware, I've added this as a bypass indicator.

Repaired and cleaned up.

It sounds exactly like a 10-band EQ sounds. The +-13.5V supplies mean that this has huge headroom, and it can be used to filter high amplitude signals without distorting as soon as a 9V or 18V unit.

Electro-Harmonix Deluxe Electric Mistress

I really like these big box EHX pedals. I understand why trends have moved away from huge pedals but I love the look of them even if they are unpractical.

This is a 90s Deluxe Electric Mistress reissue (EC 1000 Rev B) that I was asked to look at. It passed a clean signal, but no effect. You could sort of hear something happening when power was cut to the pedal but it was very weak. This seemed like a positive, it meant the BBD was still able to do something.

Going through this pedal was pretty easy thanks to Ralf Metzger's fantastic Electric Mistress site (presumably inspired by Kit Rae's Big Muff shrine). Ralf has info on identifying different versions & reissues, theory of operation, full schematic and repair & aligning instructions. This version (V4) has a Reticon RD5106A BBD and 24V DC input. Oddly enough there is no bypass indicator LED, this is usually a good way to tell old EH pedals from the reissues.

Walking through the alignment procedure for the V4 I found that I couldn't adjust the clock signal to get as low as the specified 40 kHz. I had some clock signals at the BBD but I couldn't see any modulation on them in any switch or knob settings (the modulation of BBD delay time produces the flanging effect). After buzzing out the circuit it turned out that there was no continuity across any of the terminals of the Filter Matrix switch. As far as I can tell the switch wafer was removed or disappeared somewhere, sliding the switch didn't bring anything in contact with the terminals. I wired in a spare DPDT switch in parallel and had a working flanger.

Temporary eplacement switch added across the original.

I did a quick alignment following Ralf's guide to correct any adjustments I made when it was working incorrectly. The only diversion was setting feedback, I always feel that any pedal with a feedback control should be just about oscillating at the extreme end. It will get a replacement slide switch, they are easy to source but I didn't have one at hand. The switch doesn't effect calibration so it can be swapped out later.

This is the first through-hole pedal I've written up here. I suppose not having any SMD parts makes it the least false.

Cheers to Ralf.

Klon KTR

Another quick one. Another pedal that's a lot fancier than I normally get to play with. This is the reissue/sequel Klon that "inspired" the EHX Soul Food and dozens of other klones.

Not of Bill's making, but it probably helps.

The build quality is pretty high, everything is board mounted and the footswitch is on it's own small board with a ribbon cable and snap-in connectors providing strain relief. The only wires are for the battery snap. Despite the "essential" diodes, there are no exotic or outrageously expensive parts. It looks like surface mount film counts are used throughout.

 

The circuit seems to be similar to Centaur traces found online, and pretty close to the Soul Food except for the magic diodes. The 7660 doing voltage doubling and inversion seems to add a lot more ultrasonic switching noise (~26-27 kHz) to the power rails than I remember seeing in the Soul Food.

This one had a bad TL072 opamp (U2). I would guess that someone used a power supply above 9V and the charge pump supplied a voltage above +18V (the upper limit for the TL072) and popped it. The pedal works after replacing it, so an easy enough fix once the problem had been found (I just guessed when everything else around the opamp measured OK). I forgot to take many pictures of this one, the noisy one above is an "after" shot, you can tell the alignment of U2 is a little off.

Side-by-side it's very close to the Soul Food in sound. Connecting to a signal generator and scope, both can do ridiculous 25+ Vpp output with the right input settings and signals, which should overdrive the shit out of pretty much anything.

Revisiting the Ibanez Echo Shifter

After repairing the second Echo Shifter, the PCB sat on the bench for a few days before I re-assembled it. When I did, I found that it had stopped working again. It would light up, but there was no signal in either effected or bypass modes. When I played with the controls I found I could still get it to oscillate by changing delay times with the oscillation switch on, so I knew the delay and output circuits were good.

I opened it up, connected a signal to the input jack and started tracing to see where it died. It wasn't getting past the input jack. Measuring continuity, I found that the jack tip and sleeve were shorted together. I desoldered the jack and it tested fine out of circuit. There is another choke/filter at the input jack, presumably to keep any high-frequency digital noise out of guitar cables, and it looks just like the failed one on the power input. I took it off, and the jack was no longer shorted, but no signal was getting through. Shorting across the pads as before and everything works again. I measured the part out of circuit and the pins were shorted together in a way that doesn't make sense to me at all. It seems like they fail both as open circuits and as shorts.

L6 removed and pads shorted

This is probably totally fine as a fix, and I couldn't see any clock noise or similar on the input jack. I decided to replace the jumpers with some ferrite beads to help cut any high frequency crap. 0805 parts will just about fit, the ones I had are 1 kOhm at 100 MHz.

L6 populated with ferrites

There is another one of these filters at the output jack, I replaced this part at the same time.

L7 replaced wit ferrites as well

Technically these parts have a 500 mA rating but I decided not to add them to the power supply input, as I worried about reliability.

After re-assembling and punishing the switches and jacks for a little while all seems good. This should make the ES-2 closer to bulletproof. Hopefully some more dead units can be revived as well.

Line6 DL4/MM4/FM4 firmware images

Here is my collection of firmware dumps for this series. The DL4 and MM4 are from pedals I've owned or repaired, the FM4 image I found online.

Stickers on the EEPROMs and text data in the images refer to version numbers but I've never actually seen any different revisions.


DL4 1.3:
https://drive.google.com/file/d/0ByVCt2OFhXnyVF9kZnRXTDJTNEk/view?usp=sharing&resourcekey=0-86887qF7KuqQq77nf4Vd7A

MM4 1.0:
https://drive.google.com/file/d/0ByVCt2OFhXnyNURXckFJUTlJYmc/view?usp=sharing&resourcekey=0-DM-HZXCfQB50_p-1awD_cg

FM4 1.0:
https://drive.google.com/file/d/0ByVCt2OFhXnyZUx2R0RSQTBzc00/view?usp=sharing&resourcekey=0-1ak_q2Ip8ABL4MlzcFJE2g

I am still hoping to do a mod to switch between different firmware images. I was hoping that I could find larger pin-compatible EEPROM and toggle some address pins but the larger chips seem to have different pin-outs. Making an adapter board to go in place of the socket is probably the way to go but I haven't really had the time.

Once again, if there are copies of the firmware from the AM-4 and DM-4 please get in touch.

Ibanez ES2 Echo Shifter

1 second analog delay with modulation, an oscillation switch and tap-temp from Ibanez. I really like this delay. Opinion tends to be split on these, they sound great and and have a very nice feature set but are very prone to dying. Online reviews are split between gushing praise and reports of the things crapping out:
http://www.musiciansfriend.com/amplifiers-effects/ibanez-echo-shifter-analog-delay-with-modulation-guitar-effects-pedal#reviews

https://www.amazon.com/Ibanez-ES2-Shifter-Analog-Guitar/dp/B00BFWIZGI/ref=sr_1_1?ie=UTF8&qid=1473597444&sr=8-1&keywords=ibanez+es2#customerReviews

I wanted to look at these to figure out why, I ended up with two with different faults. These seem to match the most common complaints.

Broken Sliders

This first unit was sent to me already repaired and modded. The previous owner had had someone swap out the footswitches for nicer soft-touch switches. The originals are actuators that press microswitches on the PCB. The new switches worked, but tap-tempo and bypass had been swapped functions. The slider for delay time had also been sheared off and the battery snap had been cut off.

 

The backside has some opamps (TL072) and some 74HC4040 ripple counters. I don't know what the counters are doing. The toggle switches are wired to the PCB and mounted through the PCB in rectangular cut-outs - I like this, it prevents form turning when the nut is tightened and they are strain-relieved from the rest of the board. J6 has two pins soldered together. The trimpot on the right hand side looks like a mod but may be done at the factory, it looks pretty clean.

 

The other side has two Coolaudio V3205 4096 stage BBDs, and 2 BL3102 clock drivers. The clock frequencies for the BBDs goes as low as ~3.5 kHz, which is ~500 ms for a 4096 stage delay, so if both BBDs are cascaded then a 1 second analog delay is achieved with a bandwidth in the region of 1-1.5 kHz. This is dark but still sounds very nice for long delays for guitar.

U9 is an Analog Devices ADAU1701 DSP. This device has 2 ADCs and 4 DACs but very little memory, so it seems very unlikely that it is providing any digital delay. I am guessing that this is only handling tap-tempo, bypass, generating the modulation signal and maybe doing filtering and/or compression of the delay signal before and after it hits the BBDs. U13 is 24AA128 EEPROM for the program code.Analog Devices make a visual drag-and-drop programming environment for these DSPs (SigmaStudio) but unfortunately it will not load binary ROM dumps so I can't see what exactly what this chip is doing. I can probably dump this if anyone needs it.

 

These sliders are a standard 45mm footprint.

 The delay slider is a 10K linear 45mm potentiometer, I used a Bourns part (PTA4543-2015DPB103) as a replacement.

Added a connector to footswitch wiring so I could take the board in and out of the case.

I also re-did some wiring to swap the footswitches back.

Power Failures

This seems to be the most common problem. I came across a second unit that refused to power up at all. This one had a piece of foam between the battery bracket and the back of the board, and had the original switch PCB and actuators. There was no trimpot with hot-glue this time, but otherwise identical.

L1 looking worse for wear.

 I started measuring for power at the 9V jack. No current was drawn, and L1 looked a bit dodgy. This looks like a common-mode choke on the power input, filtering noise along with C7 and C70. The top of the ceramic broke off when touched with a tweezers. I guess this part wasn't rated for the current draw of the pedal, or doesn't handle vibration or shock well.

L1 bridged.

 As this is just a choke, we can jumper straight across it and restore power to the rest of the pedal. I lifted it off with hot-air and used some bus bar to bridge the pads.

Working again, and sounds just like the other one.

Current draw is less than 60 mA on these, which is surprisingly low.

I haven't seen any problems with noise or interference since removing this choke, so I will probably go back to the first pedal and do the same as a preventative measure. It is likely that this filter is need for EMC compliance to stop power cabling radiating the ADAU clock signal. This is a simple enough repair, and only needs the back casing taken off, the board can stay in place inside the pedal.

2 More DL4s

Big Green Delays, again.

I had a couple of these Line 6 DL4s handed over for repairs and each had a different problem from what I have seen before, so I'm holding out that this will be interesting to someone.

Silver-Taped Green Delay

By now I have something of a checklist of problems for these pedals. This one had all power supplies present, and none had any strange ripple or noise superimposed. Next step is to look at the test pads - there was no clock signal anywhere. Since the MCU generates the clock, and the MCU is responsible for resetting and waking the DSP (which produces the sampling clocks) nothing will work without this master clock.

U7. Oscillator and caps swapped at this point.

I know the problem is likely to be in the area of the MCU (U7). I replaced the oscillator crystal and capacitors with parts from the scrap board to see if I got a clock signal, as that's an easy place to start. No luck. I took off the MCU with hot air and replaced it with the one from the scrapper and everything worked again. Fantastic.

After removal and wicking of U7.

Alignment of the replacement is not great, but solder joints look good and function perfectly.

These MCU are a cheap 8051 core in a "ROMless" configuration, meaning that they have no ability to be programmed and only load code from external memory. This is a Good Thing for repairs, as it means that a scrap unit isn't really necessary, any new IC of this model should work, and all the code is in the one socketed EEPROM. In practice, I have found that "ROMless" is sometimes a misnomer. MCUs often have fuses or configuration bits that need to be programmed, or even have a small area of programmable memory for a bootloader, so a straight swap isn't always possible. More on this to come.

Normally I use a Kester flux pen for surface mount work, but this was the first time I used a syringe of tacky flux paste. It is really amazing stuff and I am completely won over. It made this job much faster and cleaner (I used it again for the Whammy repair, but unfortunately I'm getting to write about these out-of-order).

The owner had cover this pedal in silver gaffer tape. I think it looks cool.

A New Power Issue

This unit didn't get as far along before something look wrong. It had a working 3.3V supply, but all other power supplies voltages were low.

Another Rev 6 board.

The 6.6V voltage doubler (MAX660, U20) follows the 3.3V supply, so I started there and removed the output cap (C48) and IC. Measuring resistance across the output cap pads (6.6V to ground) gave 29 ohms, so there is still a problem.

U20 & C48 pulled.

The 5V regulator is next (U21, LM3480) so that was pulled, and the 29 ohm to ground was gone. I pulled U21 from the scrap board and replaced it, then repopulated U20 and C48. All power supplies returned and relays clicked, worked again.

Poor focus, but U21 has been replaced.

LEDs and relays are working after C48 and U20 are back.

At this point the scrap board I have is looking pretty bare. Between the 3 pedals I have written about and another personal unit that hasn't been fixed yet I've removed pretty much every IC. This is fine though, as I found out that very little (if anything) on these pedals is pre-programmed. In nearly every case new parts will work perfectly, having a donor just means avoiding waiting a day for replacements to be delivered. This shouldn't really be surprising as these are consumer goods made in large quantities, it makes sense to keep any custom, selected or programmed parts to a minimum to keep costs down.

Digitech Whammy 4

I have a bit of a backlog of stuff I've done but not found the time to describe here. It's probably best to do so before I forget everything.

Big red pitch shifter. Another 90s digital classic. I used to own one of these these but moved it along because I didn't really get on well with no polyphony and the artifacting didn't always sound too great. I think these older Whammy pedals are pieces that need to have music written with them in mind instead of just switching them on and playing the usual stuff.

A pedal large enough that the photos look terrible

Sold as not working and had had a true bypass mod done. I was hoping that it just needed calibration or that there was a simple issue with the true bypass work. Instead it just flashed all the LEDs once when powered up and then did nothing.

Inside we have a large PCB that fills most of the enclosure. There's another Motorola 56k DSP56362 (same as some of the Line 6 and EX stuff), an Atmel 89C55 (8051 architecture), a Cirrus CS4224 Codec, the MIDI optoisolator and some CMOS logic chips and opamps. The DSP56k/8051/CS4224 seems to be a really common choice of components for late 90s/2000s digital, it's very similar to what was in the Line 6 pedals I looked at previously.

The mod work was pretty bad, the wiring used was way too thick and was under strain, there were ferrite beads cracked off the PCB and the 3PDT footswitch fell apart when I was removing the old wiring.

Main PCB and LED/encoder PCB

Atmel 8051 MCU under the sticker

This has the annoying 80s/90s practice of including an AC power supply to create positive and negative voltage supplies instead of using switching converters and running off 9V DC. There are 3 linear supplies, +5V and -5V for the analog stuff and 3.3V for digital. The 3.3V was down near 0V and the regulator looked scorched, so the heatsink was desoldered and I replaced it. No change, so there is excessive current being pulled somewhere. 3.3v to GND measured around 40 ohms. I removed any 3.3V bypass capacitors in case they were shorted, and then noticed that the main DSP chip was running pretty hot.

At this point I gave up for a little while. The DSP is a 144 pin QFP and in it can be difficult to find reliable suppliers of these 56k series parts these days. There are also no schematics for this pedal online, so I would have to trace things out or make guesses.

I tried contacting Digitech and asking about a schematic, and they replied with a PDF within an hour! Here it is https://drive.google.com/file/d/0ByVCt2OFhXnyZS1JZFlmb0Z3VTA/view?usp=sharing&resourcekey=0-vBilBFlfU0YChMUrFsavrg

I also found that this IC is actually still stocked at a lot of distributors, just not in huge numbers. I decided to get one from Farnell for around 18 euros, Digikey have them as well.

Removed the original bad DSP was difficult, as the chip is quite big and there is a large top-side ground place surrounding it that takes a lot of energy to heat up. After not much progress with hot air I flooded all the pins with some Chip-Quik low-temperature solder that I had. This kept everything melted and worked well but made a mess, there were blobs of solder all over that region of the board that had be wicked and scraped off. I realised afterwards that I should have tried using a larger air nozzle for a chip this big.

After DSP removal, there is solder everywhere

After wicking and cleaning the board

 The new chip went on easily enough with some tacky flux. The true bypass mod removes the original momentary footswitch, but this switch is still needed to calibrate the footpedal, so it is usually replaced with a pushbutton mounted to the back of the enclosure. In this case, the switch was connected to the board using the original connector and then fixed down with a big blob of hot glue (you can see this in one of the pictures above). Unfortunately, all the rework heated the board enough that this glue melted and ran all over the board, leaving a thin layer around the MCU. Now nothing happened when powering it. I could peel off  some of the larger pieces of glue but most of it needed to be scraped. At this point I gave up temporarily again.

DSP resoldered,Glue cleaned, C52 replaced, Y1 removed, cleaned and repopulated

I came back a couple of weeks later and used some IPA and cotton swabs to get off the rest of the hot glue. C52, Y1 (crystal oscillator), the momentary switch connector and some of the legs of the MCU were the worst effected, so I removed all of those except the MCU and cleaned the board as best as I could. Y1 was pretty badly coated in glue, so I cleaned it and repopulated. I tried poking around wit an oscilloscope probe and found that every time I touched one of the pads of C52 the LEDs would flash. The schematic says that this is the reset timing cap, so presumably the MCU was never being reset. I replaced it with a 1uF film cap I had on hand and the Whammy fired up. I can only guess that swapping the DSP was successful, but that the hot glue was preventing Y1 from oscillating and the MCU from getting a clock signal, or C52 was bad, or both. After redoing the true bypass wiring everything works.

There is also a 1k serial EEPROM on board, but it is connected to the DSP and not the MCU, so I think it for saving calibration settings and not the program code. I would like to dump the MCU if possible, but I'm going to leave this alone. If I come across a scrap board I'll pull the chip and try to dump it. There is also a JTAG debug port wired to the DSP, I haven't looked at that at all.

Was this worth it? Replacement parts ate up a good portion of what I would have gained from selling this, and I have more than a few hours invested in it, so probably not. The newer models add polyphony and have lowered the value on these older units too. I'll add a LED (mysteriously missing? Maybe someone killed this halfway through a mod) and sell it on. I'm glad to have swapped out a QFP144 successfully and found a schematic for this, so I'm still thinking of it as a win.

Electro-Harmonix Nano Pog

Polyphonic pitchshifter, in a small box, put into a bigger box. I forgot I did this one. I took a look at this around November 2015 for Moose Electronics who was doing a custom re-house with mods for someone. The pedal had stopped working somewhere between coming out of the original enclosure and going into the larger one he had made for it. I found some pictures on my camera and I actually took some notes, so the details shouldn't be completely hazy.

Work in progress pedal. Wiring was temporary.

ADSP-BF592

Surprisingly, EHX seem to have several different digital designs on the go, using pretty different architectures. They even have multiple polyphonic pitchshifters on the market, with the POG series competing with their Pitchfork. I guess they may be doing some market segmentation and charging more for the established Pog brand. I have also heard that they may be using different designers for different products, which might explain why they use different processors in some products instead of a single unified architecture.

The insides are typical modern EHX, board-mounted jacks, power connector and pots, with a soft-touch momentary switch for bypass. Bypassing is done with a DG419 analog switch, which is a little unusual as they normally use a 3DPT mechanical true bypass (possibly why this was rehoused?) For anyone wanting to convert these, you can short the switch pads and the pedal will always start up in effected mode and stay there. There is a huge programming/debug connector in the top-right.

The Nano Pog has an Analog Devices Blackfin DSP (ADSP-BF592 http://www.analog.com/en/products/processors-dsp/blackfin/adsp-bf592.html#product-overview) doing nearly all of the work. This is a more modern and significantly more powerful part than I usually see, most digital pedals I've looked at recently have used the older Motorala/NXP DSP56k series. The part will run at 200 MHz but I didn't think to look at any clocks on the board (I also didn't have an oscilloscope with any hope of seeing 200MHz signals at the time). AKM AK4552 24bit/96kHz codec is handling conversions.

A TI TPS61620 (http://www.ti.com/product/TPS62120) forms a step-down regulator to get a 3.3V supply from the 9V input. TPS60403 (http://www.ti.com/product/TPS60403) is using as in inverting charge pump in a neat SOT23-5 package. Apart from some opamps (TLC2272C) and an EEPROM that's pretty much it.

This thing had a pretty awful sounding distortion that farted out badly on low notes. The effect could be heard and it tracked correctly, so the digital guts were working correctly. I usually check all for power voltages first, wherever is immediately convenient. The DSP was getting 3.3V. I know what to expect with the opamps, looking there found 9V at V+ and something like 0.5V at V-. Poking about the board found that the charge pump had the same mystery voltage on it's output. The ceramic output cap (C39) is right at the edge of the board and looked like it had been cracked, maybe from taking it out of a tight-fitting enclosure. Not the best layout decision but it probably doesn't matter as most people won't be taking these apart. The datasheet (http://www.ti.com/lit/gpn/tps60403) recommends a 1uF output capacitor, and I had a sample kit of 0805 ceramics, so I tried replacing it. I now had -3.3V at the opamp negative supply pins. I don't usually see +9V and -3.3V supplies like this in pedals. Opamps will work fine with asymmetrical supplies, their outputs will just be limited by them in the usual ways. Pitchshifting should have unity gain, so there's no need for huge output drive. Low voltage charge pumps are easier to find these days too, most won't handle a 9V input. I just don't know why this was chosen over the usual virtual ground/split supply system, as that would allow similar headroom and would eliminate an IC.

Anyway, this was a quick enough fix and it sounded clean afterwards. The polyphonic tracking is cool and it doesn't have much of the shrill digital artifacts you can hear on the octave-up settings on some pitchshifters. I only played with this briefly before handing it back, and although I liked it it didn't exactly inspire many applications. I might need more time with these.

Finished rehouse.

The final version came out great, I think additional A/B outputs and some momentary switching were the custom options.  https://www.facebook.com/mooseelectronicsdublin/

Electro-Harmonix Soul Food

I don't think I need to talk too much about this one, there's a lot of info online already (not enough pictures though. Photograph your stuff). This is EHX's budget version of the Klon Centaur, apparently with very few changes from the original.

This one arrived dead, nothing happens when power is applied. Probing around with a 9V supply connected doesn't find voltage anywhere. I started at the DC jack, and found that there was no continuity between the tip and the PCB. After desoldering, the DC jack still had an open circuit internally. This was probably mechanically damaged.

The 3PDT footswitch board has the True Bypass/Buffered Bypass switch of the original Klon and KTR.

D2 & D3 do not look like the "magic" Klon diodes. Internet consensus says they are Schottkys.

With a new jack installed it still had no power to the ICs. Diode D4 is connected across 9V to ground as reverse polarity protection, but as there is no series resistor D4 had become shorted, presumably by an incorrect power supply. I pulled it and replaced with 1n4007 as I have a lot of those. Works as expected now.

I would imagine there were two incidents that caused it to end up like this. Someone used a power supply of the wrong polarity and burned out D4, and someone broke the jack by forcing in the wrong connector or stepping on the power cable while it was plugged in.

For anyone else working on these modern EHX pedals, I have found that plastic spudger tools that are sold for opening electronics (particularly phones) work quite well for pushing on the DC jacks to remove the PCB from the enclosure, and don't leave any scratches.