Sunday, 12 February 2017

Line 6 DL4 final fix

A short update on my DL4 that had come back from the dead but wouldn't run as a DL4.

I went through 3 replacement DRAM chips and none of them worked. With all of them I got distorted noise instead of a delay signal. Yesterday I sat down with a schematic and beeped out every pin of the DRAM to the DSP. The RAS line (pin 45) of the DSP wasn't quite soldered down, so the RAS pin at the memory was open. Some flux and a quick touch of the soldering iron and now it works with external memory and runs the DL4 firmware correctly.

New RAM chip. The flux mess was cleaned off afterwards.

The DRAM that I know worked was a Toshiba TC51V17805CFTS-60. I think any 16 Mbit DRAM with the same standard pinout will probably work here.

Sunday, 29 January 2017

Electro-Harmonix Deluxe Memory Boy

Another quick one. This is the high-end version of the Memory Boy that I've looked at before. The Deluxe adds an extra gain control, tap tempo, tap divide, a rate control for modulation, an effects loop and the ability to control rate, depth, feedback or delay time with an expression pedal.
I got this used with a fault, the seller said the delay signal was very quiet compared to the dry. I plugged it in, and sure enough the delay was kind of weak. The manual for this one says that the added Gain control goes from -6 dB to +20 dB. When I put it straight up at 12 o'clock the delay seems to match the dry signal in terms of volume. I can't find anything wrong with it so I'm just taking a look inside.

Both PCBs.

The guts are similar to the Memory Boy, 4558/LM324/TL072 opamps, SA571 opamps and of course the 4 BL3208A BBDs. There is an ATMega16 microcontroller doing the tap-tempo and probably modulation waveforms as well. There is a JTAG port but I don't know how to dump the ATMega through JTAG, and I'm fairly certain it will be read-protected anyway.

BBD board, front
BBD board, backside

The BBDs are on a daughterboard, as per the Boy. This time they are connected with header pins and sockets and held down with screws into brass inserts instead of soldered to the main board. On the backside is the same 4011 IC used as an oscillator for the BBD clocks.

Main board uncovered

Underneath the BBD board we can see an LM13700, most likely modulating the clock frequency in the same way that the Memory Boy worked. I was a little surprised as I thought the ATMega would generate all the clock signals, it must just generate control voltages to the LM13700 to adjust the clock rate.

One nice feature is that the full IC part numbers are on the silkscreen, which should make life easier for anyone attempting to repair one.

The tap-tempo features are nice, but the rest of the pedal is essentially the same core delay as the Memory Boy. The tap-tempo does seem to allow longer delay times than the knob, around 1 second (by ear).

I'm not the biggest fan of this series, I prefer the Ibanez ES-2 for analog delays. This version is definitely an upgrade over the original if you can afford the real estate.

Sunday, 22 January 2017

Line 6 DL4 rebuild

This is the first DL4 I ever got, bought broken almost 5 years ago. Despite working on DL4s for other people and my own MM4 repair I have never been able to fix this one. It has always been something I had lying around and would take out for an hour here and there whenever I had an idea.

Board as I had been storing it. The taped on diode was a part I had replaced.

In retrospect this was a bad example to try to start with. Most of these 4x4 pedals have power supply issues or have a bad IC somewhere and can be debugged without too much hassle. With this one, I replaced the some parts in the power supply back in 2012 and got the right voltages, but it would never start up. Very occasionally the LEDs would flash on and then it would freeze or die again. Looking at the test points didn't help, it would show a master clock but no other clock signals. The MCU would usually reset correctly on power up, but would then assert the reset line of the DSP and never release it. The only good model for what was going on was that multiple ICs had some kind of damage, maybe from a severe power supply failure.

Somewhere in halfway through the process of replacing all ICs.
I had tried swapping some parts with the spares PCB I had (which is now almost completely depopulated) without any luck. I'm not clear on the exact timeline of work as things were quite spread out, but I know that after replacing an MCU on one of the other DL4s I bought a box of new ICs and replaced the one on this pedal, which didn't help. At some point I decided to order all new ICs, and I replaced everything except the DSP (DSPB56364AF100) and the Cirrus audio codec (both end-of-life and hard to find new) - no change. After swapping a DSP56k part in the Whammy pedal I built up some confidence and decided to see if I could get a replacement part for the DL4.


I got one from a Chinese vendor on eBay, knowing that it could be a fake part. It turned up a month or so later and I put it aside. I dug out this pedal recently and tried swapping it out. The old part came off easily but I had some trouble aligning the replacement QFP, in the end I just cheated by drowning the pins in tacky flux and reflowing the board with hot air and it soldered perfectly.

It was still dead, and now the 3.3V supply was being pulled low. It looked like I had a solder bridge somewhere on the DSP, or it was a remarked/fake/dead part with a short from 3.3V to ground. My method for finding shorts to ground on a board has always been to guess where the short might be and remove parts until it's gone. I had read about a technique for finding shorts where a constant current is fed into the power rails and a DMM on millivolt range is used to measure the voltage drop across parts that connect to power and ground. The part with the lowest voltage drop is the short - this is basically a way to make low resistance measurements with a higher sensitivity than most ohmmeters can manage. I fed in 400mA to the 3.3V rail and measured all the 3.3V coupling caps. Voltage drop was lowest near the DSP (shit) but of the 4 power decoupling caps around the DSP, C39 had the lowest drop. I removed C39 and the pedal powered up, current consumption was back to normal. I don't know if C39 was bad (it didn't measure as a short outside of the circuit) or if it had a small solder bridge I couldn't see, but I was pretty pleased with this and will use this method in future.
Coolaudio parts: V1000 & V4220M. Later removed the adapter boards.

At this point the 4 LEDs would just flash on and off, but this isn't too surprising as I had removed the audio codec and the DRAM chips. I used the modulation EEPROM that came with my MM4, as I knew that program doesn't need DRAM. The Cirrus CS4223 codec is harder to source, but I happened to have some Coolaudio V4220M chips that I bought to try with their V1000 reverb chip. This looks like a second source/clone of the Cirrus chip, so I installed one. This gave me a working pedal! I tried the DL4 EEPROM that came with this pedal and it was completely dead. I moved it to my MM4 (which has been converted to a DL4) and it didn't work there either. I reinstalled the original DRAM IC and tried the MM4's known-good delay EEPROM and only got heavily distorted noise. This seems to confirm that all of the ICs that came with this DL4 are damaged.

I now have an MM4 that acts as a DL4, and a DL4 that only works as an MM4. With Behringer/Coolaudio parts in it.

I have ordered some DRAM chips that I think will work, so hopefully this can also be used as a DL4 again.


The parts were not too expensive (€20-30) but I sunk a lot of time into this. It's satisfying as a personal victory after having this thing around for so long but if I got one in this state as a repair job I would probably turn it down.

Lessons learned:
- a complete digital rebuild is totally possible
- it's probably not worth the cost of parts and time
- all program information is stored in the EEPROM, no ICs need to be programmed or configured
- DSPB56364AF100 are available from eBay seller "e-best_trade". Unfortunately, none are listed right now
- Coolaudio V4220M works as a replacement for Cirrus CS4223
- short finding with a constant current source and sensitive voltmeter can work really well

Saturday, 31 December 2016

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.

Sunday, 13 November 2016

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.

Saturday, 15 October 2016

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.

Sunday, 9 October 2016

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.