Another Ibanez ES2 Echo Shifter repair

I took another look at an Ibanez Echo Shifter that I had previously repaired, but had come back to me. This time it wouldn't light up, but did pass a clean signal. I suspected a bad power supply for the digital section.

I opened it up, and I could see ~8.5V volts powering the opamps but no voltage at any of the digital parts, where I would expect 3.3V.

Way back in July 2017 a commenter asked if I knew what the part number for U14 was, likely to be a buck-converter, and that they suspected it was a Texas Instruments TPS62056. The package, function and chips marking all looked like a perfect match so I ordered a few this week to see if it was right.

Before

Desoldering was straight-forward, I used hot air and kapton for protecting parts I didn't want to overheat.

During

The new chip has near identical markings to the old one. Re-soldering was a little harder as there isn't a lot of space, I ended up removing C73 temporarily to get better access. The bridged pins are fine, all those pins are connected together at the PCB.

After

Everything works again with the new chip. Thanks to Shane Bussiere for doing the research and sharing the part number, sorry I didn't help out.

All working again.

Looking at the TPS62056 datasheet, I can guess why this failed. The buck converter chip has a maximum Vin of 10V, the Echo Shifter runs the power jack through a series Schottky diode for polarity protection and then to the TPS. If you use a 9V power supply the chip gets around 8.6V, which is fine, but using a 12V power supply or higher will probably kill it. I couldn't find a compatible chip from TI with the same footprint and pinout but higher maximum input voltage, let me know if one exists.

I would really like to modify this to add a modulation rate control. I hunted around for modulation signals and unfortunately there doesn't seem to be a LFO onboard, it looks like the modulation is done in software in the ADAU1701 DSP. A modulated square wave is run out of the first audio DAC on pin 46.

While I had this open I desoldered the 24AA128 serial EEPROM and dumped the contents, it can be downloaded here. Afaik the ADAU1701 instruction set is not publicly documented, so I don't think the firmware can be easily modified.

2 70s Electro-Harmonic Small Stone phasers

2 vintage Electro-Harmonix Small stone phasers. The artwork for both of these is almost identical to the 90s reissue (which uses LM13700 OTAs) but the lack of any LED indicators gives away that these are older - probably late 70s or 80s.

Small Stone #1

I think this is the pedal I have owned the longest with repairing. I started buying broken stuff  to repair from eBay in 2011, and picked up a couple of vintage Big Muffs. One was a lot that came with this pedal, a Small Stone, pretty much as pictured - no knob, no pot, no switch and a lot of broken wiring.

The Small Stone has 4 phase shifting stages, using 1 OTA as an LFO and 4 more as variable RC filters that give varying phase shifts. Early versions of the Small Stone used CA3094 OTAs branded as "EH1048", a house-marking for Electro-Harmonix. This version is an "Issue J" and has the 5 EH1048 chips, dated to 1977.

Issue J

EH1048 - 1977, week 32.

I bought a new 24mm reverse log pot, installed a 3PDT switch and re-wired the pedal (true bypass, why not). It didn't pass an effected signal. I think I put it aside at this point, I suspected that the OTAs might be bad and didn't have any replacements.


I came back to this recently. The oscilloscope showed that there was no LFO signal anywhere on the board, even though most of the voltages on oscillator OTA looked reasonable. I bought an RCA metal can CA3094 and replaced it - now I had an LFO, but still no wet signal. Looking at the input and output pins of each OTA, I could see that the first stage was phase shifting, but the second had no output. I shorted together the input and outputs of the second stage, and now I had a phase-shifted signal. It wasn't quite as deep as it should be, with only 3 or 4 stages active, but it verified that the other two ICs were good. I ordered one more CA3094 to replace the dead EH1048, and complete things.

Before re-housing

I sourced a new hockey put knob that fit the new 24mm pot. Originals are hard to come by, but it fits the right aesthetic.

This was inspected by... Elsa? Cheers, Elsa.

There is usually some foam behind the PCB on the back panel of the housing. The PCBs just hang off the back of the rate pot, the foam is to prevent it from shorting out on the back panel. This foam had perished, so I taped some card down to insulate the PCB instead.

Small Stone #2

This second unit is a similar vintage, I picked it up hoping it might help repair the first one.

This is a slightly different PCB (with a phenolic substrate instead of fibreglass?), but looks to be the same circuit more-or-less. It also has 5 EH1048s, dated to 1979.

This one actually worked despite being sold as faulty (this is not that unusual). It just had a couple of quirks. It was a big help in verifying the switch wiring on the other unit, and for taking reference voltage readings off the OTA chips.

Phenolic PCB, instead of fibreglass?

The first was that the rate pot had some odd damage, the casing was partially open. I guess this could have been caused by dropping the pedal onto the knob, or by pulling on the PCB while it was still attached to the enclosure. Or during factory assembly, this was EHX after all. This was easy to close and re-crimp with pliers.

Opened pot housing...

...closed again

The second was that it would start to oscillate with the "color" switch in the up position and no input. It works fine with a guitar connected, or a buffered pedal in front of it, but with no cable or dangling unconnected cable it will start to ring at the top of the phaser sweep. As far as I can tell this is just something that this revision does, to fix it I would have to switch to a shorting input jack or modify the pedal to reduce the positive feedback when the color switch is in the up position, neither of which I really want to do.

These turned out really well. Feels good to have them done. They sound almost identical, the only change that jumps out is that the two different brands of pots don't quite match up - the plastic shaft CTS pot physically rotates further than the new Alpha pot, so the rates are slightly different when the pointers are matched by eye.
I don't have a modern Small Stone, or a Sovtek, to compare with. They definitely sound smoother than JFET phasers I'm used to.

One or both of these will probably hit Reverb in the next couple of days, get in touch if interested.

Electro-Harmonix Stereo Memory Man (EH-7811)

The classic, basic, no-frills BBD delay. This is an EH-7811 revision, dating from around 1980 based on IC codes. This version is main powered (240V), runs at +/-15V internally, Panasonic MN3005. There is no LED, I think this was the last version without one. There is an Echo/Chorus switch which probably reduces delay times, and in-phase and out-of-phase outputs for a "stereo" effect.

This is another one that I have had a for a while, and later came back to. I bought this a couple of years ago and nearly got it working, then hit a dead-end.

It was pretty dirty on first inspection, and missing a knob for the blend control. The power cable had been shortened to a ridiculous length, about six inches, making it awkward to work on.

Before cleaning...

The original eBay picture shows this off:

 

The insides show that the PCB is complete with no obvious damage. It does anchor everything off of board-mounted pots which are only on one side of the large PCB - the other end floats and tends to cantilever.

PCB as received

The delay/chorus switch is almost entirely missing, just the frame left.

Interior of case, Echo/Chorus switch.

Closer inspection of the PCB found that the Blend potentiometer's pads had all craclked off. The pot was still hanging onto the board, but nothing was electrically connected. I ran some small jumpers from the pot back to the nearby traces. I also installed a new sliding switch. I referenced a schematic for the later EH-7811B at David Morrin's excellent site. The main difference (apart from the LED) seems to be that this version has an extra 741 opamp to invert the delay signal for the out-of-phase output.

At this point, I had some signal coming thorough, but hugely distorted. All output opamps were saturated, sitting at ~ 13 or -13 volts. I socketed and replaced some of the opamps with no change. There was a DC offset being introduced somewhere.

I had a few ideas:

1. dead opamp, or opamp feedback network. No changes when swapping opamps and measuring feedback resistors.
2. leaking AC-coupling capacitors. I replaced some 1uF caps of a type I had seen fail before with modern film caps, no changes
3. Missing ground node somewhere...

This went back into the "fix later" box for a while. I dug it out and went over some of schematics for other revisions and noticed that one side of the blend knob should be connected to ground. My blend knob had been cracked off the board, I could barely see a small track below the pot's pads that should have been connecting to ground.

I ran another jumper wire to ground, and now all the outputs were sitting at 0V.

I fitted a new mains cable so that is actually usable. There is no internal fuse, so I changed the cable fuse to a 3A part. I tried the original opamps in the sockets, but the outputs got noticeably more noisy. Maybe semiconductor processing has improved to the point where new 4558s and 741s are less hiss-y.

There was some serious clock whine, especially at long delay times. Fortunately I was able to completely trim this out.

PCB after repairs.

PCB after repairs, parts replacement.

I've said before I haven't noticed huge differences between analog delays based on BBD types. I had the Aqua Puss at hand for comparison between a V3205 and MN3005. The Memory Man sounds cleaner, if that makes sense? Less distortion on each repeat, a bit closer to the original signal. Still sounds like analog delay, just not as overblown.

Reassembled.

Digitech PDS 1700 Chorus/Flanger

I picked this up out of curiosity, I'm fond of the PDS series and the off-the-shelf design. This is a digital Chorus and Flanger, where the traditional BBD design has been replaced with an 8-bit digital delay line (i.e. not DSP or modelling) and the delayed signal mixed in the analog domain. This one worked but didn't switch very well.

PDS 1700

There aren't too many surprises when comparing the insides to the PDS delays. There is the same ADC (ADC0820) but instead of DRAM it uses a single 2k 6116 SRAM. This is probably because the required delay times for a Chorus and Flanger are much shorter (this pedal maxes out at 51 ms) they could spring for the more expensive SRAM. This also means all the DRAM refresh circuitry isn't needed, and there are fewer logic chips overall.

Guts

PCB Backside

The switches were pretty flaky. The mechanical design DOD/Digitech used at the time has a poor reputation for reliability. I find that if they are maintained they work well, but I don't think they stand up to force. The classic problem is that switches that won't switch on the first try get a harder stomp the second time, and things deteriorate.

PDS hardware. Input jack looks non-original.

The paddles press down on momentary push buttons on PCBs inside the pedal. The paddles bear against two bolts that pass through slightly oversized holes in the enclosure. There is a preloaded spring on the bolts on the insides to push them against the enclosure, with nuts holding the springs under tension. There is another return spring on the switch PCB to bounce the paddle back.

I replaced the switches, I'm using these. There was some play in the paddles, the bypass one would rotate slightly in place, meaning it would not always hit the button switch. I tightened down the nuts on the inside until this play was removed, then it switched every time. You need a 3/32" hex key and a 1/4" socket wrench to make the adjustment.

The effects are cool. There is interesting play between the delay time and depth controls, the flanger side can go from covering high to lower-frequency ranges, kind of like the difference between a Boss BF-2 and a HF-2. The chorus does odd (cool) detuning effects at high delay times, modulated chorus in the middle and traditional chorus at minimum delay.

I have this one listed on Reverb if anyone is interested.