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.