EH Deluxe MemoryMan 4 knobs SAD1024 Distortion

[dicklorent]

I recently aquired an old EH DMM(EH Deluxe MemoryMan 4 knobs SAD1024) and any repetition is distorted no matter how i set it.
Tried to replace the compander chip (NE570N) but it's not it.
Any ideas?
Thanks,
O.C.

[Mark Hammer]

Every single BBD known to humankind, including the venerable SAD-1024, reqires a DC bias voltage in order to pass signal.  The audio may be much less than a volt but it needs to ride atop a DC voltage that can be more than 12v sometimes.  For this reason, many BBD-based circuits will show a trimpot tied to the BBD input in the schematic.  This trimpot allows you to set the bias voltage just right.  A little more or a little less and the sound gets distorted.  Even more or even less than that and you lose delay sound altogether.

Most BBDs will also provide dual complementary outputs which need to be balanced to cancel out noise and audio grime.  As a result, you will usually (though not always) find two trimpots adjacent to any BBD, regardless of how many stages it has.  In some cases the trimpot is replaced with fixed resistors but many manufacturers stick with trimpots.

It is not out of the ordinary for the bias voltage to "drift" over the years and require resetting of the trimpot.  One of the inherent difficulties of the SAD-1024-based Memory Man is that it has multiple 1024s in order to attain a longer delay and each one of these is subject to bias drift from its own trimpot.  Where you can retweak a delay pedal using a single MN3005 by adjusting the trimpot until things sound nice, you need either a scope or intelligent use of an audio probe to adjust 1024s earlier in the chain since you still won't hear anything decent at the pedal output unless ALL the chips are adjusted right.  I'm not saying it is impossible to do by ear, just much trickier.

The good news is that you won't blow anything up by adjusting these trimpots.  All you will screw up is the sound quality, which is completely fixable.  In the majority of instances where people report a delay signal that sounds substandard, trimpot adjustment is the source of problem and the cure.