memory man adjust

[michael_krell]

Does anyone know the process to adjust the trimmers in a memory man. I am trying to do it by ear and its basically impossible. I have access to a scope and all I just need to know how to do it.

[Mark Hammer]

Depending on the issue and chips used the trimpots will do different things and the advice will change.  What's in there in the way of BBD's?

[michael_krell]

THere are 4 reticon SAD1024's with 2 trimpots adjacent to each of them.

[michael_krell]

I can almost get the whole thing adjusted  by ear but the delay signal is distorted. I replaced the opamps with jrc 488p's. does that have something to do with the gain??

[Mark Hammer]

Now I'llbet you know just exactly how happy people were when 4096-stage devices came out!

One of the trimpots is for the bias, and the other is for the channel balance.  

Pins 5/6 and 11/12 provide complementary outputs of a sort.  Each channel is driven by a clock pulse that is opposite polarity/phase to the other.  When those signals are mixed in equal proportion the clock signal is cancelled out and all you should get is audio.  The trimpot/s tied to these pins are for achieving that balance, for noise-minimization purposes.  Having it set wrong will not prevent it from working, but will result in poorer-than-possible performance.  If you have a scope, you can set these just by sticking your probe on the wiper and tweaking until you see little or no HF clock signal.  You can also set them by ear, or just set them as close to the midpoint (or equal resistance on each trimpot leg) as you can.

All BBD's require a DC bias for the audio signal to ride on.  The other trimpot, which will have its wiper tied to pin 2 and/or 15 (either directly or via a resistor) provides that DC voltage, which is summed with the AC signal on the BBD input pin.  At extremes of the trimpot rotations, you won't hear any audio coming out of the BBD (again, the wiper of the OTHER trimpot), and somewhere, just one side or other of the midpoint, you'll begin to hear a grungy version of the delay signal at the output.  With perfect adjustment of the bias voltage, the delay output will be clearest.

Obviously, in this case, the signal has to be provided externally, instead of just relying on the clock circuitry to provide a signal (which we used in the other trimpot adjustment).  You can play your guitar, or you can just patch in a radio or CD-player or something with a suitable level, and check the scope for the largest amplitude *AC* signal on the BBD's output.

Did I explain it clearly?

[michael_krell]

Yes that is definetly explained clearly. Thank you very much.

Maybe you can answer my next post?