Vintage Synth Repair

At 17:16 Uhr +0000 05.04.2013, alesisfusion8hd wrote:
> thanks for your advice. Output of the slider seems ok (1.36 V w/ KBD CV way
> up, LFO & EG down), but input of LM301 still 0. I removed the LM301 from its
> socket -- I already changed it several years ago, and then put a socket
>-- and
> try to get a chance to test it this week-end.

There probably wil be very little keyboard cv directly at the input of the
LM301 since it passes through a 100k resistor before.
If you look at the implementation of an inverting opamp you'll see that the
gain of the opamp is defined by the input resistor and the resistor in the
feedback loop.
The feedback loop has R9 and T5 as resistors; together they form
theoretically a resistance between 88.7k and 113.7k. The input resistor
(R4) for the keyboard CV is 100k. It's obvious that we're looking basically
at a gain of 1 here (so the signal amplitude stays the same, just the
polarity is reversed); and T5 is used to finetune the gain (because we want
proper tracking over the whole keyboard).
R4, R5 and R6 are not just there to decrease the control voltages; they
each define together with R9/T5 the gain for the control voltages.


I believe the keyboard CV should be 2V for the lowest key of the Odyssey
without any transposition and with the tuning knob centered.