[sdiy] Moog 901A questions

[Terry Michaels]

Message text written by Turner Wallace
>I'm puzzled by Q10.

>According to the notes with the schematics Q10 may or may not be fitted
but
>is required as part of the calibration procedure.

I'm curious where you read this, I could not find that wording in the 901A
Service Manual.

>However, I have convinced myself that Q10 is needed to complete the
feedback
>loop on the exponential converter during normal operation. 

>If this isn't the case, how are the two long tailed pair differential amps
>stabilised with respect to gain etc.?

>One day I will understand the design of this module! <


Q10 must be in place for the circuit to work.  You are correct in that Q10
is the output stage of the op amp that supplies current to the diode
string, the op amp cannot close the loop without Q10.  The voltage needed
by Q10 to balance the diode string is sent to the 901B's where the same
current is produced by transistors matched to Q10.

Terry Michaels