[sdiy] Moog 901A questions

[Turner Wallace]

I now have the Technical Service Manual with me:
The 901A Oscillator Controller section B Checkout Procedure point 2 states

'Place a 2N4058 transistor in the Q10 socket, if one is not already there.'

I now now happy that Q10 is required for correct operation. It samples the
output voltage and provides current feedback.

That accepted, my next question is what is the Low- compensation trimmer
compensating? Is the primary offset due to the differential amp or the
reverse leakage current of the CA3019?
It is interesting to note that the manual indicates it is this value that
will require trimming the most often!
I can work out the dc offset performance of the diff amps but I don't have a
spec on the CA3019. What is the reverse leakage current?

>From rough calculations, I think the low offset trimmer provides ~+/-4uA.
The CA3019 would appear to operate in the current range ~1uA to 250uA (from
memory) over the range of the fixed control voltage selector. I have only
achieved good state of tuning in the range 0V to +5V input to the module.

  


-----Original Message-----
From: Terry Michaels [mailto:104065.2340 at compuserve.com]
Sent: Monday, March 26, 2001 2:34 PM
To: Turner Wallace; synth
Subject: RE:[sdiy] Moog 901A questions


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
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