[sdiy] Moog 901A questions

[Terry Michaels]

Message text written by 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.

I would say the text in the service manual could have been written more
clearly, and is a little misleading.  It gives the impression you might
find a 901A without Q10 installed!  This would ordinarily happen only if
you were replacing the 901A/Q10/901B/Q7 matched sets.

>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.

  
<

The CA3019 has a typical reverse leakage of 0.0055 microamps, so I doubt if
that is the reason behind the Low Compensation adjustment.  It is almost
certainly there to supply bias current for the discrete opamp (Q6 thru
Q10), which I estimate to be around 0.5 microamp, and possibly the emitter
current needed to fire the unijunction transistor, which is about 1
microamp.

Terry Michaels