I may publish the whole Kicad project(schematic, pcb, footprints etc), but first I would like to have it "ready" and probably have better names for all the nets and power rails etc.
I have tried to calibrate the unit and the common mode adjustment seems to be quite coarse. Maybe multiturn trimmer would be one improvement for this unit.
Wictor
It is meant to be a 4.5 - 5.5 digit calibrator. To get to real 6 digits will need at least better operational amplifiers. There is a tendency to be stuck in an endless loop calibrating this (hence their recommendation to not calibrate it unless absolutely necessary). What I found works well is when you back and forth two pots (e.g. zero and zero balance or range and range balance) to calibrate the pot 3/4 way to desired outcome and then do the other pot. It tends to close in faster on the calibration.
Short term, however, it does seem able to do six digits (I was able to use it to prove my 34401A's reference was noisy using the Datel).
I do have some suggestions for better clarity and naming of the nodes (I didn't want to be pushy - but since you said you are interested in doing this anyway....):
1. Power supplies: There are two 33v power supplies in the Datel. You mark 1 as +V and -V. The second is TP4 and -V2 (+V2 you call the name of the rectified voltage, inconsistent with +V1).
a. So the suggestion is to call the Emitter of Q12 "+V2".
b. V1 and V2 are identical (except 1-2 resistor values & the overcurrent detector formed around Q7) - so draw V1 and V2 supplies with identical layout in the schematic - it is easier to read.
2. TP1_10V - I'd call this 10V_ref or Ref_10V or something like that. TP1 is just one thing that is attached to the 10V_ref and is inconsistent.
3. The feedback around U1 (Comprised of inverting voltage TP1_10V through divider network of R32 and R33-R37 & R39 and noninverting voltage DIV_G through R1) is unclear. 3k01 R_3011 is actually misleading where it is as it is a bias resistor for the LM399...
4. DIV_E-DIV_H - I'd draw the matrix switch. It flips the LM399's zener between COM and DIV_G. (That in turn - via Q3 and Q4 - slide V2 supply to the opposite polarity - all D1-D2-D3 seem to do is ensure the diode is always properly biased). Perhaps Q3 and Q4 deserve a nice area near the 10V buffer.
5. AFAIK - DIV_A is not related to DIV_B DIV_C and DIV_D. It is the same as COM and should be labeled COM. The connection between Div_A and COM happens at the big pad under the transformer.
6. DIV_B C and D should be drawn as a potentiometer so people understand the functionality. Here is how it works:
Between Div_C and Div_D - 8.139K resistor (fixed)
Between Div_B and Div_C - Variable. At 0V - open (infinity). At 10V - 8.139k resistor. At 1mV 89.7k resistor.
Hope this isn't confusing...
-al
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