Side project - MV864A restoration - reverse engineering challenge

PICKED UP MILLIVAC MV864A AND PAID FOR MANUAL FROM THE MANUFACTURER

Having purchased a used high accuracy meter on eBay, it was time to restore and calibrate it so that I can use it on my bench. It generally works, but when I used my bench power supplies to deliver given voltages and currents to compare with the meter reading. The readings were close but not exactly on. 

This meter has a grid of potentiometers on the rear which allow for calibration of each scale of voltage, current and resistance. I found that on some ranges I couldn't get the meter all the way to the target reading even at the extreme of the pot movement. I also didn't know if I had to do the calibrations in a certain sequence or could adjust any scale independently of the others. 

I contacted Millivac and asked if I could buy the manual that has the schematic and calibration instructions. I provided the serial number of the unit to have them find the proper version of the manual. I was told that the oldest manual they could provide was for a newer version but that the schematics and other information should be quite close. I spent the $100 and began the restoration when it arrived - see earlier posts about this side project for details. 

MAJOR DISSONANCE BETWEEN MANUAL AND THE UNIT ON HAND

Unfortunately for me, the schematics and other diagrams are very different - you can see that the general scheme is the same but the parts used and layout were very, very different. The major reason is that my unit uses primarily germanium transistors whereas the manual shows the meter after they had re-engineered this around silicon transistors.

The parts numbers are all different, because it was a redesign. The layout on printed circuit boards is different. In order to put a scope or voltmeter on various points to compare against the values shown in the manual, you need to know where the point sits. Further, the waveforms and voltages likely have changed between the germanium and silicon designs. 

Here is the portion of the schematic that I have been reverse engineering - I still have a few values to add to the drawings and some cross checking, but it is close. First, the portion of the schematic from the manual. 

Next is the version I captured from the actual unit:

Lets zoom in one two sections to see how different they are. First we will look at the input filter that sharply notches powerline noise. Second we look at the preamplifier that finds the difference signal between the filtered input and a reference signal from the photocells of the chopper. 

For the input filter, I have the following component values and connections:

Compare that to the schematic portion from the manual. These parts should be very close since a notch filter has component values determined by the frequencies and rolloffs, which wouldn't depend on germanium versus silicon techology at all. In other words, this is the best situation for a match.

Even here we have a few discrepancies in the values of parts although most are close. The output resistance is much higher, which may be based on the change in transistor technology. The parts for the filter are visible on the board:

The variations would be tolerable here, even if I had to map out that 3R1 on the manual schematic is R201 on my PCB. Once I move into the areas where the redesign is more substantial, this becomes less feasible. 

The preamplifier as I found it on the unit has a germanium FET and two germanium 2N414 NPN transistors. 

The manual has a more complex circuit with the same FET but three silicon 2N2907 NPN transistors. I am guessing that they chose a lower gain for the three stages to achieve the same result as the two germanium transistors provided on my unit.

DIVERGENCE IS WIDENING AFTER THIS POINT

There are ten silicon transistors on the rest of the schematic and two coupling transformers in the version in the purchased manual. However, on the actual unit, there are fifteen silicon transistors and four coupling transformers. A second adjusting potentiometer is on my board, but not in the manual; it has no marking on the PCB to hint at its purpose. 

Here is the top of the PCB with the previously discussed sections partially covered to show the remaining components that must be reverse engineered. 

REVERSE ENGINEERING IS TEDIOUS BUT STRAIGHTFORWARD

The huge advantage I have is that the PCBs in the unit are single sided - traces only on the bottom and components only on the top. I have flipped the picture of the bottom so that I can match it to the parts in the top picture, allowing me to figure out the wiring between parts. 

I will keep at this until I have the entire schematic of this PCB captured. I figure I can use LT Spice to figure out the corresponding voltage and waveform values I should see at the reference points to correspond to the ones marked on the manual's schematic. 

In addition to this PCB, there is a second PCB that has the power supply and range attenuator components. This also is different from the manual version. Therefore I have to reverse engineer at least the power supply portion. The range attenuator and the front panel selector switch should match up fairly closely, so I won't bother drawing that out unless I run into problems during restoration that indicate a divergence.