Side project - MV864A restoration - reverse engineering power supply section

 WAITING ON PCB AND PARTS TO RESUME MAINSTREAM 1130 ACTIVITIES

The 1130MRAM memory PCB is scheduled to arrive on October 10th and the new components will be here on the 8th. Until I have the new board ready to final testing on the 1130, it does not have working core memory. That is required to begin testing the 1132 printer and 2501 card reader controller logic as well as to resume testing of my virtual 2315 disk cartridge project. 

In the interim I will keep busy doing these side projects - the Millivac MV864A meter and the IBM 3278 terminal restorations for example. It was worth $28 to speed up the delivery of the PCB, since I am eager to work on the top priority 1130 restoration tasks. 

POWER SUPPLY VASTLY DIFFERENT FROM THE MANUAL AND SCHEMATIC

The manual shows a power supply with four silicon transistors, four diodes, a zener diode, an LED for the main power indicator and three test points to measure +8.5V, -6V and ground. The actual board in my meter has one transistor-like device plus a large germanium power transistor, two diodes, and an incandescent main power indicator. 

Even odder, the three test points I found, marked TP63, TP62, and ground, measure -14.5V, -6V and 0V respectively. There is an 8.5V delta between the first two, but nothing shows up as +8.5V. The meter appears to work, thus this must not be a result of failure in the supply. Rather this is likely due to the older germanium technology used on my meter versus the redesign with silicon semiconductors. 

CHALLENGES FINDING DATASHEETS FOR BOTH TRANSISTOR DEVICES

I have completely failed to find any data sheets for the MHT3030 power transistor, although it is almost certainly a germanium PNP device. I did find the smaller device, a General Electric RA-1, in an old GE manual found on the web. It is a compound device, four terminal, with a transistor plus a zener diode attached to the transistor emitter. It was used in voltage regulator circuits and there are reference designs in the GE manual. 

This manual "GE Transistor Manual - Circuits, Applications, Characteristics, Theory" published in 1964 is a gold mine of information. The section on Regulated DC Supply and Inverter Circuits covers the Reference Amplifier (RA) devices and circuits using them to build 'precision' voltage regulators. Here is the sample circuit in the manual:

This corresponds extremely well with the recovered design from my reverse engineering. A few differences - the meter uses a half wave rectifier rather than the full wave design from the GE manual and the GE manual uses a fixed resistor voltage divider to provide the 7V reference voltage to the RA1 base when the output is (almost) 12V. 

Due to resistor tolerances and a more limited choice of resistor values in the 1960s, GE chose the resistors to give 11.73V which was the close enough (for a 'precision' voltage regulator). The inclusion of a potentiometer in the Millivac circuit allows for adjustment to hit a target output voltage, with the midpoint of the pot setting -14V and enough range to achieve -14.5V. 

Here is the schematic I created from reverse engineering the board of my meter:

Two things remain a mystery to me after this work. First, the method of creating -6V appears to be a simple voltage drop over 540 ohms of resistance - this only works out to an 8.5V delta if the current through the resistors is 15.7ma. Until I see how this line is connected on the other PCB I can't assess whether this makes sense.  Second, the existence of -14.5V and -6V rather than the +8.5V and -6V of the manual's power supply. 

Here is the schematic from the manual for comparison: