EML 101 repair -- reference voltage circuit

The EML 101 on its test fixture on the workbench.

I wrote in my previous entry (before unemployment and tornado disasters intervened) that I have not been able to get the EML 101 to scale. I think I figured out why. The 101 has a circuit that generates a 4.4V reference voltage that several other circuits, including the expo converters, use as a standard for developing other control voltages. Here is the circuit in question, from an old set of photocopied schematics:

The zener diode at the left, marked 1N823A, is supposed to have a reverse bias drop of 6.2V. The two resistors to the right of it comprise a voltage divider that drops this down to the 4.4V reference. That is filtered by the cap and fed to the 741 configured as a unity-gain buffer.

I had to hunt around for where this circuit physically was. I knew from a legend on the schematic page (not visible in the scan above) that the circuit was on board #1, the expo amp board. In the photo below, showing the reverse side of the panel, board #1 is at right, just to the left of the power supply:

EML 101 guts. The power supply is at farthest right, then the three main boards are numbers 1 thru 3, right to left: the expo amp board, the oscillator board, and the filter/VCA board. In this photo, the expo amp board has been detached from its standoffs for ease of access. The rear of the patching jacks can be seen across the top.

I figured it had to be near one of the 741s on the board, but I was confused at first because I didn't know what the 1N823 actually looked like. There were many ordinary glass-bodied ordinary diodes on the board; I figured it didn't look like those, but I wasn't sure what I was looking for. The data sheets I found for the 1N823 were no help because none of them had an actual photo.

I finally focused my attention on what appeared to be an oddly-marked resistor at the lower right edge of the board. In the photo below, the screwdriver points to the vicinity:

I had been looking at this "resistor" because the marking didn't make any sense. If you double-click on the photo below to expand it, you can see that it is marked sorta-blue, red, orange, and then a blue T that points towards the right end. It finally dawned on me to decode the stripes: if you allow that the sorta-blue might have been gray 35 years ago, they spell out "823", the part number of the zener I was looking for! I've never seen a diode marked like this. The blue T points towards the cathode end. Sure enough, this was it. The 2.2K and 5.6K resistors are right above the zener; the 1K resistor is to the left, and the round metal thing that appears in this photo to be a four-legged transistor is the 741. (This is how ICs were packaged in the early days, before the DIP package came into use. The other four legs of the 741 are obscured by the green wire.)

The 4.4V reference circuit. The "708" stamped into the board is this 101's serial number. This photo was shot through a 2X inspection magnifier. The finger is not part of the circuit (hopefully).

I measured the voltage across the zener and got 6.39V, higher than the spec'ed 6.2. Doing the math with this input voltage, I got 4.6V for the voltage divider output, which, sure enough, is what it measured. I considered replacing the zener, but researching the 1N823 further, I noted two things about it: (1) it is a temperature-compensated part, and (2) it appears that the only present-day suppler is NTE, and Mouser has it marked "not recommended for new designs", which suggests that it may not be in production much longer. And there doesn't seem to be an exact current-day substitute. So I figured that as long as the one that's there is still working at all, it would behoove me to find a way to make the circuit work.

The obvious answer is to tweak the voltage divider by replacing one of the resistors. I did the math for what value I would need in place of the 2.2K resistor, and I got 2.53K. Off to the Mouser catalog to see what they had in the way of 1% resistors. 2.53K isn't an available value; the two closest are 2.51 and 2.55K. So I ordered 10 of each, and I'll measure each one to see which one gets closest to the 2.53K value I need.

I've got the shipment now, and next week I'll replace the resistor. And then we'll see if my grand theory is right: that the 4.4V reference being off by 0.2V is what is preventing the 101 from playing in scale.