Recently, I got more R&S instruments for repair than ever before. This time, an URE RMS-Voltmeter. It can measure AC and DC voltage, in true RMS values, from 50 µV to 300 Volts. 0.5% basic error. So it is about 1% accurate, with a frequency response from 10 Hz to 20 MHz.

It showed a failure message, “error d” which is a combination of errors that aren’t a good sign. So I was not sure how to approach it first. Checking the manual, I decided to proceed first with some basic test to see if the analog circuit is reacting to commands. A simple test involves the check of the reference voltages: there are 1 V, 10 V, 0.1 V and other voltages derived from a main reference on the analog board, and by digital command you can switch either of these to a test point. However, I could not get all of these voltages switched correctly.

Doing some analysis of the logic chips and circuit, the analog board didn’t get a valid command – there was no signal on the A0 address bus. Tracing back the signal path, this bus originates from an optically isolated driver circuit on the CPU board.

There are 12 optocouplers, fortunately, socketed. So I changed the B12 (A0) and B11 couplers, and indeed, the reference voltage issue showed a different behavior. Obviously, something in wrong with these couplers. I tested a little, and replaced the B12 coupler by a 4N28, and this brought the unit back to life, without and failures. Run the self-calibration (necessary to remove a wire bridge to execute calibration command).

Then, after checking more carefully, decided to replace the 4N28 by a 4N35, because this is a part of virtually identical performance to the (obsolete and old type) of optocouplers installed in the unit. Also checked the signal integrity and slopes with a scope, can’t see any difference of the new B12 coupler to the other couplers.

One major advantage was that I got a whole file with all big schematic copies, easy to work with it. I like to put these on the floor for easy reference without damaging the paper.

A final test with a precision level generator. All seems good, even after a few hours there is barely any drift.