Suddenly, my SG-811 microwave generator started to play up, running hot, and then, stopped working with power supply failing. Too bad! Fortunately, the Micro-Tel power supplies have a fairly consistent design for all their various instruments, and I have fixed already several of them, so it will be an easy task. In this case, just needed to replace the main transistors to get it working again (MJ12002 replaced by BU208A, which are much more easily available). But switching on, I immediately noted that something was wrong, too much heat and current around the oscillator control board. This board has a LH0021CK power (1.0 Amp) opamp that is driving the tuning coil of the YIG oscillators. The part seems to be shorted out.
Looking at the schematic, there are current sense resistors for each band, with the sense wires switched by an analog multiplexer.
The LH0021 are not quite rare, but expensive – fortunately, a kind guy from the US offered a pair for these on eBay, NOS or used, for a reasonable price. And some weeks later, they made it to Germany.
These are really nice parts, all gold plated and solid pins.
Replaced the LH0021, and the SG-811 is basically working, but still too much current on the LH0021. What is going wrong? Turns out, there is a oscillator control board inside the RF unit, which is switching the coils depending on the band selected. Probing around, this doesn’t seem to work, because one of the coils, band 3, stays energized all the time.
Easy to find the troublemaker – a shorted switching transistor, a medium power PNP, 2N5193. These 2N5192 are not very common, so let’s do a search for similar parts in by basement archive of obsolete parts – and, in fact, there is a bin of BD438, including a note about their characteristics, and a not from the former owner (a generous old man who didn’t need any electronic parts any more, and had several lifetimes’ worth of supplies).
With the RF unit open and the board pulled out, it’s a good idea to check all the transistors and diodes, and in fact, another one found shorted as well (not a tuning switch, but the main power for one of the oscillators).
With both of these transistors replaced, the SG-811 can be put back into service. Didn’t take all that long to fix, not much longer than to deal with repair quotes, shipment, and other hazzles when repairing more modern units.
…plenty of power, up to 18 GHz…