Elad Orbach : fix repair ebay find SG382 SRS rf signal generator and SG385 power supply

I bought this Stanford Research Systems SG382 as a malfunctioning eBay item.

The two main symptoms were no 10 MHz signal at the rear reference output and a front panel that did not respond.

The photos were taken later. When I first opened the SG382, the inside was covered with a heavy layer of black soot or residue.

I did not have a complete schematic, but the circuit description in the manual was enough to follow the relevant sections.

The missing rear 10 MHz output was caused by resistor R101 on the main motherboard. It should be 4.99 kOhm. I did not have the correct SMD part, so I fitted a normal through hole resistor instead. It was not pretty, but it restored the output.

The front panel started working after a short reflow of the SPI interface devices U308 and U309.

During the repair I accidentally shorted something on the SG385 power supply board, which provides 15 V, 5 V, 3.3 V, minus 5 V and minus 15 V.

Several rails were measuring about half of their expected voltage. After some probing I traced the problem to an LM358 in the temperature control circuit. The LM358 had failed short circuit, so I replaced it.

I also replaced the cooling fan because the original one looked tired.

Repair and internal photos below.