HP-8640B Repair

In the summer of 2005, I purchased an almost mint condition HP-8640B signal generator on E-bay.  Unfortunately, the RF final amplifier was defective, and finding a replacement is almost impossible these days.  I found this most excellent website which provided a little more insight into how to go about repairing this section of the signal generator.

Apparently, my version (Option 1) uses a bias voltage from the hybrid amplifier to control the output signal level, where the above link was designed for the avionics version (Option 4), which does not.  I could see on the schematic, (provided by one of the best sites around for older equipment manuals), that the hybrid contained a small RF detector circuit.

Discrete Amplifier

After many attempts using various MMICs from Sirenza, I was ready to throw in the towel.  The MMICs worked fairly well, as long as the output load was 50 ohms.  But when adjusting the output step attenuator, it appears that the SWR rises during the switching process, instantly blowing out the miniscule amplifier.  After destroying about a half-dozen of these pinhead sized devices, I decided to try going with a heftier amplifier using discrete components.

The circuit is a standard broad band amplifier with an added RF detector.  The 4:1 transformer was created with 8 turns of wire wound on a 3/8" toroid.  I am not sure about the properties of the core material, as it was salvaged from some surplus microwave equipment.  The amplifier seems to provide a good signal up to about 300 MHz with this transformer, but I can still get a usable signal up to 512 MHz.