Back in April of 2016, I bid on a Hewlett Packard 8640b signal generator to upgrade my test bench at home. This was from a local seller, and I won the bid for $115.00 + $20.00 shipping, for a total of $135.00. I figured for that price, I couldn’t go wrong. In working condition, these typically run about $500.00, depending on condition.
When the unit arrived, it was not packed great (they used packing peanuts, instead of snug foam), but it was double boxed, and there appeared to be no damage in shipping.
I plugged in the unit and brought it up slowly on the variac, in case there were shorts that might cause further problems in the unit. To my surprise, the unit came up and appeared to mostly work. I could not turn the FM Deviation knob through all of it’s ranges, and the generator indicated that the deviation was out of allowed limits when I turned the FM modulation on. Also, the FM Deviation vernier knob was broken completely off, and the Fine Tuning control was inoperable. But the unit was generating RF on all bands up to 512 Mhz, and the output level seemed to be well calibrated.
I removed the bottom cover to investigate why the FM deviation knob would not go into all levels. I found the dreaded “broken gear set” syndrome on the FM Deviation and Range module. I wasn’t too surprised at this, as it is a very common failure in these generators. Seeing that it was in pretty good shape aside from that, I looked for info about repairing the module.
Here is a PDF guide for the following steps, adapted from Hewlett Packard’s Service Sheet D.
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