Early in the repair cycle of the 8640b, when I had just finished the brass gearset installation, I ran into a problem during the first power up after reinstallation of the deviation and range module. When I powered up the unit, I saw a flash from under the main board, and the unit came up with no led frequency display. I suspected that I had lost one of the power supplies. After finding the manual for the generator, I saw that there were 3 power supply boards with led indicators on them to indicate the various voltages were being generated.
Another item on the 8640b refurbishment project was the broken Fine Tune control. I could rotate the control from the front panel, but it had no effect on the frequency generated.
After removing the bottom cover, I could see the problem. The Oscillator Module was missing the shaft that connected the fine tube knob with the control on the rear of the main oscillator
The shaft should be near the top of the picture above.
After replacing the gearset in my 8640b signal generator, the next item on the list was to repair the FM Deviation potentiometer, which had the shaft broken off where the knob would attach to the front.
Since the pot is still electrically good, and I cannot locate another pot with the same value and length of control shaft, I thought that I could somehow repair the shaft.
My first attempt at this was to remove the old shaft at the potentiometer body, and replace it with a longer one. It looked like I could remove the retaining clip near the pot’s body, and perhaps the shaft would pull out. Unfortunately, after removing the retainer clip, the shaft remained held to the body, so that did not pan out.
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.
In support of the Arizona Near Space Research group, I have agreed to set up a new ground station for tracking the balloon flights. We do this in order to point directional antennas at the balloons as they float across the state on their missions. I had done this about 10 – 12 years ago with a PIC processor programmed to read the telemetry from the balloon beacons and calculate the azimuth and elevation to point the directional antennas at the balloon for optimum reception of live video feeds.