8640b FM Deviation Pot Repair

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.

FM Deviation potentiometer

FM Deviation potentiometer

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HP 8640b Signal Generator Gear Set Replacement

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.

Gearset Kit

Gearset Kit

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ANSR Ground Station Tracking System

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.

Ground Station Configuration

Ground Station Configuration

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Restoring the Kenpro KR-5400 Rotator

As mentioned in my prior post, I found that the elevation rotator for my Kenpro-5400 set had corroded internally to the point where it was no longer working.  It had been sitting outside in the weather for over 10 years, with no maintenance or activity, and the seals had allowed water to leak into the rotator, resulting in the corrosion.  The Azimuth rotator just needed a bit of fresh grease on the bearings to work smoothly.  This post will show some pictures of the cleaning/rebuilding process for the elevation rotator.

Kenpro KR-500 Elevation Rotator

Kenpro KR-500 Elevation Rotator

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ANSR – Arizona Near Space Research work

After having taken a break for a few years, I have renewed my membership in the Arizona Near Space Research group.  This is a club that specializes in high-altitude balloon launches that carry payloads typically designed and built by students ranging from Jr. High to Post Graduate levels.  I was involved in the club shortly after it was formed in the early 2000s, then found myself juggling for time to work on our cabin in Northern Arizona, Golf and family matters.

When I was involved years ago, I created an automated balloon tracking/antenna pointing system to keep our antennas pointed at the balloon as it floated downrange from the launch site.  These antennas were high-gain 430 Mhz or 2 Ghz directional arrays on which we captured live video feeds from the balloons.  My original incarnation was controlled by a PIC Microprocessor and a few I/O devices, driving my Kenpro KR-5400 Azitmuth-Elevation antenna rotator.  It would listen to the GPS position info on the balloon’s radio link, and based on the ground station location, would calculate the correct azimuth and elevation to point the antenna at the balloon.  It then sent appropriate signals to the antenna control box to steer the array.

Original PIC-Based ANSR Balloon Tracker ~ 2003

Original PIC-Based ANSR Balloon Tracker ~ 2003

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