Refurbishing a General Radio Company 1531AB Strobotac unit built in 1975.
A friend of mine was retiring and closing his HVAC test and balance company, and mentioned that he had some old test equipment he wanted to get rid of.
Of course I would be interested in free vintage test equipment, so I made a trip to his location to see what he had. On the shelve, one piece of equipment stood out as something that I definitely recognized. It was an old, well used, and dirty strobotac unit.
The Strobotac is a calibrated stroboscope that uses a flashtube to produce repetitive short-duration bright flashes of light. It is often used to measure the RPM of rotating objects such as motors and fans, by adjusting the flash rate of the Strobotac until the object looks stationary, and then the calibrated dial will read the RPM or submultiple of it.
the GR 1531AB is capable of producing flash rates from 110 to 25,000 flashes per minute, and can measure speeds up to 250,000 rpm.
The unit was dirty, missing a few parts and had a heavily deteriorated AC power cord but seemed to be in repairable condition.
So my first step was to clean it up and check inside the unit for visible damage and other missing parts.
This version of the GR 1531 consists of the components mounted to the panel and two circuit boards. The two boards are the oscillator trigger circuit board which uses two vacuum tubes, and the switch circuit board with high voltage capacitors.
Overall the internal parts were clean and in good condition for being 45 years old. The main power switch fell out of the front panel as I raised the oscillator timing board, due to it missing its front panel nut. I needed to take it out anyway to replace the AC power cord, and was able to source a replacement nut from some older spare switches I had in stock.
I used extreme caution while working on this unit as the power supply produces plus and minus 400 volts (total 800 volts) along with a 250 volt supply rail. There are also several 1,000 volt rated capacitors that are capable of storing these voltages for long periods of time.
I replaced electrolytic capacitor C13 with a Nichicon 10 uF 500 volt unit, and rebuilt the multi-capacitor can C12 with a 56 uF and two 27 uF 500 volt rated Nichicon capacitors. I also replaced C8, with a 100 pF 500V NP0 ceramic capacitor. C8 is specified as a ceramic capacitor in the parts list, but had leaked a significant amount of a waxy substance on the circuit board, and tested at 87 pF which was outside of it’s +/- 10% range of 100 pF.
The unit powered up with no issues after about a 20 second delay while the tube filaments warmed up. I followed the fairly easy calibration instructions in the manual and was done with the electronics side of the refurbishment.
The next item to tackle was the missing cover hinge pivot assembly. Looking thru all my spare washers and screws, I was able to find a nylon shoulder washer that was a good fit for the large hole in the handle, along with a nylon flat washer to sandwich it in. A stainless steel pan-head screw with two nuts and a lock washer was used to attach the handle to the case. A pair of 6–32 X 5/16″ stainless steel pan-head screws were used to replace the missing case circuit board mounting screws.
The clear reflector cover was very scratched from many years of use and required several hours of scratch removal and polishing using Novus plastic polish #3 thru #1. I did not get out some of the deeper gouges and scratches, but they were mainly along the outer edge and didn’t greatly affect the reflected flash light beam path.
The mirrored reflector was was in very good shape along with the Strobotron flashtube which looked like it had been replaced in the not too distant past.
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I may try using the unit for some stop motion photography in the future if I can find the right subject.