General Radio Co. 1531AB Strobotac Refurbishment

GR 1531AB Strobotac front pan­el with strobe and reflec­tor after restora­tion

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.

GR 1531AB Strobotac unit miss­ing some screws with a pro­tec­tive coat­ing of grease and dirt
GR 1531AB Strobotac unit oth­er side miss­ing cov­er hinge screw assem­bly
GR 1531AB Strobotac unit with dete­ri­o­rat­ed pow­er cord and scratched reflec­tor cov­er

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.

GR 1531AB Strobotac unit removed from case and cleaned up
GR 1531AB Strobotac unit removed from case. trans­former side
GR 1531AB Strobotac unit bot­tom boards and trans­former mount­ing brack­et

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.

GR 1531AB Strobotac bot­tom of oscil­la­tor trig­ger board with orig­i­nal capac­i­tor
GR 1531AB Strobotac top of oscil­la­tor trig­ger board
GR 1531AB Strobotac side view of the switch cir­cuit board wit 1.15 uF 1,000 volt capac­i­tor

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.

GR 1531AB Strobotac capac­i­tors C12 and C13 replace­ment

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.

GR 1531AB Strobotac Schematic (click to enlarge)

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.

GR 1531AB Strobotac case hinge after repair, with new cir­cuit board mount­ing screws
GR 1531AB Strobotac with mod­i­fied mount­ing screw and wash­ers

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.

GR 1531AB Strobotac reflec­tor and Strobotron Type 1538-P1 flash­tube

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.

Click on any image in this post for a larger image
GR 1531AB Strobotac unit orig­i­nal cov­er hinge assem­bly
GR 1531AB Strobotac Hinge brack­et and label
GR 1531AB Strobotac unit opened and rest­ing on base
GR 1531AB Strobotac unit front pan­el after clean­ing
GR 1531AB Strobotac Tube bases and com­po­nents
GR 1531AB Strobotac Capacitor C13 and pow­er sup­ply diodes

I may try using the unit for some stop motion pho­tog­ra­phy in the future if I can find the right sub­ject.

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