Tony Faiola's GPT-750 HV Arcing Fix


The Technical Materiel Corporation GPT-750 Power Supply

I thought trouble shooting the GPT-750 power supply would be a simple project, however as usual for me, a “Tough Nut” came up with this power supply.

After using this transmitter with basically no problems after 50 years of use, the plate and/or the main circuit breaker would actuate.  The design had no special circuits, it was a straight forward full wave rectifier center tap high voltage transformer.  The plate and screen circuit breakers protected their respective circuits and opened the key line to prevent the high voltage from being enabled.

During my trouble shooting process of disconnecting almost all of the high voltage components, I was bewildered that only the 872 mercury vapor tubes were in the circuit blowing the breaker.  The 872 filament transformer was even replaced.  Removing the plate caps on the two 872s did not cause the breakers to blow.  Thank goodness this result eliminated the huge power transformer.

Next thought was it had to be the 872 tube socket platform and filament wiring.  I replaced the filament wire with a 30KV number 14 wire going from the filament transformer to the Johnson ceramic tube sockets.  Breakers still blew.

Next and final step was to insert a General Radio 240 Volt variac or variable voltage transformer between the ac line voltage going to the high voltage transformer.  Starting the power supply voltage reading at 1000 Volts DC and slowly bringing it up to 2000 volts, the breakers opened.  It had to be a break down in the 872 tube sockets. 

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Careful inspection of the inside diameter  of the two sockets revealed carbon build up.  To correct the build up, I used a Dremel tool with a 3/32 inch diameter file bit (used for chain saw sharpening).  Not only did this process clean off all  the carbon deposits off the inside socket diameter, it made some additional distance from this inside diameter to the tube socket metallic conductors.  If the ceramic was porous, it could absorb moisture.  To finalized the socket repair, I used two separate coats of SUPER CORONA DOPE by GC electronics 4226.  Each coating provided 3800 v/mil minimum breakdown protection.

Slowly bringing the variac up to the 240 volt ac line voltage and a 3000 volt DC reading on the amplifier deck hv meter finally passed the test.  After a few weeks of constantly keying the high voltage, there was no sign of breaker failure.  Solid state replacement of the 872 tubes also produced the same results with a slightly higher high voltage.

I hope this article will save any owner with this problem a headache.  One can be easily sidetracked with a faulty 872 tube.

Tony Faiola, K3WX (W3GBS)