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.
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
I hope this article will save any owner with this problem a headache. One can be easily sidetracked with a faulty 872 tube.
Faiola, K3WX (W3GBS)