I am rebuilding the power supply on a pal's L-4B and am stumped at a
problem that has developed.
Original symptom was no high voltage. Looked like the fuse resistor
had opened in the supply. So I felt the equipment was old lets rebuild the
supply.
First I bought the Harbach Power Supply kit and installed it. I also
replaced
the two 50K 50 watt resistors in the bleeder. All looked Ok.
After about 15 minutes in standby, one of the bleeders flashed over to the
mounting
bracket. Evidently the mica spacers broke down. This is between #1 and #2
bleeder.
So I changed the resistor, adding more mica.
After about 10 minutes in standby, big flash, resistor arcs to the mounting
screw internally.
So I look at this and think about how disappointed I am at Drake
engineering.
Apparently the screw must be exactly centered or bad things happen.
I ponder this and do the following on the replacement resistor. I wind
Scotch #27 tape
around the screw building enough on each end to act as a spacer centering
the screw.
Rated for 250 degrees so I'm not worried the tape will melt making goo all
over. I had also
reinstalled the mica spacers. Now I take nylon washers and screws and lift
the mounting
bracket above ground. I'm now confident the arcing will go away.
Hook everything up and now I see negative grid current, even with no tubes
in the sockets
and the HV not connected. When the HV is connected a 5K 10 watt resistor at
the bottom of the
bleeder string runs very hot. There is 125 or 95 volts at the junction of
the bleeder stack
feeding bias to the tube. This looks normal.
There is 1950 or 2600 volts on the HV output depending on setting of the
CW/SSB switch.
I'm stumped. Help!
ed K0KL
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