Low PA Grid Drive? Well, Maybe Not...
The following hint was recently submitted by Don Chester, K4KYV, regarding the PA grid drive on a BC-1T transmitter:
A number of hams attempting to use these transmitters have
mentioned low grid drive as a problem. The grid drive may be OK,
while the problem is actually in the metering circuit.
My BC-1T suddenly showed low PA grid drive (90 mA maximum), and
the cathode current of the driver stage wouldn't exceed 140 mA.
Normally, it is closer to 200 mA. I checked out voltages and
resistances in the driver and PA grid circuit, and everything was
normal. So I temporarily wired my digital voltmeter in the
circuit, and while the transmitter multimeter read 90 mA, the DVM
read 155 mA! I double checked with my analog volt-ohm-meter, and
got the same result. The culprit was the 240 ohm resistor in
series with the meter movement (fundamental range 0-1 mA). It
read 450 ohms on the DVM. I couldn't find a 240 ohm resistor in
the junk box, but I did find a couple of 500 ohm precision
resistors and a 7000 ohm one, wired them all in parallel, and
ended up with measured resistance of 241.2 ohms. With the new
resistor in place, the meter reads 164 mA grid drive with no
plate voltage on the 833A's and the driver cathode reads over 200
mA, dipped. That's a little high, so I checked the total
resistance of the paralleled 3-ohm resistors used in the Driver
Cathode and PA Grid meter positions. Sure enough, the carbon
composition resistors had increased substantially above their
nominal values. If I can't find replacement resistors or a 1 ohm
precision resistor to take their place, I may have to purchase
some new replacement 3 ohm resistors. That should bring the
reading close to the nominal value.
Don K4KYV
If you have any service or conversion hints on either the Gates or Collins broadcast transmitters, please e-mail them to the Webmaster.