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.



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