04-12-2013, 08:48 AM
I've now finished everything on this radio, except for clearing up the problem in the audio power output section. This includes the two 0.1uf caps in the IF cans. Replacing those did nothing for the voltages, but did seem to help the IF slopes. Once doing an IF alignment, the sensitivity is actually quite good.
Now for the output issue:
This evening I checked this whole section thoroughly. The input to the grids from the driver transformer is quite clean until the volume is turned up far enough to overload the input (RF) signal. Unfortunately, that's where the normality stops. Going to the output tubes themselves, I find that there is far too much current being drawn. Inserting a DMM into the center tap connection of the output transformer, I get 120mA of current draw, where the total according to the service literature should be 56mA. This draws the voltage down to ~110V on each plate, where it should be 275. The cathode to ground is ~91V, where it should be 45. No grid to ground voltage, other than the AC from the signal. It seems to me that there should be some tiny DC voltage there, too, given the DC resistance of the driver transformer. But perhaps not.
Experimentation shows that if I put a series resistance in the center tap to the output transformer, it lowers the voltage to the plates, but the voltages to all other points on the chassis did come up to near normal.
I've tested DC resistance (leakage) on the output tube sockets, and find no significant leakage (actually, immeasurable, as my DMM registers only to 20MOhm, and I get no reading (OL) on the meter from grid to cathode, grid to plate, or plate to cathode. (this with removing the wires from the grids to the driver transformer and the plates to the output transformer). Despite this, I am getting some ceramic sockets to replace the two phenolic sockets.
Also have some 45's on their way, hopefully these will help. (Just for giggles, I put in a pair of matched 2A3's, it made little or no difference.. too much plate current I'm sure).
Now for the output issue:
This evening I checked this whole section thoroughly. The input to the grids from the driver transformer is quite clean until the volume is turned up far enough to overload the input (RF) signal. Unfortunately, that's where the normality stops. Going to the output tubes themselves, I find that there is far too much current being drawn. Inserting a DMM into the center tap connection of the output transformer, I get 120mA of current draw, where the total according to the service literature should be 56mA. This draws the voltage down to ~110V on each plate, where it should be 275. The cathode to ground is ~91V, where it should be 45. No grid to ground voltage, other than the AC from the signal. It seems to me that there should be some tiny DC voltage there, too, given the DC resistance of the driver transformer. But perhaps not.
Experimentation shows that if I put a series resistance in the center tap to the output transformer, it lowers the voltage to the plates, but the voltages to all other points on the chassis did come up to near normal.
I've tested DC resistance (leakage) on the output tube sockets, and find no significant leakage (actually, immeasurable, as my DMM registers only to 20MOhm, and I get no reading (OL) on the meter from grid to cathode, grid to plate, or plate to cathode. (this with removing the wires from the grids to the driver transformer and the plates to the output transformer). Despite this, I am getting some ceramic sockets to replace the two phenolic sockets.
Also have some 45's on their way, hopefully these will help. (Just for giggles, I put in a pair of matched 2A3's, it made little or no difference.. too much plate current I'm sure).
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