The primary and secondary circuits share the same chassis, but they are not electrically part of eachother. Electrolytics usually have the negative connected to the chassis (or items at a potential similar to the chassis). The secondary circuit (B circuit) is usually wired in the conventional manner. Maybe i will resolder the caps.Positive ground has to do with the primary circuit only. I can't find a loose connection but i will try again. When i turn on the radio it sounds good for a couple of minutes then it is so garbled or there is no noise. Then the reversal cooling procedure with a drop of liquid CO2 from an inverted “Can of Air”. The developed AVC voltage will shift upon a frontal RF gain malfunction or its abrupt failure.Ĭomponent checking of higher value resistors with a controlled mid level heated air flow from a blow dryer. That way you would have a very sensitive monitoring point of all of the frontal RF circuitry performance of the set. Now should that problem be RF related with a complete loss of audio and had the audio section passed I would move on to monitor the highest level of the AVC developed for the set, which would be at the top/high end terminal of the volume control. The troublesome audio out’s 1st grid coupling capacitor seems to be a composite feature within a PC couplate (C7)_ and, probably, the couplate was never touched…but the good thing is that their ceramic construction is VERY reliable compared to the typical utilization of a std paper cap in that application. With particular consideration to the 1st grid’s voltage swinging more positive, with the coincident reduction of plate voltage. Then after warm up, when the trouble onsets, do a voltage check again to see what voltages are shifting. (That may take two attempts, with cool own time in between) Taking the readings quickly, before approaching that trouble time threshold. I think that I would then be taking/logging down voltage readings on 1st grid, cathode, screen and plate voltages (with the volume an minimum for that one measurement) and the companion audio preamp/driver stage on its equally applicable G1,K and P. If all passes the tappa-tappa-tappa tests. Then I would place stabilizing downward thumb pressure atop each tubes evacuation tit and then enact some healthy buffered taps on the sides of each of the tubes progressively to confirm no loose tube support infrastructure or loose floating contaminants/slag/crud/bad internal spot weld that could be thermally activating. Then after experiencing the distortion onset, I would take my trusty ¼ in nut driver that I have a 3 inch length of rubber heater tubing installed within its shaft center area. I would start by seeing that there is not a gassy 35C5 audio output tube involved. Using just only what information that you have provided…I interpret your description as saying that there is an onset of audio distortion, progressively getting worse after thermal warmup. That’s one nice little unit that you have your self., a full house tube complement and even with the desirable RF front end stage, only being topped with utilization of another variable tuning condenser section devoted to the plate circuit tuning of the RF amp stage.
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