EH Scott Radio Enthusiasts

The Fine Things are Always Hand Made

Today I finished the replacement of the outer jacket on the 12-wire cable from the radio-tuner chassis to the power supply. I took time to ohm out each wire from the 12-pin plug to its destination in the radio-tuner chassis. I had continuity to each destination place in the tuner. There were no shorts on B+ or other leads, so my re-connection of each wire was successful. The plastic strands that make up the new outer jacket are extremely tough. It took the highest temperature settings of my soldering iron to melt the strands and halt fraying at the ends. They are also highly resistant to abrasion - a real plus for any cable that may move against a cabinet surface as the chassis is moved in and out. I will do the 4-wire cable next. When that is done the cables will look better, work well and be protected.

I decided to take some voltage measurements while I had the set operating. I was interested to see what the AC input voltage was and the resulting DC voltages at the power supply were.

AC input was measuring 122VAC in. This is higher than most days. Usual voltage is 119VAC.

B+ at the 4uF capacitor in the power supply was +413 VDC.

B+ at the plate of the 6L6's was 388 VDC and at their Screens was +302 VDC.

I just received some 10pFd N2200 disc ceramic capacitors that I am going to try in the FM Oscilllator circuit to see if I can stop the frequency shift during warm-up. These might be too much compensation. I do not have a highly accurate RF signal generator in order to check the actual amount of frequency shift, but results will tell whether I need to select some lower amount of compensation. The shift that takes place is all within the first 5 minutes of operation. That is not really severe. Most manufacturers of tube tuners and receivers did not tame their units until the early 1960's. By the time I built my second FM tuner, a Dynakit FM-3, they had refined the compensation of that set until from cold to hot it needed no retuning at all. I still have that tuner and it is still as stable as the day I put it together back in the middle 1960's. I also have a Fisher 500C receiver that has no drift either.

Joe

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Today I installed a 10pF N2200 disc ceramic capacitor in the FM Oscillator circuit of my Early 800B. This appears to have solved what drift the unit has during the first 5 minutes. After being fully warmed up, I let it cool off and when turned back on it was right on station. I will do several more tests with the cover back over the FM front end and report again.

Joe

Today I tried the 16uF metalized Polypropylene AC capacitor that I had bought to try in the 800B. With the 120VAC input present today, the B+ rose to 439VDC at the 4uF electrolytic. The slight hum that I had been hearing in the sound was completely gone, so using one of these will likely take care of other units that may exhibit hum even after the modifications for hum that Scott had developed years ago.

The specific capacitor I chose was an AC motor capacitor rated at 470VAC made by Arcotronics and purchased from Allied Radio a month ago.

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