Restoring the FM Front End on the 800B

[Joseph W Strickland]

Today I worked on the FM Front End. There was an intermittent connection at the plug to the IF circuit, so some cleaning using Deoxit took care of that along with multiple insertions and removals of the plug. There were lots of static crashes and pops in the sound from the front end. Unplugging the IF strip brought complete silence, so the noise was definitely coming from the front end.

R51 which feeds B+ to the RF amplifier was burned. I replaced it with a new 1 K ohm 1/2 watt resistor. I also replaced C47, a 390pF silver mica which I believe was the cause of the burned resistor. I used a .01uF disc ceramic capacitor there. I found that R49, a 56K ohm was reading way high nearly 4 meg ohms. I replaced it with a new 56K 1/2 watt resistor and also changed C41 a 390pF silver mica with a .01 disc ceramic.

R55 a 220K ohm had increased to 979K ohms, so it was replaced by a new 220K 1/2 watt resistor. I plan to replace C51 a 300pF silver mica with another .01 disc ceramic capacitor tomorrow when I have better outside light. That will be followed by replacement of R56 a 2.4K 1/2 watt resistor and C53, a 6200pF will be replaced with another .01 disc ceramic.

Already the FM signal is now stronger and more noise free - just some minor residual noise. I will also try try another set of tubes.

Joe

[Joseph W Strickland]

Yesterday late, I tried adjusting the trimmer capacitor, C43, a 3-25pF air trimmer with a shaft that has a screwdriver slot. The high end of the FM band was somewhat low as to where it indicated a given station versus its actual frequency. This is something that can change when an oscillator tube is changed. When I tried to adjust it the small hex-nut that surrounds the shaft popped off. This released the spring tension that maintained a good ground for the stationary vanes of the capacitor. Unfortunately the moving parts were no longer stable and the distance or spacing between vanes was no longer stable either. The nut that popped off has no threads on it. It turns out that it is pressed onto the shaft with the back tension spring in compression so that the vane to vane spacing is correct.

After several attempts to press the nut back on the shaft and get correct vane to vane spacing failed I realized that the only repair that would be successful would be to replace the trimmer capacitor. This is not an easy task, although there is easier access to the inductor and capacitor leads in this chassis than some I have worked on. The part number shows to be identical with the trimmer capacitors used in the AM RF tuning according to the parts list. However if you look closely you will see that as installed these are not identical. For instance the same capacitor is used in the Mixer and RF amp. circuits of the FM front end. The ones used there have one post on the rear of the capacitor that is long on the side toward the rear of the chassis, while the one toward the front is trimmed back. For the FM Oscillator circuit it is necessary to trim back that long post in order to use it, otherwise it may bump against the oscillator inductor which is physically very close to the capacitor. I trimmed it back by using a micro-file. I did not want to use wire cutters as I was afraid the shock could shatter the ceramic material that the body is made from.

Once I got the part in place, soldered to the inductor and 100pF capacitor, C44, that couples the signal to the grid of the oscillator tube, I installed the tube and turned the set on. This time I was able to easily adjust the high end tracking of the oscillator with no problem and the adjustment remained stable with vibration etc.

These trimmer capacitors are normally sturdy, but being a press fit, the nut that is pressed onto the shaft of the capacitor can come off and it is not likely that it can be put back successfully. It seems that once it comes off, there is a looseness that will not go away if an attempt is made to put it back on.

If I had not had a parts unit, I would never have gotten this up and operating again without finding a new or other used capacitor made the same and of the correct capacitance range.

So far the additional parts changed have not corrected the small amount of residual intermittent noise I hear on quiet passages on FM. I may try another round of cleaning the tuning capacitor (better grounding of the main tuning shaft). The only resistors that have not been changed are R26, R48, R50, R53. The 5 ohm grid resistors are not likely the cause of a problem as they have no real power applied to them. I may change R48, the cathode resistor of the RF Amp. tube and R50, the grid leak resistor of the oscillator as it has drifted higher in resistance and check again for residual noise. Another set of tubes made no difference at all.

Joe