Can someone shed some light on my 800B FM front end?

[Tom Jardine]

Admittedly, I have not replaced any components on the tuner chassis yet. AM is strong and audio is good. I have replaced electrolytics and other leaky caps & OOT resistors on the amp chassis. The FM front end is querky when tuning across the dial. Stations will come in suddenly and with the slightest touch of the tuning knob and disappear as well. Basically, tuning is extremely erratic. The strange thing is the front end coils E14, E15, & E16 have little or no affect in alignment of the low end of the dial. C39, C43 & C49 do what they're supposed to do. Any suggestions? 

[Norman S Braithwaite]

If the problem persists after replacing leaky capacitors and out of tolerance resistors on the tuner chassis, consider replacing the 100pf capacitors at the input and output FM IF transmission line matching networks.  An acquaintance has recently found a bad one (tan Sangamo).  The symptom was erratic FM signal strength.  All other components known to be problematic had already been replaced.

Norman

[Tom Jardine]

Thanks, Norman. I've found lots of OOT resistors on both chassis and have replaced some leaky caps. I found the discriminator is a very tough one to align, but finally got it. I now have very good sensitivity on FM and AM. I miraculously solved the erratic FM tuning problem. I sprayed some Cramolin on the movable shaft of the tuning capacitor and the erratic tuning completely disappeared. Apparently I was getting intermittent grounding of the non-stationary tuning capacitor stator. I do want to try your tone control mod. Does it make that much of a difference? My 800B is the later (S/N3187) version, has a Thorens changer and the H H Scott dynamic scratch suppressor. It has the Magnavox coaxial speaker without the bell covering the field coils.

Norman S Braithwaite said:

If the problem persists after replacing leaky capacitors and out of tolerance resistors on the tuner chassis, consider replacing the 100pf capacitors at the input and output FM IF transmission line matching networks.  An acquaintance has recently found a bad one (tan Sangamo).  The symptom was erratic FM signal strength.  All other components known to be problematic had already been replaced.

Norman

[Tom Jardine]

The 800B is working pretty well now overall. I added the tone control mod. The sound is a overly bright on mid-range highs, even with the treble set for mid-range. Any suggestions for taming that? I checked and re-checked my work for errors. The drawing asks for a 220 pf cap on the CCW end of the treble control. I only had a 300pf, but I shouldn't think that would make much of a difference.

Norman S Braithwaite said:

If the problem persists after replacing leaky capacitors and out of tolerance resistors on the tuner chassis, consider replacing the 100pf capacitors at the input and output FM IF transmission line matching networks.  An acquaintance has recently found a bad one (tan Sangamo).  The symptom was erratic FM signal strength.  All other components known to be problematic had already been replaced.

Norman