Philharmonic : motorboating and harmonics

[Ken Carr]

My Philharmonic is running since I installed a new voltage divider.

I am still in the process of replacing capacitors. All power supply capacitors are replaced and about half of the receiver caps. The four-part electrolytic on the receiver has been re-stuffed.

The radio motorboats over segments of each band. The sound can be affected by the volume control. Also, one local strong station shows up as harmonics at several frequencies to either side of the primary frequency.

I know I should just be patient and replace all those capacitors but if anyone knows a likely source of the motorboating I would like to address it right away.

The voltage regulator tube, VR100, and the RF converter tube behind it do not have any shields at all. All of the tubes on either side of the main tuning capacitor have inner and outer shields.

Suggestions?

[Ken Carr]

Update:
After more investigation I think the problem described above may be in the circuits feeding the first RF tube, a 6u7G.
If I place my finger on the grid cap of this tube all motorboat noise stops and even the weakest, previously unheard, stations come in clearly. Touching the cap with a non-conductive wooden rod has no effect.

The tube tests fine. The 1k resistor to 100V is fine. The paper caps in the main tuning variable capacitor were replaced.
There appears to be a 25 ohm resistor attached to the grid cap. I believe I will have to remove the cover to the main tuning capacitor to get to it again. Before I do this (tedious) does it seem I am going in the right direction or can someone suggest a more likely source of the problem?
Thanks.

[Norman S Braithwaite]

Ken:

It sounds as though you may be encountering a problem similar to one that I recently addressed.  The RF AGC circuit is designed to limit the output of the RF amplifier stages to a level within the normal operating parameters of the first detector/converter tube.  If the RF AGC circuit is not working the output of the RF amplifier will greatly overwhelm the converter tube or if improperly working may well cause motorboating that would likely be stopped by placing a finger on the first RF tube.  The RF AGC circuit, however, derives its control voltage from after the 2nd IF transformer therefore the first two stages of the IF amplifier must be reasonably well aligned for the RF AGC circuit to work.  Before beating yourself up trying to fix the problem of unknown cause, I strongly recommend aligning the IF amplifier.  Hopefully this will resolve the problem but if not a likely cause will be eliminated.

Norman

[Ken Carr]

Norman - Thanks for the suggestion. I will give it a try. It bugs me to be so close. Each time I come up with a new 'solution' I get a little closer. Hopefully the alignment will do it. I have your excellent hookup drawing to use for the setup.
Thanks again!