The PHILCO Phorum

I am finishing up the restoration of a Philco 42-360.  I have replaced all of the paper caps and electrolytic caps, out of spec resistors and a couple of bad tubes. 

I have aligned it and is working nicely... well most of the time.   The oscillator tube is an XXL loctal tube and I have to wiggle it just so to get it to oscillate.    I have cleaned and deoxed the contacts and socket and tried 3 different tubes.  The result is the same.  Once I wiggle the tube or remove and reinsert it it works fine until I turn off the radio off and let it cool and turn it back on. 

That is one problem --- the other is with the volume control part 39 on the schematic.  It is a 0.5 meg ohm pot.  The volume increases as expected until it is about 3/4 clockwise.  At that point the volume drops off.  I have cleaned the the pot with deoxit.  I am wondering if it is the pot or perhaps something wrong with the AVC circuit.

I tried replacing the pot with a 0.8 megohm pot that I had on hand.  It did not work so well -- perhaps the taper was not correct.

Thanks in advance for any suggestions.

John w.

Some contacts/controls will not be "cleaned" with just De-Oxit... De-grease the socket thoroughly with an oil-less contact cleaner, then follow-up with 91% alcohol (not denatured) dry thoroughly, use a gentle blast of compressed air if available. Find a dud loctal tube, use very coarse sandpaper on the pins of this tube. The extreme roughness of the pins from the coarse sandpaper can be used as a tool to burnish the side of the socket contacts where it is needed most. Gently insert/remove the tube burnishing tool. Some brands of Loctal sockets are fragile especially the spring locking mechanics. At least ten insertions and gentile wiggling about should suffice. Using the same tube-tool apply the de-Oxit as thin film (cotton swab) on the pins reinsert and wiggle to distribute. Remove the tube-tool, then insert the working tube. That should do the trick, if not then replacement of the socket is in order. I don't envy that as it is an RF stage so there will be a plethora of connections...

The control has a flat spot or a grease laden area. It may be possible to repair but there is some risk of ruining the control. Inject that same contact cleaner in the control, back it up with a rag, flood the control and work the control dozens of times, again, compressed air. That may correct it, if not remove the control noting where the noise is by marking shaft position on the body. Take the control apart, and find the spot, if the carbon is completely gone the control is hopeless, if not it may be possible to gently bend the contact arm to an unused area,. Test with the ohmmeter for success. Again, a bit of de-oxit as a film on the carbon will be fine.

If the control is spoiled, Oldradioparts.net will have a NOS one, he is the control guru, Mark Oppat.

Good Luck!

Chas

Thanks Chas for the help.

I replaced the 400K volume control pot with audio taper with a 500K pot with some unknown special taper.  It seems to work just fine now.  I think I will open the old one to try and repair  (just for the experience). 

Of note: the original Philco volume control has one side of the pot grounded to the body of the control.  So when it is attached to the chassis one side of the pot is grounded.  With the replacment pot that is not the case... so at first it did not work -- took me awhile to figure that out.  I ran supplemental ground to the appropriate leg of the replacement pot and it works just fine now.  Perhaps that is common knowledge to the Philco Phorum gang. 

I also bent the pins on the oscillator tube just slightly -- in hopes they would make better contact.  So far that maneuver is working -- I will need to cycle the radio on and off numerous more time to know for sure since it was an intermittent problem.

Thanks again for the help.

John w.