[Raleigh]

I have mostly completed restoring the electronics of a 40-150. It was a mess when I started, with some previous work I had to rip out. I restuffed the electrolytic capacitors, replaced all of the paper caps, most of the mica caps and many resistors. Once I brought it up, it operated well, but it had one odd problem. The frequency shifts abruptly. For example, I would be listening to WPTF in Raleigh at 680 AM. Suddenly the station would go away as if I had tuned to an open place on the dial. I re-tuned, and WPTF would be at about 620 or so on the dial. The same thing seemed to happen with other stations. For example WDOX would shift from 570 to about 530 or so. The sift to lower frequencies would (mostly) not be reversible until the radio had been shut down and started at a later tiime.

This is not a drift, but an abrupt shift in frequency.

When I was working on the chassis, I could often make this downward shift happen by wiggling he tuning condensor, but usually no amount of wiggling would restore to the original frequency of 680. Once the radio is off for a while, the frequency woften starts as it is supposed to be, but sometimes it starts shifted down 20 or 30 KHz.

My question is--does anyone recognize these symptoms? Does it have anything to do with the grommets that isolate the tuning condsensor from the chassis? The grommets all seem to be worn out, allowing the consdensor to be pulled a little out of true by the pulley and tuning cords.

Any advice would be appreciated.

[planigan]

Raliegh, it sounds like your on the right track. Since its a sudden shift, not a drift, I would think it's mechanical and may also be heat related since it resolves itself if radio of for a while. The only other thing I can think of is a tube shorting/opening when it heats up. Try substituting tubes in RF curcuit and replacing grommets. PL

[Raleigh]

Thanks, planigan. I figure the problem must be in the local oscillator/mixer circuit because when the frequency shifts there is no reduction in volume, so the IF must be the same.

I've replaced most of the rubber-insulated wires. There are a few that are both hard to get to and still have supple insulation, so I didn't replace those. If I wiggle those wires I don't get the frequency shift effect. I've also replaced all of the capacitors associated with the oscillator circuit.

I can often get the frequency shift to happen if I wiggle the tuning condenser and sometimes if I only touch it, so I am focusing on that component as the problem. As you say, it may be affected by warm-up.

Does anyone know where to get replacement tuning condenser grommets for the 40-150? Renovated Radios has some grommets for Philco tuning condensers at this site http://www.renovatedradios.com/parts.html#rubber but at 7/8 inch, they seem way too big for this radio.

Also, can you use anything besides compressed air to blow out the tuning condensers? I've cleaned it as best I can. If I knew of a product that would clean it but not hurt anything I would try it.

By the way, I did replace the mixer tube and the RF amplifier tube. I thought that since the volume isn't affected by the frequency shift, the IF should be the same, so I didn't replace the IF amplifier, detector, or audio amplifier tubes. I think I have spares so I could replace all of the tubes just to see.

[TonyJSenior]

If your grommets are worn I would replace them. They isolate the frame of the tuning condenser from electrically contacting the chassis. I had a set of worn grommets on a 51-932 and while it didn't cause the drift as in your case, it did cause the audio to become distorted, much like it was a bit off station and overloaded sounding. You should be able to take a quick resistance check between the chassis and the tuning condenser to see if in fact they are worn to the point that the condenser is making contact and possibly causing your issue.