[PAradiogeek]

Hi,

I restored the electronics for my 48-206 a month or two ago and since then I have been using it in my garage as I work on other cabinets.
However, I'm not very happy with the performance.  If I tune in a station after it powers up, I find that I have to go back to it and adjust the tuning four or five times over the next 20 minutes.  After about 20 minutes or so the tuning stabilizes.  I do not see this behavior in any of my other radios.  Is this indicative of a faulty alignment, or is it just that it was an inexpensive model with so-so performance, or something else altogether?
Thanks,
J

[Mondial]

I don't think the drift is normal, as the oscillator circuit is basically the same as many other Philco's of the 1940's. I could not find a schematic for the 48-206, but looked at the 47-204 which Ron says is similar.

The first thing I would try is a new 7A8 mixer-osc tube. If the tube is gassy, its characteristics could change as it warms up, affecting the osc frequency. 

There is not much else to the osc circuit, just the osc coil and the tuning cap. The only other part I see is the 47 pF coupling cap to the 7A8 osc grid which you might change. You could also try hitting these components with a bit of freeze spray and see if the tuning changes.

[PAradiogeek]

Thanks for the suggestions Mondial

My 7A8 tested at 96/100 on my tester, but I realize that mixer tubes can be very finicky. Unfortunately, I don't have a spare in stock, so I will have to order one. In the meantime I'll try freezing that 47pf cap and replacing it if it is affected by the temp change.

Now that I think about it, those small value caps were usually mica weren't they? I wonder if a previous owner replaced the original cap with a cap that had a different temperature coefficient than the original. If so, that might be the cause of the problem. Sound reasonable?

[Mondial]

Yes, if the previous owner replaced the cap with a poor quality ceramic, it could certainly cause the drift with temperature. 

It should be a mica or a NPO ceramic if original. Ceramic caps were just coming into general use in the late 1940's so it is possible that a ceramic might have been installed originally. If it was ceramic it would be tubular or "dogbone" style, not the usual disc ceramic caps you see today.

Low grade ceramic caps have a huge temperature coefficient, so it will be obvious when you hit it with freeze spray or heat with a hair dryer.

What freq are you tuned to when you notice the drift? The effect will be much greater at the high end of the band than at the lower freqs.

[PAradiogeek]

I'm pretty sure I've noticed the drift across the dial. My two most listened to stations are at 1210, and 610. I can't say that I've noticed more drift at the top, but I wasn't really paying that much attention. I just noticed that it did drift... everywhere.

I think I'll try testing for that tomorrow too.

Thanks again!

[codefox1]

Sometimes it is a carbon resistor that has drifted and become "noisy" and sensitive to temerature changes. You know where about to look; shouldn't be more than a few suspects. I'd just replace 'em making sure to follow original layout and lead dress. I'd also blast out the tube sockets and pins with deoxit if you haven't already done so.

[Kenneth F. Besso Jr.]

WOW,,610 in my back yard,,,in the phone(hard line) sometimes when you pick it up,,,,screams in the 81 philco ,,I redid,,hope you find the resistors

[mikethedruid]

I would look at all the capacitors and resistors in the mixer/ oscillator circuitry. If I had not done so already, I think I would replace them all with new ones of equal or higher temp and voltage ratings as the originals. After that, a re-alignment, and you should be good.

[tom.seeger]

Actually. You should never change the mica or ceramic capacitors in the oscillator tuned circuit. These are almost never NP0 or C0G caps. They are almost always chosen by the original manufacturer to drift in exactly the right direction to counter any drift by the oscillators tuned inductor/transformer. So if anyone changed them for highly stable caps thinking they were doing a good thing, well they were not.
However I am not to saying this is the case in your radio. I have seen weak mixer/oscillator tubes do a bit of this, or quit altogether when they warm up.
See if you can determine if your oscillator caps have been replaced. Good luck.

[mikethedruid]

I should have specified all the PAPER caps, although some of those old mica and ceramic can fail. In those cases where I have replaced them, I have not found any problem that a thorough alignment didn't cure. If you have replaced the paper caps, and left the resistors, it MAY be an old resistor that has become heat sensitive, which happens too.

[PAradiogeek]

Thanks for the responses everyone.

There are two mica (or at least domino-like) caps in the radio. One of them is that 47 pF cap. Spraying that cap with freeze spray after the radio warms up doesn't seem to affect the tuning. I also tried cooling other components in this circuit, but none of them being cooled affects tuning.

I cleaned the tube sockets and pins as suggested and I believe the severity of the problem has lessened somewhat. One other thing I've noticed since I've been investigating this is that the volume also seems to go down a little over the warmup period. I'm not sure if this could be related to the 'tuning' problem, could it?

Thanks,