Hi All!
I think I've seen it all now! I have a Philco Transitone 48-200 that I restored, it would play great for awhile then drift off frequency, first thing to check obviously would be a problem in the oscillator or AVC sections, I replaced the parts I suspected, same result, one strange thing I noticed all along was the radio would not come up on station, even though I shut it off with it tuned in, well after exhausting the possibilities with the oscillator, AVC, and the IF sections, I took a chance on checking the 14B6 tube again, which by the way is the Detector/first audio/AVC, and found out from a cold start the tube was extremely unstable, then as it warmed up it would straighten out somewhat, I have never seen a thermally sensitive tube until now, what a weird problem! I since replaced it with another 14B6 which tested good and was rock solid stable from the very get go. Just thought I would post this strange situation for any others who may experience the same problem.
Mark

Strange that the audio/detector would cause such a problem. This tube has absolutely nothing to do with frequency stability. I would think that it'd be a drifting condenser either in the oscillator or IF section. For instance, in a very small Sentinel radio of mine, I replaced the original IF micas with ceramics. The radio probably drifts about 5 KC during warm-up. Doesn't bother me much, though.

T.

:Strange that the audio/detector would cause such a problem. This tube has absolutely nothing to do with frequency stability. I would think that it'd be a drifting condenser either in the oscillator or IF section. For instance, in a very small Sentinel radio of mine, I replaced the original IF micas with ceramics. The radio probably drifts about 5 KC during warm-up. Doesn't bother me much, though.
:
:T.
Hi Thomas
Well Thomas after further checking I found that a bypass capacitor was missing in the IF Section, apparently someone else has worked on this radio in the past and did not bother too put it back in, it is C302 on the Philco 48-200(121) Schematic, it is a .1uF, now the radio comes on and on station, further it does not drift anymore, unbeleivable what some people do too these radios!, thanks for the tip about the capacitor in the IF, guess I should have looked more closely at the schematic, must have just overlooked it:)
Mark

I wonder which is more stable: a silver-mica cap or a ceramic NP0 cap? I've used silver-micas for tuned ckts, but I assume that NP0s would be cheaper.

Ceramics probably aren't as stable as micas. I've heard that they aren't. Could be other things in my radio, but after replacing the micas with ceramics, the radio drifts an obvious amount.

T.

:Ceramics probably aren't as stable as micas. I've heard that they aren't. Could be other things in my radio, but after replacing the micas with ceramics, the radio drifts an obvious amount.
:
:T.
Hi guys!
Well, earlier I had stated I found a bypass capacitor that was missing and replaced as per the schematic, thought I had this problem solved, so nice to find out I don't, still drifts off station, and when turned off when tuned into a station, then powered back on after sitting for a period of time all I get is static, no station, any help would be greatly appreciated at this point, I have no idea what is causing this.
Mark

Well, the condenser you talk about, .1 MFD, is definitely not inside of the IF can. I'm talking about the small mica condensers (fixed or trimmer type) within the can. They're usually around 200 MMFD. The condenser you talk about probably won't cause frequency drift. Things that cause frequency drift are oscillator tubes, IF transformers and their condensers, oscillator condensers (any small value condenser that's associated with the oscillator coil), and RF coils (including the oscillator coil...though wax covered coils don't usually move much at all unless heated so that the wax melts). Occasionally tuning condensers can cause frequency drift, but they have to be near a very hot tube. You can check your oscillator condensers for leakage. I don't recommend replacing them unless they're leaky. They don't usually drift. You can try substitution to see if the problem goes away. It is slightly possible that one could be opening up, causing instability, especially during temperature changes. I more strongly suspect condensers within the IF cans. If these are trimmers, the plates can move and change shape slightly when warming up. This changes the intermediate frequency, and changes the incoming frequency necessary to combine with the oscillator frequency in order to pass through the IF circuitry. There is nothing you can or should do to fix this (well, technically you could parallel the trimmers with very small value condensers that drift the same amount in the opposite direction...this would cure the problem perfectly). If the cans contain no trimmers, fixed condensers can cause trouble depending upon their design. If the drift is significant for local stations, you may have to look for trouble. If it is only slight, and only affects distant stations, don't worry about it unless the radio lacks sensitivity, or is irratic sensitivity wise. Most of my super heterodynes do not drift at all. Some do, but they perform very well otherwise. Their drift is insignificant and is always the same. My RCA 55X2 drifts slightly during warm-up (perhaps by 2 or 3 KC). My car radio (1949 Mercury radio by Zenith....I drive a Chevrolet, not a Mercury...wish I had a Mercury) drifts significantly sometimes, which makes re-tuning after using push buttons necessary when the radio is cold (I adjust the buttons when warm). Considering what environment the radio must work in, though, drift is understandable. Drift is especially noticable when it is around 10 degrees outside.

Drifting is an inherent problem with super heterodynes, though. It is something that is quite possible due to circuit design....if the oscillator drifts or the intermediate frequency transformers drift, there's going to be station drift. This is not a problem so much with TRF receivers. Better super heterodyne radios have compensating components built into them. Not all do, though. Again, unless the problem is quite severe, the drift in your radio could be normal.

T.

:Well, the condenser you talk about, .1 MFD, is definitely not inside of the IF can. I'm talking about the small mica condensers (fixed or trimmer type) within the can. They're usually around 200 MMFD. The condenser you talk about probably won't cause frequency drift. Things that cause frequency drift are oscillator tubes, IF transformers and their condensers, oscillator condensers (any small value condenser that's associated with the oscillator coil), and RF coils (including the oscillator coil...though wax covered coils don't usually move much at all unless heated so that the wax melts). Occasionally tuning condensers can cause frequency drift, but they have to be near a very hot tube. You can check your oscillator condensers for leakage. I don't recommend replacing them unless they're leaky. They don't usually drift. You can try substitution to see if the problem goes away. It is slightly possible that one could be opening up, causing instability, especially during temperature changes. I more strongly suspect condensers within the IF cans. If these are trimmers, the plates can move and change shape slightly when warming up. This changes the intermediate frequency, and changes the incoming frequency necessary to combine with the oscillator frequency in order to pass through the IF circuitry. There is nothing you can or should do to fix this (well, technically you could parallel the trimmers with very small value condensers that drift the same amount in the opposite direction...this would cure the problem perfectly). If the cans contain no trimmers, fixed condensers can cause trouble depending upon their design. If the drift is significant for local stations, you may have to look for trouble. If it is only slight, and only affects distant stations, don't worry about it unless the radio lacks sensitivity, or is irratic sensitivity wise. Most of my super heterodynes do not drift at all. Some do, but they perform very well otherwise. Their drift is insignificant and is always the same. My RCA 55X2 drifts slightly during warm-up (perhaps by 2 or 3 KC). My car radio (1949 Mercury radio by Zenith....I drive a Chevrolet, not a Mercury...wish I had a Mercury) drifts significantly sometimes, which makes re-tuning after using push buttons necessary when the radio is cold (I adjust the buttons when warm). Considering what environment the radio must work in, though, drift is understandable. Drift is especially noticable when it is around 10 degrees outside.
:
:Drifting is an inherent problem with super heterodynes, though. It is something that is quite possible due to circuit design....if the oscillator drifts or the intermediate frequency transformers drift, there's going to be station drift. This is not a problem so much with TRF receivers. Better super heterodyne radios have compensating components built into them. Not all do, though. Again, unless the problem is quite severe, the drift in your radio could be normal.
:
:T.
Hi Thomas
Well I think I've put my finger on the problem, the way this radio is designed and the way the tubes are layed out spell trouble, one tube in particular, the Rectifier Tube is located right next too the Tunning Condensor, so my guess is as the tube heats up it is changing the condensor enough too cause the drift, when I say it is next too the condensor I mean right up against the body of the Tunning Condensor and one of the associated trimmers, unfortunately there is not much that can be done, its just a bad placement of the tube and condensor the way I see it. As far as any condensors in the IF cans there are none, except for the trimmers, however one of the IF Cans does have a 47K resistor in it, unfortunately it is not the problem, I checked it and it is right on the money as far as resistance, guess I'll just have to live with this problem, not much too do about it as far as I see.
Mark

The rectifier next to the tuning condenser is most likely your problem. The resistor within the 2nd IF transformer will not cause frequency drift. This resistor simply mutes some of the high frequencies...noise and such that you don't want to hear. Actually, I'm learning more and more as I read more of these old technical radio books. I am not so sure anymore that resistors actually mute high frequencies (I read this somewhere else). From what I read, resistors simply lower the current available from the audio signal, giving it less force (voltage remains the same, though due to the lower current, it can be loaded down more easily). Capacitances in the circuitry which cannot be avoided (such as element to element tube capacitance) are what actually mute the high frequencies. Placing a resistor in series with the audio signal allows capacitance to react on it more readily, which mutes the high frequencies. Whatever the case may be, resistors tend to make a circuit softer and more spongy, which affects the high frequencies...when put in series with an audio signal (or any signal). When paralleling an audio signal (or any signal), they shorten the discharge time for all capacitances within the circuit, making the circuit favor low frequencies less, which is why shunting resistors reduce bass. This really has nothing to do with your problem, but since you brought up that tone muting resistor, I thought I'd share this new-to-me information that I just found out.

Regarding the trimmers in the IF cans, these can be affected by heat, too....they change position slightly due to expansion of the metal pieces. Your problem is most likely due to a heated tuning condenser and its trimmers, though.

Thomas

:The rectifier next to the tuning condenser is most likely your problem. The resistor within the 2nd IF transformer will not cause frequency drift. This resistor simply mutes some of the high frequencies...noise and such that you don't want to hear. Actually, I'm learning more and more as I read more of these old technical radio books. I am not so sure anymore that resistors actually mute high frequencies (I read this somewhere else). From what I read, resistors simply lower the current available from the audio signal, giving it less force (voltage remains the same, though due to the lower current, it can be loaded down more easily). Capacitances in the circuitry which cannot be avoided (such as element to element tube capacitance) are what actually mute the high frequencies. Placing a resistor in series with the audio signal allows capacitance to react on it more readily, which mutes the high frequencies. Whatever the case may be, resistors tend to make a circuit softer and more spongy, which affects the high frequencies...when put in series with an audio signal (or any signal). When paralleling an audio signal (or any signal), they shorten the discharge time for all capacitances within the circuit, making the circuit favor low frequencies less, which is why shunting resistors reduce bass. This really has nothing to do with your problem, but since you brought up that tone muting resistor, I thought I'd share this new-to-me information that I just found out.
:
:Regarding the trimmers in the IF cans, these can be affected by heat, too....they change position slightly due to expansion of the metal pieces. Your problem is most likely due to a heated tuning condenser and its trimmers, though.
:
:Thomas
Hi Thomas
Yep, thats what I've concluded as well, after a period of time, when the the radio has reached its peak, so too speak, as far as the heat issue is concerned, it is completely stable and stays playing loud and clear, its a great looking Philco Flying Wedge Radio in a wooden cabinet, just a shame that they had too lay out the tubes the way they did, oh well, just have too live with it, thanks for the info on the resisitor usage, very interesting.
Mark