Unusual for both detector diodes to go bad. FM does use a different detector circuit so this could be your problem. Most often it's the osc/mixer tube. Even though they test good may not operate on FM frequencies.
Resistor in series with selenium rectifier will be very warm. It's there to take surges and protect the rectifier. Best to replace the selenium rectifier with silicon diode. Over time voltage drop increases and these things give off a bad smell when shorted.
Norm
:hi i have been working on this set and am is great but when switched to fm there is only static the 12s8 discriminater tube checks bad on both diode tests and the 33ohm res. on the selenium rect. gets very hot when switched to fm i ordered a new tube but wonder if the diodes not working are a direct cause of the res. overheating one diode shows some emission but very little the other one nothing. i temporarily bypassed the selenium rect. with a silicon diode but nothing changed exept a slight increase in volume thanks. butch
If the resistor only gets hot when the set is put to FM, here is a likely problem that I had with a set once, using the miniature tubes: Remove each FM IF transformer from its circuit completely. These transformers are usually rated to operate at 10.7 MC, so if you see this stampped on the can, then this is an FM IF can. With quite a few of the 1950s/1960s sets that have slug tuned IF transformers, the condensers, one for each coil, will be molded into the base of the transformer (plastic part with the terminals). Check to see that neither of these condensers has shorted, and see to it that the primary of each transformer is not shorted to the secondary. Each coil should have several ohms of resistance. No resistance (zero ohms) indicates a shorted condenser across that coil. If there is any connection between the primary and secondary of a transformer that is not shown in the diagram, then there is a short. A short in a primary coil condenser will cause FM not to operate and the power supply may get warmer, and a short from a primary coil to ground (B-) will not only kill the FM, but will also cause a high drain on the power supply. On the last IF transformer, depending on the circuit, some circuits call for a condenser leading from some part of the primary coil to some part of the circuit in the secondary side. This would kill the FM if it shorted, would overload the power supply, and would blow your discriminator diodes.
Apart from the above, check for other condensers that have shorted in the FM section. Sometimes if condensers are in a multi component globule, finding the proper component is somewhat difficult. Usually these epoxy multi-section units of the 1950s and 1960s have numbers on them that correspond to the circuit diagram.
Let me look at your circuit diagram if I can find it here on Nostalgia Air, and let you know what I come up with on that discriminator circuit.
Thomas
The discriminator diodes in the 12S8 tube could cause FM trouble because of their weak condition. This would not cause the power supply to overheat. They carry an extremely small amount of current. Actually, if they are weak, this usually doesn't affect the quality of the sound at all. An extremely weak tube can be used as a detector just as well as a strong one.
Anyway, try replacing the tube just for the heck of it, and let me know what you come up with as far as condenser shorts, etc.
Also, when I was talking about multi-component globules, I originally thought your AM-FM set was from the 1950s, but from reading the diagram I see that it's from the 1940s. You're not too likely to find any multi-section components in these radios, though there were some by the late 1940s. By the 1950s and 1960s these were common, though.....sort of like early microchips.
Thomas
Other than that, all I can say is to look for shorted and leaky condensers, or shorted coils, etc. Does the resistor always get hot or just for FM? Have you tried working backward with a signal generator? You do this by starting with the last FM IF transformer with a signal generator set at 10.7 MC. Work backwards until you get no signal. Use .001 MFD condensers between the generator leads and whatever you connect them to.
Thomas
At any rate, be thorough. I cannot stress enough how important it is that no condensers have leakage of any kind. The only condensers that can have a small amount of leakage are electrolytics. These shouldn't be less than about a million ohms. All others must drop to infinity--PRECISELY where the needle starts--NO LESS. In your case, however, it sounds like you have a lot more than leakage. It sounds like you have a good short going on somewhere. Also, don't overlook some fault in the grid section of the second IF tube that may be causing this tube to draw too much current. If you absolutely cannot find any trouble with the components I listed above, and this general area, it is slightly possible that something is wrong on the other end of the tube, causing it to draw too much current. If all components check out incredibly well, though, then rule this out for now, and keep your focus where it was before...with condenser #30 and the IF transformer, and things in that area.
If you cannot find trouble, don't go modifying circuits or anything of that nature. There is something wrong. You just aren't finding it yet. The set worked when it was new. It'll work with the same circuit now if you find the trouble. Just be patient and extremely thorough. Don't be afraid to disassemble things and inspect them...just make sure you can put them back together...study them well.
Thomas