Philco model 42-327,this AM/SW works good except the am quits working , after checking it it appears by all accounts to be a bad solder joint at the oscilator coil, the terminal in question has the antenna transformer, and a couple mica caps attached to this joint, at first i thought was bad conection at the osc coil, so i removed and cheked it out found that it had good conection so I cleaned up the old solder and re soldered the caps and the antenna transformer ( this to appears to be ok) to it, it will play for hours but then slowly fade away, it also kills the push buttons as well, but SW play fine, if I take a screw driver and just touch the connection(at the osc coil) it pops back on, what the heck can it be? I have resoldered this connection and the other componets attached to this and corasponding joints but still the same problem sometimes will play for hours and sometimes only a few minutes, but either way I just lightly touch the lug and the radio comes back to life could it be one of the micas? since there is no way to check them and unfortuneatly I do not have any lying around to sub them so I have not persued this avenue as of yet but could it be one of these caps? Any ideas is appreciated.
Thanks

It is easy to check any condenser of any value for leakage. Just use the most sensitive setting on your resistance check positions on your multi-meter. It say something like X 10k or X 1k, which means multiply reading by 1 or 10k, whichever your's says. You should be able to touch the leads gently and read the resistance in your hand. When testing a condenser of any value, you want the needle to go back down to infinity (where it started). With larger condensers, the needle will jump up and fall back down as the condenser charges up with the current from the meter. With really small condensers (about .001 mfd or less), the needle may not jump up at all with a good condenser, as the charge-up period of these small units is really short. However, with any condenser, if the needle goes up, it must come back down. If it's a really small condenser, and the needle goes up, this is likely a leak, and you should pay close attention to see that the needle goes EXACTLY back to where it started. If the needle stays up even at several million ohms, this is enough to throw many circuits off, as the biasing of many tubes is received through several megohm resistors. If your radio drifts around volume wise, then you need to check for leaky condensers in the RF section, and also check for drifted resistors, especially in the AVC circuit and any grid biasing circuits. Your trouble could also be caused by a gassy or weak tube. Sometimes oscillator tubes do this when they are failing.

I described a better condenser tester a couple of months ago. The procedure I described above is usually all you need to find a bad unit. If you wish, I'll explain the condenser tester circuit to you, though. It's easy, and if buit as a solid state unit instead of the tube I originally suggested for the rectifier, all the parts are available at Radio Shack for very little money.

Thomas

:Philco model 42-327,this AM/SW works good except the am quits working , after checking it it appears by all accounts to be a bad solder joint at the oscilator coil, the terminal in question has the antenna transformer, and a couple mica caps attached to this joint, at first i thought was bad conection at the osc coil, so i removed and cheked it out found that it had good conection so I cleaned up the old solder and re soldered the caps and the antenna transformer ( this to appears to be ok) to it, it will play for hours but then slowly fade away, it also kills the push buttons as well, but SW play fine, if I take a screw driver and just touch the connection(at the osc coil) it pops back on, what the heck can it be? I have resoldered this connection and the other componets attached to this and corasponding joints but still the same problem sometimes will play for hours and sometimes only a few minutes, but either way I just lightly touch the lug and the radio comes back to life could it be one of the micas? since there is no way to check them and unfortuneatly I do not have any lying around to sub them so I have not persued this avenue as of yet but could it be one of these caps? Any ideas is appreciated.
:Thanks

Thomas, I understand you can check caps with a good VOM, but as far as micas are concerned I was always told you cannot trust a VOM to give a accurate reading, as a cap could read infinity, but not proof that it is good , I thought to really check a mica that subsitution was the only way? if not I just learned something new.?
:It is easy to check any condenser of any value for leakage. Just use the most sensitive setting on your resistance check positions on your multi-meter. It say something like X 10k or X 1k, which means multiply reading by 1 or 10k, whichever your's says. You should be able to touch the leads gently and read the resistance in your hand. When testing a condenser of any value, you want the needle to go back down to infinity (where it started). With larger condensers, the needle will jump up and fall back down as the condenser charges up with the current from the meter. With really small condensers (about .001 mfd or less), the needle may not jump up at all with a good condenser, as the charge-up period of these small units is really short. However, with any condenser, if the needle goes up, it must come back down. If it's a really small condenser, and the needle goes up, this is likely a leak, and you should pay close attention to see that the needle goes EXACTLY back to where it started. If the needle stays up even at several million ohms, this is enough to throw many circuits off, as the biasing of many tubes is received through several megohm resistors. If your radio drifts around volume wise, then you need to check for leaky condensers in the RF section, and also check for drifted resistors, especially in the AVC circuit and any grid biasing circuits. Your trouble could also be caused by a gassy or weak tube. Sometimes oscillator tubes do this when they are failing.
:
:I described a better condenser tester a couple of months ago. The procedure I described above is usually all you need to find a bad unit. If you wish, I'll explain the condenser tester circuit to you, though. It's easy, and if buit as a solid state unit instead of the tube I originally suggested for the rectifier, all the parts are available at Radio Shack for very little money.
:
:Thomas
:
::Philco model 42-327,this AM/SW works good except the am quits working , after checking it it appears by all accounts to be a bad solder joint at the oscilator coil, the terminal in question has the antenna transformer, and a couple mica caps attached to this joint, at first i thought was bad conection at the osc coil, so i removed and cheked it out found that it had good conection so I cleaned up the old solder and re soldered the caps and the antenna transformer ( this to appears to be ok) to it, it will play for hours but then slowly fade away, it also kills the push buttons as well, but SW play fine, if I take a screw driver and just touch the connection(at the osc coil) it pops back on, what the heck can it be? I have resoldered this connection and the other componets attached to this and corasponding joints but still the same problem sometimes will play for hours and sometimes only a few minutes, but either way I just lightly touch the lug and the radio comes back to life could it be one of the micas? since there is no way to check them and unfortuneatly I do not have any lying around to sub them so I have not persued this avenue as of yet but could it be one of these caps? Any ideas is appreciated.
::Thanks

This is very true. When you test condensers with a VOM, you are not checking capacitance, but merely leakage. If the condenser is open, and it's small enough, you won't know it. With larger value condensers (.001 mfd and higher), you can usually watch the needle jump up just a hare's breath, and fall back down, if you watch really closely. If you want to know about a condenser being open, you can either use substitution, or you can get a condenser checker (AES sells them). Keep in mind that with radio frequency circuits, you can't parallel condensers in some circuits, because you'll change tuning.

T.

Well as far as this radio is concerned, I figured one of these caps was messing up and when I touch the lug with a screwdriver I am somehow discharging one of them and the radio comes back to life , could this be a true statement?, While the radio is working I cannot make it not work by tapping around the area, so this was the only conclution I could come up with , is it a possibility?
E.
:This is very true. When you test condensers with a VOM, you are not checking capacitance, but merely leakage. If the condenser is open, and it's small enough, you won't know it. With larger value condensers (.001 mfd and higher), you can usually watch the needle jump up just a hare's breath, and fall back down, if you watch really closely. If you want to know about a condenser being open, you can either use substitution, or you can get a condenser checker (AES sells them). Keep in mind that with radio frequency circuits, you can't parallel condensers in some circuits, because you'll change tuning.
:
:T.

This is very true. You could have a leaky condenser, that is slowly charging a circuit incorrectly (leaking B+ from one stage to another, or leaking AVC voltage to ground), or you could have a very high drifted resistor. You could even have a faulty tube (gassy, weak, weird other phenomenon), but I'd rule this out until everything has been gone over with a fine toothed comb. A problem I had with my Zenith portable (5G-500) was a leaky ballast resistor. Never would have guessed the problem had I not dug in at the unlikely components. The chassis, being an AC-DC radio, is isolated from the line through a .1 mfd condenser. The chassis always showed a rather high direct current voltage of about 60 volts or so as compared with B-. Sometimes the set would break into oscillation and stop working, and every time I shorted out the chassis condenser, it'd come back to life. Don't know why this had any affect on the circuits because nothing else connects to the chassis. The over all field affect must have been doing something, though, and all of the loctal tubes receive their shielding value through the chassis (the shell keyway on each socket is connected to the chassis). Well, one day I decided to go about things differently and search out the unobvious. I removed all connections to the filament balast resistor (this is a 3 terminal resistor that drops down the rectified DC from the 117Z6 tube to about 6.5 volts for the 1.5 volt battery tube filaments that are all connected in series). When I tested from each lead of the resistor to its metal case (which attaches to the chassis), I found leakage! Interesting! So evidently the insulation in the resistor broke down. I, being the originality freak that I am, took the hard way, and instead of simply replacing the resistor, I bent open the metal housing of the original resistor and replaced the insulation with some sheet asbestos. As things turned out, I would have been better just replacing the resistor, because now it looks like hell. Well, the radio works well, though.

So, yeah, check for stray voltages, resistors that have drifted or opened up. Those that connect to the control grid of a tube bias the grid, and if they drift high, the grid will eventually collect so many electrons from the cathode that it will become saturated, and the set will stop. Touching the grid will relieve some of the electrons through your finger, and set operation will begin again. Keep in mind that other things do this when resistors are drifted, or condensers are leaky.....not just grids.

Sometimes a screen grid buffer condenser can open up and allow the screen grid to oscillate. Screen grids, when connected indirectly to B+ through a resistor, can drift and throw a tube into oscillation if there isn't a condenser of significant value from the grid to B- to keep voltages stable.

Let me know what you are touching to get the radio to come back to life. Let me know if other terminals have the same results. Let me know the model number of your radio so I can look up the diagram (I forgot it if you already gave it).

T.

P.S. You know how in the old days they used to bias the grids negative with a battery? Well, another method used with output tubes puts a resistor in series with the cathode so that the cathode is slightly positive with respect to B-. That biases the grid negative. Understandable.

Still another method is used in high impedance circuits, where you find the cathode tied directly to B-. I don't know if I ever explained this to you before, but by very nature of tube operation, as electrons stream past the control grid of a tube to the plate, some collect on the grid. If a condenser passes audio or RF currents to the grid, the grid will receive these currents, but as you know, condensers don't pass direct current. That said, the electrons that build up on the control grid from natural tube operation will eventually build up to such a degree that the grid is so negative that the tube stops functioning. If a resistor is placed from the grid to B- of a relatively high value, some of the electrons can be leaked off, and the grid can be kept at a fairly constant negative level (with no signal applied). Often the grid biasing (leak) resistor that you find on the first audio amp. tube will be around 1 to 10 meg. On the primary grid of the oscillator tube, it'll be more around 15 to 100k. (Some circuits lay out the oscillator differently, and no resistor is used.) If the resistor specified for a certain tube drifts in value, the grid will be allowed to go more negative. If the condenser that connects to the grid leaks, whatever voltage is on the other side of this condenser will also affect the grid voltage either positively or negatively, depending on what's on the other side. So if either the resistor or condenser are faulty, this will affect the bias of the grid.

The antenna grid on the oscillator, however, receives its negative voltage through the antenna coil from the AVC circuit. The AVC circuit serves as a sort of negative C battery. The antenna coil is of such low impedance that electrons cannot normally build up on the antenna grid.