Hi all. I'm still very new here and hope this is posted correctly.
My Philco PT-25 has been playing well since I replaced capacitors. I turned it on earlier today and just as the audio was coming up as the tubes warmed up the pilot lamp got really bright and went out. I could hear a little drop and then return to the volume level as this happened. The set still plays just fine but I thought I remembered reading somewhere to try to keep the pilot lamp operational to help tube lifetimes.
This evening I replaced the lamp and turned the radio on and the same thing happened. The lamp has always been brightest when the power is first switched on, then dims until the tubes warm enough to start playing, and then maintained a fair brightness. I think it's odd that the lamp behaves normally until the sound starts becoming audible.
What should I do next to figure this out?
The schematic (Part 11 1940 RMS Year Book) shows the pilot lamp directly in the power and filament circuit with what I believe is the metal cased resistor pair mounted to the chassis. The resistances in this were ok when I replaced the caps.
I may not be able to get back to working on it for a couple of days. My furnace is getting worked on tomorrow... whole other problem.
Thanks in advance.
-Dave
Check that resistor which is across the pilot light. My bet is it opened up. That would be the 33 ohm section of R29. If it is the metal cased resistor that is not surprising. That is a Candohm resistor, and they do fail. Ring it back from the bulb socket and you should find one side going to one end of R29, and the other side going to the middle terminal of R29, and the resistance of R29, with the bulb removed, should read infinite, or very high if that section is bad, if it is good it should read about 33 ohms. The other section should read about 175 ohms. If a section is open, one can solder an appropriate resistor in place of the bad section. The current the resistor will see, max, would be about 0.15 amps, so 0.15 x .015 x 33 = 0.7425, you will need a 33 ohm resistor, 1 watts MINIMUM, 3 is better, to replace that section of R29 Just remember, the bulb will also be taking some of that 0.15 amps, so not all of it will be going through that resistance usually. That resistor is there to keep the radio playing when the bulb burns out, but the bulb should always be replaced as soon as possible to prevent strain on the resistor. Unfortunately the schematic does not list the bulb type, and the parts list only gives a Philco part number, but I am sure Ron, or one of the others here, more familiar with these radios than I, can tell you what the actual bulb type should be. Based on the voltage the 33 ohm resistor would see WITHOUT the bulb, about 5 volts, my guess is it is a type 47. I'm sure you can figure it out.
Correct. Bulb is a 34-2068, which is a #47, 6.3V @ .15 mA.
And if plugged into 120V AC and only rated for 115V (do not know if this is the case; check) the extra voltage will stress the bulb.
However I also am leaning towards what Mike said.
Thanks all.
I finally got around to checking out the dial pilot lamp issue. I took the chassis out of the case (again) and removed a couple of tubes to check resistors. The R29 candohm (sp?) checked out per schematic for 53 ohms parallel to the lamp and 175 ohms to the rectifier tube. So, I replaced the lamp (#47 is correct), turned the set on, and waited to see what would happen.
This time, the lamp started bright, dimmed as the filaments warmed up, came back to normal brightness as sound began coming from the speaker and stayed on as it should.
Then, I put the chassis back in the cabinet, turned it on, normal up till sound from the speaker and the lamp flashed out again.
Head scratching time...
Trying the same thing over and over again and expecting different results is the definition of insanity, right?
Now, I don't know if the following makes any sense or not. I am getting really good at taking the chassis out and putting it back in. This all started after I got this little radio working and put it all back in the case and the dial string broke!
There are three screws that hold the chassis to the cabinet. One in the front center and two on the rear about an inch in from either side of the radio. Two of these screws appear to be copper plated originals. One is a newer shiny hot dipped self-tapping screw like one might use for sheet metal. It fits and tightens up perfectly, and it is maybe a couple of mm's longer than the original two.
I have found that if I don't use the longer screw in the front center, and if I use either of the other screws, I can only tighten that center screw to just contact the case, then the lamp will not flash-pop!
I think the metal can resistor may be affected by a slight warping of the front of the chassis, where it is mounted, when the long screw is used or the short ones are over-tightened, and that may be the cause of the lamp burn out. Could it be this touchy?
Anyway. I am thinking about bypassing the metal resistor with a couple of ceramics (I think I can find a 5w 57 ohm and a 10w 175 ohm) just to see if the problem goes away. I haven't been able to think of a way to "warp" the chassis and measure the resistance at the same time yet.
For now, the radio is back together playing fine. I have been listening to long distance stations tonight. The dial lamp is working just fine also. I have turned it on and off a couple of times just to see how it goes and everything seems fine for now.
-Dave
p.s. The furnace is fine now, too.
The Candohm resistor "strips" are a known failure point. But, also check to make sure the long screw isn't contacting any components under the chassis when its installed. Sometimes things get shifted a bit under the chassis when work is done to it, but also it's quite common for a chassis screw or bolt to be lost during the past lifetime of the set, and someone puts in a substitute that is too long, and it causes a short by touching wires, etc.