Gentlemen,
I am currently working on a 1932 Philco model 51. I have changed all the capacitors, most of the resistors and checked the tubes. Sometimes when I apply power the radio will not play. The only way to get it to operate is to momentarily short pin 2 of the oscillator and 1st detector tube (# 224) to ground. Then it comes alive and operation seems to be normal. I just discovered this by accident when I was checking voltages on that tube and it is very repeatable. Works every time. I understand that this radio has some weird characteristics but this should not be required to get it to play. Anyone have any ideas to solve this problem?? thanks
walt
Moved thread to Electronic Restoration section.
I'll bet you can guess...
Swap the 24A tube for a different one.
You can try lowering the value of #35 a K or two if that doesn't do it.
I'd pull the oscillator coil and give it the hairy eyeball looking for green dots on it. Then I would rewind the primary just to eliminate the possibility of it being intermittent.
Thanks for the suggestions . But I'm not sure which is the oscillator coil. Is that the one mounted to the side of the chassis ( # 12 on the schematic) or is that the 1st IF transformer? And by looking for green dots on it I guess that is corrosion I'm looking for? as you can see I'm pretty new at this. thanks for your help.
walt
#8 is the osc coil.
I would as Terry said start from swapping the 24 tube. If it oscillates but does not start well, it is either the tube or maybe the bobbin of the coil needs baking.
I doubt even with green dots those would be the reason as it does not look intermittent if it starts every time with that pin 2 to GND.
Also have a look at the service notes on pg 2-11 in Rider's.
Thanks so much for your inputs. Sounds like I ought to pull the coil and have a look at it after I swap the tubes. Here again, I am confused as to what the green dots mean , that you refer to.
walt
(12-16-2016, 04:14 PM)wally73 Wrote: [ -> ]Thanks so much for your inputs. Sounds like I ought to pull the coil and have a look at it after I swap the tubes. Here again, I am confused as to what the green dots mean , that you refer to.
walt
If you can see any green areas on the copper wire on the coil it's a good bet that it will be open or intermittent. Most of the time it's on the smaller outer winding. Has to do with a reaction with the wire and it's insulating materials causing it the rot.
AAAAHHHH that's what I figured but wasn't sure. I'll check it out . Nice to talk to the experts. thanks
walt
OK , I pulled the oscillator coil and inspected it . I cleaned it up with a little alcohol and a soft bristle brush. It seems that there are tiny microscopic green "dots" but I know I am not going to attempt to rewind that beast. Looks like a job for a brain surgeon . I didn't pull out the inner coil but I checked all contacts for continuity , put it back in the radio. Swapped # 24 tube with another one I had . The only thing I haven't tried is to reduce # 35 resistor from 8000k to 6000k since I don't currently have one.
The radio essentially behaves as before. Sometimes (not every time) when I first apply power I have to short pin 2 of # 24 tube to ground and it fires right up . I may have to live with it that way . If you all have any further suggestions I'd welcome them . I'll even get a 6000k resistor if you think it would help. thanks
walt
A lot of those coils aren't too bad to rewind. A little patience. There are quite a few threads here that document this, and may even be one on your coil. But, I would recommend swapping tubes, maybe even replacing with a new one, or even a couple different brands. Osc circuits can be kinda quirky, and sometimes it takes trying several different tubes to get it to work. Doesn't necessarily mean the tube is bad either, as it may work fine in another radio, or stage.
The resistor could be redced by parallelling a 25K resistor to it. Doesn't have to be precise.
OK , I'll look for the coil winding threads on the forum and can I find the Rider service notes on-line somewhere?
walt
As Tim mentioned these early tetrode mixers can be a bit fiddly. Reading the service notes tells me that it's a bit touchy in regard to the wiring layout. Sometimes doing an alignment to the IF and osc stages will help after you've done some parts replacement as these are slightly different from the originals. That's the long answer.
If you want to experiment a little take two pieces of insulated hookup wire and make a gimmick capacitor by twisting them together like 3-4 times. Connect by soldering(clip leads add stray capacitance and inductance) one wire to what you where touching to make the set play and the other to the chassis. Add more twists and see if you can get the osc to start reliably. If you get a bunch of twists and it works ok you could replace it with a trimmer cap.
Have Fun!