Hi all. I have just finished recapping this radio. New resistors where needed, all paper caps, lytics, etc. I replaced all rubber wiring and was careful as to lead dress and placement. I took out the push button mechanisms in order to do a thourough job. On first power up after recapping, R36 (1,000 ohm) began heating up and smoking after about 15 seconds. I did not replace this first time around as it tested good. I replaced it with a 2 watt that I had on hand. It still is heating up. Rather than undoing all the tuning stuff again needlessly, can someone point me to possible problems to look for? I did not power this radio up when I first got it, I just jumped right into it. But I did notice that the electrolytics had been changed sometime ago and that the neg. of the 12uF cap had been grounded. (I fixed this). Since I don't know if the radio ever worked after this mod was done, could this have done some damage I am not aware of? Thanks for any help!
Welcome.
1. Have you checked all your tubes? In particular 7J7? Maybe shorted. If not, see if anything around it is shorted.
a)Try to remove the tube and measure from the Plate of this tube (or from the end of that 1K resistor closest to it) to GND (chassis). See if there's a short.
b) If there was no short put the tube in and repeat the measurement. If yes - replace the tube.
c) If there is no obvious short with or without the tube, with the tube out power it up and see if it still smokes
2. When a resistor smokes, replacing it with a higher power one is usually not an answer.
3. Actually before you even put the tubes in and all that why don't you first see if your rectifier is OK etc.
Thanks for the quick help morzh. Here is what I have found: Plate of the 7J7 (pin 3 of the 6J8G in this one), shows a short to ground with and without the tube in. R36 heats up just as bad with tube out. Pin 3 of the 6J8 only goes to white of the 1st. IF trans. (to C-30A inside the can). other lead of C-30A goes to red lead of 1st. IF. Junction of red lead, R36, R16, and C17 also shows grounded. Should this be so? I have re-checked all tubes and all read good with no shorts. I did notice that C61 (12mf.) shows grounding on the negative even though it is not physically grounded (floating ground thru R60 and 59?). I should add that the radio produces no sound at all (speaker hooked up, manual dialing knob in, broadcast band, but no antenna wires attached. Any ideas? Thanks, Tim
I think you will find that capacitor 17, 0.05uf, is shorted. Try cutting the lead on either side of this cap and see if your B+ comes back.
Yep, cap #17 is shorted.
As for the C61, if you are sure it is a short see what shorts it, it's easy/ For instance there may be a paper sleeve around it that the clamps goes around and it may be broken through.
Thank you both very much! I will check C17. C61 is a new cap.(as is C17, but it may have shorted for other reasons) I wasn't thinking and wired C61 the same way as the last "tech" did which I believe was wrong (chassis ground). It is now wired neg. to back side of R60, green lead from mains trans., R49 and 54. But this junction shows a short to chassis ground even though it is not directly connected.
Tim
Since we are remote the only sensible thing to do is disconnect C61 and then keep disconnecxting things one by one until you see what it is there from the things wired together at that R60 point that is connected to GND.
Once you figured this out then we can proceed. Guessing won't help much. Anythings possible, even shorted to the chassis transformer, though it is not likely.
Also see if you have old rubbed through wires touching chassis etc. BTW C17 may be shorted for that same reason. Or if wiring is new - if something is touching the chassis. You have to be diligent.
The only thing I know for a fact is that finding shorts in large systems like an old radio is extremely easy, if it is a non-intermittent reliable short. There are only sop many things that can short.
Thanks morzh, Here is what I have found: When all connections at the junction of R59,R60,R44, and pin 4 of 1232 are disconnected, it is R59 showing the ground. (other side of R59 is grounded according to schematic). Disconnecting R59 at this junction cures the ground problem at neg of C61. Odd, R59 showed 15.2 ohms when connected, but now (disconnected) shows shorted; I will replace it and recheck. As for the C17; when that junction is disconnected, it is the red lead from the 1st IF trans that shows ground. Disconnecting red lead cures the ground problem at that point. So I guess I will look into that deeper once I replace R59. BTW, all tubes are out. I'll post back with my findings. Thanks for yours and Brenda's help! Tim
OK, I'll let you do your own investigation at this point - I think it is better for you to find it on your own as you are on the right track now.