Hi Guys,
I dug into my neighbor’s Philco 60b last night and found what I expected- open antenna and oscillator coils. I’ve seen a sketch of the coils earlier on the Phorum but haven’t found it. Can someone steer me in the right direction? Looking for readings lug to lug. Thanks for the help. - Rob Q.
Hi Guys- ……been working on this Philco 60b and using my functioning 60b for comparison. Rewound antenna and oscillator primaries with capacitor replacement and tube check. My radio draws .42 amps at 110 v. and the non functioning one reads .3 amps. Having hit the main trouble spots does anyone have suggestions as to how to proceed? If only these old cathedral radios weren’t so dang cute! Thanks! - Rob Q.
After digging further I got the following numbers on the antenna coils. The first number is from the working 60b- ground to lug 4 (7M/.8), lug 1-3 (1.0/1.1), lug 3-2 (.1/.1), lug 2-5 (.7/.8), top coil band switch to tube cap (8M/1M). My first check of the coils was for continuity. Obviously something is wrong, but what? - Rob Q.
Here are some things to check:
Do you have audio? Do you get a hum in the speaker if you touch the top cap of the 75 tube.
If not, do you have plate voltage on the 42 output tube? An open output transformer primary would cause no plate voltage and low power supply current draw.
If the audio system is ok, inject an IF signal at 460 kHz into the top cap of the 6A7. Do you hear the signal? If not problem in the IF stage.
If the IF is ok, check if the oscillator is working. Set the dial on the 60 to 600 and listen on a nearby portable radio for the osc signal at around 600 + 460 = 1060 kHz. If you don't hear it, there is still a problem with the oscillator coil or circuit.
Hope this helps.
Thanks Mondial! I’ve got my check list for tomorrow. We’ll see how things go! - Rob Q.
Well……..I’m getting plate voltage on the 42 tube but no audio. At least it seems like we’re narrowing the issue down. Thanks. - Rob Q.
Ok, so now you have the trouble localized to the audio stages.
Check the voltage across the wirewound bias resistor #47. You should see around 16 volts across the end terminals. If significantly less, possibly a bad 42 tube.
Check plate voltage at the 75 tube plate. You should see 100 to 200 volts there. Otherwise an open plate resistor #34 or #36 or shorted bypass cap #20. If closer to 250 V there, possibly bad 75 tube.
This is all assuming you have around 250 VDC across the second electrolytic filter cap #45.
Cathode to plate or cathode to ground? Or should they be the same? So many questions! Thanks!
On the 75 and the 42 tubes the cathodes are connected to the chassis ground. If you measure from the cathode or the chassis it will be the same. Unless you have a bad rivet.
The control grids are using whats called fixed bias which is developed across #47.
The 6A7 and 78 use cathode bias using the voltage developed across #'s 8 & 9. In this case the cathode is not grounded and the bias voltage is not coming directly from the power supply.
Hi Guys- Here’s what I got- bias resistor end lugs- 5.7 v, 75 tube plate to ground- 336 v., electrolytics 380 & 405 v.
#47 & #75 tubes switched out with good tubes and no difference. #34 & #36 resistors were out of spec. & replaced. Electrolytics are new. I see that the voltages are out of whack. Does that lead us somewhere else? Thanks! - Rob Q.
Seems like the 42 output tube is not conducting current which is why your power supply and plate voltages are high and the voltage across resistor #47 is low. You tried another 42 tube so its not the problem.
Check and see if the cathode pin of the 42 is actually grounded to chassis. As terry mentioned, sometimes the ground connection is made through a tube socket rivet. If the rivet is loose or corroded you may lose the ground connection and the cathode will be left floating and unconnected. If that's the problem, solder a short wire from the cathode terminal to the nearest good ground terminal.
Bingo! Another save. Thanks for the detective work Terry & Mondial! It all came down to a bad ground. - Rob Q.