I am refurbishing a 1940 Philco 40-170CS I have the schematics. The radio was in a farmhouse with a wood stove so it was covered in soot. After cleaning the chassis with CRC I recapped and replaced two cracked resisters. Also some of the wiring insulation was cracked . I kept these wires but replaced the insulation with spaghetti tubing. Two tubes failed the life test and were replaced.
After slowly powering it up with a variac and dim bulb tester resister 20 overheated and it was clear from the dim bulb it was pulling more than 34 watts. The resister measured 1500 ohms but should be 1000 so I replaced it but still got the overheating. Also, the RF transformer was overheating I noticed when I disconnected the speaker the overheating stops and the wattage is better but most of the tubes don't light and without a speaker, I don't know if its working. I checked the speaker field ( Part 35) and it's within spec at 1700 ohms. Any suggestions?
Hey! I have a couple of quick questions to get some more information!
What do you mean when you say resister 20 overheated? Did it just get warm/hot or did it smoke?
How many volts were you at when you were pulling 34 watts? The set is rated at 35 watts and from a cold start, you may pull considerably more than that until the tubes warm up.
One easy thing you can check is to see if anything along the path of that resistor looks or could be shorted.
When you disconnect the speaker you are probably removing the field coil from the B+ power supply, thus no B+ to any of the tubes. Your clue is the warm rf transformer and warm 1k resistor. These two are in the plate circuit of the 7C7 which is probably conducting more current than it should or there is a short somewhere near the tube.
I cannot read either the Rider's sch (their numbers are illegible) or the one scanned here (it is not very sharp) so I could not say much.
If you re-scan it well, more folks could join the discussion.
The resister and RF were actually smoking. Attached is a clearer pdf of the schemo . I will in the meantime look for shorts.
There is no resistor 20. There is a coil 20.
Can you circle in red the pats in question?
It feeds the plate voltage to the mixer and the rf amp, 1000 ohm.
Mike, it's #27 in Rider's schematic, 1k resistor. His schematic shows the same resistor as a fuzzy 20.
My apologies attached is the correct schemo which more closely matches what there. The Rider's one is not accurate for this radio . Don't know why
Well, if BOTH the R27 and the RF coil in 7C7 plate are smoking, the 7C7 could be shorted.
Pull it out, see if they still heat up.
Given that Philcos have a reputation for corrosion in things like antenna and oscillator coils in the earlier sets .... what are the odds tat the RF coil is shorted?
If you unsolder and disconnect the plate lead at the 7C7 RF tube and the RF coil still overheats, you'll have narrowed it down quite a bit.
John
Coil shorted will not cause that. Coil is a few ohms in the first place, and it is in series with that 1,000 ohm resistor.
Shorting this 10-50 ohm resistor equivalent which coil is with 1000 ohm still there will barely affect anything.
If it were just the 1000 ohm resistor I'd say this is the #13 cap is shorted. But with the coil smoking too this has to be something after the coil.
Potentially the short is between Primary and Secondary of that coil, but then I am not sure what is there after it.
Also 7C7 shows some sort of screen grounded; check for short in the socket pins.
Last thought: the coil is shorted to the case that is grounded. Unlikely but is worth of checking.
OK here's an idea:
power down, unplug,
In the 7C7 socket measure Ohms between the plate pin and the chassis. See if the short is there.
pull out, measure again. See if it disappears.
If no short, put the tube back in, measure again, see if the short appears. If does change the tube.
If short is still with the tube pulled, look at the socket. Or around. Or in the coil.
Quote:John
Coil shorted will not cause that. Coil is a few ohms in the first place, and it is in series with that 1,000 ohm resistor.
Lifting the plate lead eliminates any issues on the tube side of the plate lead because it removes the the RF tube and socket, etc from the equation and leaves you only with coil, coil leads, internal shorts to ground, shorts between pri and sec, solder blobs etc. It's the easiest way to eliminate the tube and everything ahead of it in one action.