Philco G978

[dtvmcdonald]

I'm new to this forum, and almost new to vintage electronic restoration, but not that new to vintage repair.

I recently got a nice 1959 model Philco G978 AM-FM radio. Its a very very
odd mechanical thing with the dials on top, speaker in front,
and chassis vertical, the tubes pointing to the front. Seven tubes,
selenium rectifier. It works actually fairly well as-is, but is a tad weak
and shows hum on AM only. Remarkably the dial calibrations
are spot-on.

A few questions:

1) is there anywhere I can get a schematic without paying money?
I found several for sale online.

2) It appears by inspection that the only electrolytics are in the usual
can. Since its printed circuit with no parts below the board, I think I'll
just remove the present one electrically and place tubulars behind
the board. Does this sound OK?

3) more bothersome is the selenium rectifier. The current one
works fine and does not get more than barely warm. But after
too many episodes of H2Se gas poisoning in my youth due to
bad ones, I'm going to leave it in place but electrically use
a Si diode. Presumably this means a dropping resistor. Is the
best way to calculate the resistance to measure the DC voltage
drop and (after disconnecting one end) the (DC) current, and calculating from that? Of course its AC so perhaps I should add a 1 ohm resistor
in series and measure the (instantaneous) voltages across the
1 ohm and the diode with a (differential) scope and calculate the
resistance from that? What do you people do?

4) The safety line cord, the kind that is attached to the bottom
with a plug that comes undone when the bottom is removed to
replace tubes, is dried out and dangerous. Is there a good place to
get replacements (polarized)?

5) Are the small capacitors that isolate the FM antenna
terminals from the chassis (possibly hot without a properly
working polarized plus) "safe" or should I add a pair of
recent ceramic ones in series with the wires?

Doug McDonald

[radiotvnut]

1. Someone might have the Sams Photofact service information that they could copy for you.

2. I'd replace the electrolytic capacitor, along with any tubular paper-style capacitors. By this time, there were many paper capacitors housed in plastic or ceramic cases. Despite the case style, they still need to be replaced. Since this is a PC board chassis, I'd remove the old capacitor can and mount the new capacitors where the old can was located. You could cut the traces leading to the old capacitor and mount new capacitors under the board; but, it might look messy. It will be neater and easier to remove the old cap and mount the new caps in it's place.

3. There are mixed opinions on rather or not to replace a currently working selenium rectifier. If you replace it with a silicon diode, you will indeed need to add a series dropping resistor to compensate for the voltage drop provided to the original selenium rectifier. In sets like this, I find that something in the 68 ohm, 2 watt range will work. However, you may have to try various values until you obtain close to the correct B+ voltage. I always install the resistor on the AC input side of the rectifier.

4. You should be able to find a polarized plug to attach to the existing line cord. There were polarized cheater cords made for TV's; but, the female end will also be polarized, making it not practical to use on this radio. In most radios, the line switch is wired in the neutral side of the line. Even with a polarized line cord, the chassis will still be "hot" when the switch is off because of the path to ground provided by the tube filament string. To get around this, the power switch should be wired into the hot side of the AC line. This way, assuming the AC outlet is wired correctly, the chassis will not be hot regardless if the radio is on or off.

5. Those FM antenna isolation capacitors are usually the ceramic disc type and are probably OK and will remain OK.

[dtvmcdonald]

Originally Posted by radiotvnut

4. You should be able to find a polarized plug to attach to the existing line cord.

OK, thanks. I can't replace just the plug, the cord itself
is falling apart and a hazard. I'll have to replace the whole
cord just keeping the socket that connects to the radio.
Did they put the switch in the neutral end so that the 115v
would not be near the volume control?

Interestingly, until about 10 years ago I actually had 110 VDC
in my office! (We had a motor-generator to make DC to run
things like big magnets. AC-DC radios worked just fine on it.
The generator is still there, turned off.)

Doug McDonald

[Reece]

It's hard to find interlock cords today. Sometimes they show up at some of the surplus houses but there are various types of them, pin spacings, one pin larger, etc.

In the good old days with non-polarized plugs you could turn the plug over at the wall receptacle to see which way gave you less hum. Yes, the ideal situation for reception was to have neutral go to the switch to reduce hum. Whichever way was safer WRT the chassis is an ongoing discussion.

[radiotvnut]

You may end up having to do away with the interlock and hard-wire a modern power cord to the radio. The whole AC interlock idea started in the late '40's, mainly on TV's, in order to help prevent the set owner, who often knew nothing about electronics or the dangers of HV, from shocking or electrocuting himself while poking around in the TV. Of course, all one would need would be a "cheater cord" in order to bypass the interlock. Interlocked power cords continued to be used up until at least the early '80's on TV sets. I'm currently working on an '82 Sylvania that uses an interlocked power cord.