[CAUTION: LONG WAFFLE POSTING THAT MAY INDUCE SLEEP.
DO NOT READ WHILE DRIVING OR OPERATING MACHINERY]
This weekend I spotted a nice little 1956 Philips AM/FM radio in a junk
shop. At 10 pounds [~$15] I had to take it home.
As usual with British AM/FM sets, it did not work on FM.
Why do these old sets never seem to work on FM? Here's my theory:
Until the last few years, the only programme material on FM was
BBC, duplicating what was also broadcast on AM. So if your radio
packed up on FM, you simply retuned to the same programme on AM and
carried on listening.
As the tubes age or the HT voltage drops [because of an aging rectifier],
the first thing to pack up is the FM oscillator.
Most AM/FM sets in Britain seem to use an ECC85 in the front end.
One section is used as rf amplifier, the other as self-oscillating mixer.
This includes sets made in Britain and sets
[Blaupunkt, Grundig, etc] imported from Europe.
ECC85 is a double triode quite similar to 12AT7/ECC81 but with screening
between the sections.
AM/FM sets that don't use the ECC85 seem to use a pair of EF80 pentodes.
[these are TV pentodes *very* widely used in early 50s Brit TV sets]
One pentode is used as rf amp and the other as self-osc mixer.
So far as I can remember, every AM/FM set I ever saw that had packed up
on FM could be got working by replacing the ECC85 [or the EF80 oscillator] or
getting the HT voltage back to normal. [Or both.]
FM broadcasting only started in Britain in 1955, so FM sets from before
then are unknown.
The little Philips was true to form - working fine on medium wave and
Long wave but quiet as the grave on FM. Replacing the oscillator
EF80 got signals to come through on FM but very quiet.
The HT rail was at 200 volts - should have been 250 volts.
A new EZ80 took care of that.
Replacing the other EF80 had it working as good as new. When I checked the
EF80s on the Avo valve tester, both were at about only 20% of normal mutual
conductance. Both had the same factory codes and both had red wax on
them - accidentally dropped by the person who alinged the set in summer
1956. So they had evidently been there since summer 1956.
While I was fishing around for a replacement EF80, I cam across an EF184.
This was the TV successor to the EF80 - same connections, similar
capacitances but around twice the gm. I tried plugging the EF184 in
as rf amplifier. Things were louder but when I tried to align the
input circuit, the set went unstable. So back went the EF80.
The set is now working as good as new, except for a crackly volume
control. As soon as I can find a 2 M ohm control tapped at 400k,
it will be as good as new again.
So there you have it. If your British AM/FM set does not work on FM,
whack in a replacement ECC85 and, nine times out of ten, the FM band will
come back to life.
Martin A | "It doesn't matter how bad things are,
| as long as we know whose fault it is"