Post by analogdialPost by philoThanks for the great explanation! As "analogdial" pointed out, it's
actually an S 36 I did not examine the photos carefully enough
The S-36 has a panadaptor connection on the back of the chassis.
Altough I don't think it was often used.
It does? I don't see that in the schematic on bama. Also, the IF frequency on the S-27/S-36/S-36A is 5250 Kc, not 455 or near there. So it would require a different panadapter, intended for use at that frequency.
I had, for several years an S-36A, which is almost, but not quite,
the same as as the non-A version. The S-27 prewar original was
also not all that different until you got into the detectors and
audio (used different tubes).
Post by analogdialIt's a mediocre radio by today's standards. Not particularly sensitive
and the narrow bandwidth is way too wide for just one AM channel. The
wide bandwidth is a bit too narrow for fully modulated broadcast FM but
it wasn't too bad back in the day when some FMers broadcast with SCA.
Worked well for TV FM audio. The audio was very good.
I tried stagger tuning the IF transformers for wider bandwidth but that
hurt the sensitivity on wide and nearly killed it on narrow.
The damn thing was built to last. There's only one
paper capacitor, in the power supply filter. 8 ufd of
paper in a sealed steel can, if i recall. It's huge. I checked mine
for leakage and it was still no worse than an equivelant electrolytic,
so I left it alone. There's also one electrolytic (also in steel, looks
like an oil and paper, mine was no good) for the cathode bypass in the
audio amp. Everything else is silver mica. The resistors checked out
100%. Didn't look like any under chassis work had ever been
done on the radio. Amazing.
It has a really nice zero backlash gear drive. Probably the nicest
thing about the radio.
Yes, it was "quite the radio" for its day. I spent a good deal of
time on my set. In general, on the lower two bands it had
reasonably good sensitivity and performance, but really got into
trouble on the third (highest) band, which started at the audio
carrier for TV channel 6 (postwar). That IF frequency was neither
fish nor fowl---too low for postwar commercial FM signals with 75
Khz deviation. As I recall, postwar FM receivers with 10.7 Mhx
IF's have a bandpass of 200 Khz or more, but with the S-36A, I
couldn't find a way to get flat response much above 160 Khz. That
meant that tuning was very critical. I tried loading resistors and
staggered tuning, but both just killed performance without
broadening the bandpass that much.
I had the IF transformers out of the set so that I could check the
coils on a Boonton 260A Q meter and a 250A RX meter. The IF's are
slug-tuned, and one section of one of them simply did not tune with
its slug. Rather than fuss too much with it, I simply replaced the
fixed capacitor in the can with a lower value and hung a trimmer
outside across the terminals. The 250A let me determine the values
and pretune the IF's before I reinstalled them. I see from the
schematic that the plain 36 used trimmers rather than slugs for IF
tuning.
The RF front end on my set when I got it was a mess. I took the
assembly out and took most of it apart to get it cleaned up. Once
again, the Boonton 250A and 260A were a great help in "getting the
Q back." One important thing about the S-27/36 is that it uses low
oscillator on band 3, but high on the other two bands.
One comment in the above discsussion, about a bypass cap in the
output tube cathode circuit, puzzled me. The S-36A did not have
one, but I see one in the non-A and S-27 schematics. Leaving that
cap out aids push-pull linearity, as any imbalance causes the
high-gain tube to act as a cathode follower driving the other half
as a grounded-grid amplifier. That configuration is at the core of
a lot of Tektronix vertical amplifier circuits.
The audio on this radio was incredibly good. The specs talk about
10 Khz at the high end, but my set would pass closer to 20 before
it hit 6db (voltage) down. The FM deemphasis filter installed in
the set was the prewar value---100 microseconds, as I recall.
Changed that to the postwar 75 microsecond values.
Yes, the tuning mechanicals on that set were very nice. That was
**after** I took it all apart, cleaned and relubricated it, and
replaced all the balls with new ones from a bearing supply place.
That was about 25 years ago, so I've probably forgotten some
things. As a radio, it was an interesting foray into studying 1940
state-of-the-art as well as quite impressive-looking. But compared
to even early postwar designs, the set simply fell flat on its face
at around 100 Mhz---not particularly sensitive, poor image
rejection, and either too broad on AM or too narrow on FM. As I
recall, that set ended up (with an early stereo converter) in the
lobby of a non-com classical station as a lobby monitor.
Hank
