[Collins] 75S-1, more adventures.

[Dr. Gerald N. Johnson, electrical engineer]

Sprague was the originator of the Black Beauty molded paper capacitor.
They made good capacitors with that name also, but they have a polyester
film alongside the paper, but most manufacturers like Collins used the
plain oiled paper capacitors which had a short lifetime at best, I've
found very few worthy of the capacitor name. Most are unreasonably leaky
for audio coupling and AGC circuits.

A leaky audio coupling capacitor will make the output tube draw excess
plate current and can raise the hum level of the receiver.

If all you have for soldering is a 100 watt soldering gun, Please, don't
dig in to replace the filter capacitor. You will only make a mess.

When many wires connect to lugs like those on the electrolytics it is a
workable repair to cut the lug loose from the old part and then solder
the assembly to the new lug. It wouldn't hurt to use a bit of bare
copper wire to hold the old and new lugs together before soldering with
a better soldering tool than a 100 watt soldering gun. A 100 watt
soldering iron, perhaps. I prefer the Weller W100PG or W100P-3
temperature controlled soldering iron for such work. Its often sold for
soldering lead cames in stained glass work. Its not cheap, but its less
expensive than a temperature controlled soldering station. A 3/16" wide
screwdriver tip is appropriate.

The PTO frequency shifting is only set up to maintain the carrier
frequency when shifting side bands, it would take more complexity to
adjust the shift for CW. It would be practical to tack something onto
the cathode of the PTO tube such as was used for transmitting FSK.
Unless its VHF where modes are often mixed, the frequency shift when
changing modes isn't all that much of a problem. Its a whole lot more
annoying in Yaesu VHF multimodes where the display remains the same but
the receiver frequency shifts to indicate transmit carrier frequency on
CW. Even worse in Icom and many Tentec HF radios where CW is received on
LSB while the upper HF bands use USB for SSB. Its a better receiver
design when filter switching can be independent of mode. And even
handier when there's passband tuning like the 75A-4 and many Tentec
have.

Later S-line receivers had a BFO frequency adjustable from the front
panel, though I didn't like the adjustment in my 75S3B. It was a pot
with a switch to disable the frequency shift at the end of rotation.
Back about 1964, I replaced that pot with one having a pull switch so I
could switch back and forth from crystal to variable BFO without moving
the variable BFO frequency significantly.

The gear mesh is adjustable to minimize the rattle.

73, Jerry, K0CQ, Technical Advisor to the CRA
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.