[Hallicrafters] S-20R IF transformer trimmer caps

[Roy Morgan]

On Jan 25, 2015, at 9:27 PM, Jeff <jeff at podengo.com> wrote:

> Well, after tearing down the IF cans again, meticulously cleaning, and changing a few more resistors, the radio is now working great!

The mysterious disappearing fault!  hooray!

>  I found a .01 paper cap in the band switch that I had missed,

The earlier S-20 has a number of caps in and around the band switch and coil area.  Tricky work.

> I need to find a PM speaker for it, but right now just have it hooked to an R-42.
 
At Parts Express, I find:

Boss Audio Systems
| Model: BRS65
6-1/2" dual polypropylene cone replacement speaker.
Part #  265-378
Weight: 1.45 lbs.  $13.50

But you can find less expensive units.

> Any thoughts on leaving the resistor as is, or perhaps looking for a choke with 1.5k resistance to match the job that the field coil was doing?

Find a choke. Any choke.  Add a power resistor if you need to to tame the B+ voltage (less B+ is better).  Or reduce the value of the first filter cap  (C28) to approach a choke input filter configuration.  If there is any hum that creeps in, increase the second filter cap as needed.  Put the first filter cap in parallel with the second after the choke and replace C28 with a smaller cap - maybe much smaller. 
See this web page (referenced on the Playthings of the Past speakers page) for more info:
http://radioremembered.org/edspeaker.htm

The Hammond chokes at Parts Express are as good as you can get new.  This one might be just right:

Hammond 156L Filter Choke
Hammond Manufacturing
Model: 156L
Overview
Economical "open bracket" channel mount filter chokes are perfect for high voltage power supply filtering. Tolerance of 15% on both inductance and resistance. Inductances measured at rated D.C. current. 5" long leads. Inductance: 5H, DC Current: 75mA, Resistance: 135 ohms, Max DC Volts: 400V.
Part #  122-310
Weight: 0.5 lbs.  $17.50

There must be cheaper chokes out there - check AES.  I did.  They have the Hammond 156L for less money, and also a 20 ma choke, 20 H for about the same price.  I’m not sure what the total B+ current drain is. Seems that the 6F6 will draw 30 ma or more.

IF you wish to use solid state rectifiers, put the 5 volt filament winding in series with the line to buck down the primary voltage on the transformer.  Or add a filament transformer with in the radio to buck the line down.  This radio was meant for 115 volt line, the Ryders S-20 (non-R) schematic shows.

To reduce heat/wear on the 6F6 audio output, ensure it’s cathode bypass cap is new, and increase the cathode resistor.  Aim for 80 percent of rated plate dissipation or less. (Subtract cathode voltage from plate voltage to get plate to cathode voltage.  Measure cathode resistance and voltage to figure the actual plate+screen current. Or measure plate transformer DC resistance and measure it’s voltage drop to get plate-only current.)

Note: type 80 rectifiers are *very* expensive.  Make up a 4-pin to octal adapter to use the electrically identical 5Y3.  (Or even a 6X4 if you check the ratings and current needed.)  If your type 80 is good, you might sell it and buy a whole second radio for the income.  My security contractor advises me to neither confirm nor deny that I have any spare type 80’s, and to never even mention the existence of any globe type 80.

Sorry, no extra knobs here.  The one you need is the old style small pointer knob.  There was a report that Playthings Of the Past was going to retire.  He’d have what you need.  His picture at:
http://www.oldradioparts.com/knobs.htm
under pointer knobs shows the knob you need.
http://www.oldradioparts.com/knob7.jpg

http://www.oldradioparts.com/
The site says:
"CLOSED.... Jan. 20th to ? (illness) 
Please do not send any emails until this notice is removed.”

By the way, he might have the exact field coil speaker you need for direct replacement.  When he’s back, just send the mounting bolt diameter and field coil resistance.


Roy


 
Roy Morgan
RoyMorgan at alum.mit.edu
K1LKY Since 1958