S-meter for Hallicrafters S-20R

6/21/2009 12:28:43 AMLeslie

Hello All,

I am trying to fit an S-meter on a Hallicrafters S-20R; the original set had this available as an accessory unit, which plugged into a five hole socket at on the rear of the chassis (i.e. an SM-20). I have been unable to source an SM-20 unit, and have found bits and scraps of technical advice from the WEB (including an article on adding a tuning eye tube to an S-20R). Unfortunately I can't quite get the information I need.

I have an old S-meter in my pile of junk, and have tried various combinations. The socket at the back has 5 pins, according to the schematic. Starting at the top, pin 3 is used. Moving clockwise, the next pin is 2 is the B+ (I get readings of 270+VDC; pin 1 is noted on the schematic as 'to heaters' and outputs 6.3VAC. Pin 5 is the ground. Pin appears to go to the RF Gain Switch.

I've tried hooking the unit across pins 2 & 4, with no success; no matter whether the AVC switch is on or off or whether the RF gain switch is on max.

I'm not sure what I'm doing wrong, and if there is anyone out there who is into S-20Rs and/or has done this before, I'd appreciate some advice.

Have a great evening and a good summer.

6/21/2009 5:57:56 AMWalt

I believe that the rear socket was only for B+ / filament supply or battery operation.
The bama.sbc.edu website has the manual for your S-20R Sky Champion as well as the Hallicrafters factory service bulletin for installing S-meters in various models. All you need is to tap into the RF or IF stage and they show you how it's done. You only need a 0-5 ma meter and 2 or 3 resitors to do it.
The bama website downloads can be baulky at times. Mozilla Firefox works better than Internet Explorer at times. If you have trouble downloading from the site, just read the help section.

6/21/2009 7:19:51 AMEdd

Sir Leslie:

Since S meter circitry was not the norm on that 20 receiver series that you have, what say lets step forward a series and examine the SX21 which did incorporate an S meter.

In that series, assuredly the same design concept was to have the set running at full RF gain and then let the AVC loop feed back
control biasing back to the frontal RF stages. That would then have the effect of letting one monitor the plate circuitry of the frontal RF amplifier stage as also being an adjunct indicator of received signal strength.

Sooooooooo. . . . just study the accompanying mark-up and establish its circuitry to your S-20 series receiver.

73's de Edd

HALLICRAFTERS WORKING SCHEMA:

6/21/2009 7:39:28 AMLeslie

Hi Edd,

As usual, great info--the problem is that the circuit diagram you provided does not display; there is a title visible (Hallicrafters Schema), but just a small picture box with an 'x' in it; for whatever reason the graphic file is not opening (is it JPEG or BMP?). Could you send it to me as an email attachment? Maybe I'll have more luck...

Thanks,

Leslie
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:Sir Leslie:
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:Since S meter circitry was not the norm on that 20 receiver series that you have, what say lets step forward a series and examine the SX21 which did incorporate an S meter.
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:In that series, assuredly the same design concept was to have the set running at full RF gain and then let the AVC loop feed back
:control biasing back to the frontal RF stages. That would then have the effect of letting one monitor the plate circuitry of the frontal RF amplifier stage as also being an adjunct indicator of received signal strength.
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:Sooooooooo. . . . just study the accompanying mark-up and establish its circuitry to your S-20 series receiver.
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:73's de Edd
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:HALLICRAFTERS WORKING SCHEMA:
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6/21/2009 8:23:55 AMWalt(75101:75099)

Leslie - Can you open Sir Edd's schema as a .png file?

6/21/2009 11:16:53 AMLeslie(75105:75101)

Hi Walt,
All I can do is right click on the red x and the drop-down menu says 'show picture'. I double-click that but nothing comes up. What codec application do I need to use to open a .png file? Thanks
: Leslie - Can you open Sir Edd's schema as a .png file?

6/21/2009 12:50:15 PMLeslie(75109:75098)

Hi Edd,
I've been able to open the circuit diagram, which I can't quite match to the corresponding bits on my S-20R.
As you know the S-20R has an s-meter socket at the back, so that the accessory s-meter unit could simply be plugged in--i.e. the rear socket allowed the accessory meter to work off the radio circuit leading up to it. Perhaps the s-meter unit chassis had additional resistors etc., but it would basically (I guess) work on a 'plug and play' basis (allowing for the RF Gain adjustments etc. before the meter would work). On this rear socket, I am assuming that pins 1 & 5 are for a 'dial lamp' in the s-meter assembly; pin 1 outputs 6.3 volts and 5 is the ground. I've run a lamp from these pins to test the theory.
This means that the only active pins remaining are the B+ (pin 4--270VDC)) and pin 2. As I said, no matter how I hook up my meter I can't get any functionality. Using a digital voltmeter, the readings actually seem to ripple (they rise, then fall back to 0 etc.).
Sorry for the Homer Simpson questions, but I'm sure I'm missing something basic, as pins 4 and 2 must surely have allowed plug and play with the optional s-meter unit.
Have a great day,
Leslie
:Sir Leslie:
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:Since S meter circitry was not the norm on that 20 receiver series that you have, what say lets step forward a series and examine the SX21 which did incorporate an S meter.
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:In that series, assuredly the same design concept was to have the set running at full RF gain and then let the AVC loop feed back
:control biasing back to the frontal RF stages. That would then have the effect of letting one monitor the plate circuitry of the frontal RF amplifier stage as also being an adjunct indicator of received signal strength.
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:Sooooooooo. . . . just study the accompanying mark-up and establish its circuitry to your S-20 series receiver.
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:73's de Edd
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6/21/2009 5:20:59 PMMike C.(75118:75109)

On an S-40 or 40A,the later version of the s-20R,that extra pin supplies screen voltage,only when the RF control is rotated fully clockwise until it clicks and actuates a switch.One pin is not used.Although the schematic may show no connections to it,Halli used that extra pin for a tie point for a resistor.I had an old National meter(reverse reading,like the sm-40) hooked up to my s-40A for a while,it worked just fine.The needle has very little damping however,and it made me sea-sick to watch it.