[Hallicrafters] Questions on S-38A Restoration

[Rodney Bunt]

Michael

The "dummy antenna" that they talk about, aproximates a "long wire" antenna used as a broad band
receiving antenna.

Any antenna will be "shorter" in relative to wavelength the lower the frequency. So the RMA
antenna was dreamed up and is a standard for receiver alignment.

The circuit is like this

o-------||---+-/\/\/\/\/-+--------o
             |           |
             +----||-----+

So the circuit at BC frequencies has a resiistor in series with the capacitor at input ass the
frequency rises the effect of the capacitor in parallel comes into play and "shorts" out the
resistor. This at frequencies above 2 Mhz there is very little added impedance from this circuit,
as the frequency drops first the resistor and then the input capacitor reactance gives a higher
and higher impedance. Sort of looks just like a real world long wire antenna.

If you are feeding the receiver from a dipole approx 72/50 Ohm then you can use your HP sig Gen. 

Not many people I know have a dipole at 600kHz on BC band....

Regards,

Rodney
VK2KTZ

PS: All this is explained in "Elements of Radio Serviceing"
Marcus, William and Alex Levy 
Elements of Radio Servicing 
1947 McGraw-Hill book Co USA.
Hardback

Search for a copy on www.abebooks.com very good for the Glowbug bug.... 


--- Michael Melland <w9wis at charter.net> wrote:
> About two years ago I picked up a pretty nice (but dead) 
> Hallicrafters S-38A at a hamfest.  Sunday I finally had an 
> opportunity to work on it ( yeah... I know I'm slow ) and 
> got it running very well.  It's amazing how sensitive this 
> little rig is, and how good it still sounds... but band 
> two seems a bit low on sensitivity and I wanted to check 
> the calibration on all the bands to see if I could tweak 
> them a bit closer (they aren't too bad as they are).
> 
> This S-38A is in pretty nice shape.  I replaced the multi 
> section electrolytic as well as all the paper tubular 
> caps.  I can still detect a low level of hum which may 
> mean I have a bad mica cap somewhere or it's picking up 
> hum from the isolation transformer someone added to the 
> chassis years ago (nicely I add).  The speaker had also 
> been replaced and almost touches the transformer from 
> above.
> 
> I have two questions for the sages on the list.
> 
> The manual calls for fabrication of a dummy antenna (RMA 
> antenna) that you run a sig generator through to perform 
> the alignment and sensitivity adjustments.  What purpose 
> is this RMA dummy antenna ?  I have a HP8640B... why can't 
> I simply feed low level RF into the antenna connection on 
> the S-38 to tweak things up ?
> 
> I also found the schematic and parts list to indicate only 
> a 4 section electrolytic capacitor ( 40/30/30/20) which I 
> replaced.  However, I also located a 150V 30 MFD "dry" 
> Electrolytic under the chassis not indicated on the 
> schematic or in the parts list.  If I remember right it 
> was to the bottom and right of the multisection filter cap 
> along the rear of the chassis and I think connected to 
> either the the audio amp or 12SQ7.  It was under the leads 
> and wires of other components... like it was present when 
> built.  But then why not on the schematic or parts list ? 
>  Anyone else have this ?
> 
> Regards,
> 
> Mike 
> 
> --
> Michael Melland, W9WIS
> Winneconne, WI USA
> _______________________________________________
> List Administrator: Duane Fischer, W8DBF **for assistance**
> dfischer at usol.com
> ----
> Hallicrafters Collectors International: http://www.w9wze.org
> ----
> http://mailman.qth.net/mailman/listinfo/hallicrafters


__________________________________________________
Do you Yahoo!?
HotJobs - Search new jobs daily now
http://hotjobs.yahoo.com/