[ARC5] Looking at a 70 year old BC-457

Mon Jan 20 19:44:49 EST 2014

   Hello Bob,
  Your posting is one of the most informative posting I've read for some
  time.
   I have a graph I plotted of measured frequency (from a BC-221) and
   I'll post that on the ARC5-radio files section as a result of your
   work here.
   In summary I can say the drift I measured in "my" BC-221 was (in the
   order of) 160Hz per degree C at "around" 4 MHz.

  Returning to "these 70 year old critters" I spent yesterday evening
  working an a solid state version of a crystal controlled converter -
  Like McCoy's -  to put in front of an R-23.
  Many believe that the age of home-brewing electronics is gone.   I
  believe THIS is the golden age for home-brewers.  Quality test gear
  never was so readily available.
 Transistors and solid state devices are easy to build.
  They eliminate the need for chassis punches and heater wiring.   12
  (or 24v) is much easier to work with than 250V DC.
  The only missing ingredient is knowledge.  THAT is what makes
  home-brew difficult.

  So - thank you for posting these figures.  
  I will post my figure for BC-221 stability within a few days, as I
  said a moment earlier.

  73 de Les Smith
  vk2bcu at operamail.com

On Tue, Jan 21, 2014, at 6:57, MillerKE6F at aol.com wrote:
> With the continued interest in these critters I thought it might be of
> some 
>  interest to see how well this rascal was holding up after almost 70
>  years, 
> most  of which was spent sitting on my shelf.  This unit had not seen too 
> many  modifications those being the filaments rewired for 12 volts, a
> final 
> amp key  jack installed, the eye tube circuit disconnected.  Otherwise no 
> other  changes. The frequency range remained in the 4-5.3 MHz category. 
> My  
> measurements were made at a nominal 3.950000 MHz.  The first performance  
> parameter to come to mind was how stable was the VFO.  I used a regulated
>  12 
> vdc power supply for the filaments and a regulated 200 vdc for the
> oscillator  
> plates.  No final 1625s were installed during these tests.  Frequency  
> measurements were made with an HP 5340A frequency counter with an
> external  
> rubidium standard.  I should mentioned that the VFO remained operational
> on  
> Plate Voltages as low as 12VDC although the output level was marginal.
>  
> Short term stability did not seem too much affected by the plate voltage
> be 
>  it 12VDC, 100 VDC or 200 VDC.  The remainder of the frequency stability 
> was  done with a regulated 200 VDC source.
>  
> 1. Drift (Downward) over a 14 hour overnight period was 1359 Hz with an  
> ambient temperature range (rough measurements) from 58 degree F down to
> 42  
> degrees F.
>  
> 2. Short term drift over 30 minutes (58 degrees F)   4 Hz
>  
> 3. Variation with plate voltage 150 VDC Ep  shift upward 190 Hz  (from
> 200 
> vdc Ep reference)
>  
> 4. Variation with plate voltage 100 VDC Ep  shit upward  442 Hz  (from
> 200 
> vdc Ep reference)
>  
> 5. Filament voltage lowered to 10VDC  Ep 200 VDC (10 minutes to soak  up 
> change) 56 Hz drift downward
>  
> 6. Filament voltage raised to 14VDC  Ep 200 VDC (10 minutes to soak up 
> the 
> change) 11 Hz drift upward
>  
> Conclusions at this point in the adventure are that the VFO section is 
> not 
> affected all that much by operating parameters such as plate voltage or  
> filament voltage.  Temperature seems to be the major player and when the 
> unit 
> is fully operational with final tubes in place (Big Heat Source being  
> located next to the VFO can) I will rerun the stability tests.  
>  
> I will be running the transmitter on a home brew power supply that  
> duplicates the dynamotor voltage employed in the original scheme.  The 
> Filament 
> string will be returned to Nominal 24 vdc and powered from a regulated 
> DC 
> module in the AC power supply.   I hope to substantiate (for my own  
> edification) just how practical it is to use these units as built and
> also look  into 
> a number of the modifications that were made to these units over the
> years  
> to include keying and final amplifier tweaks.  Also spectrum analysis of  
> the output signals when feeding typical 50 ohm loads and methods used to
> match 
>  these loads.  I have a fairly large collection of "Modified Units" which
>  I 
>  can modify further to check the viability of TVI modifications, Buffer  
> amplifiers to mitigate chirp and pulling and other sane changes that
> might  
> enhance these gems from the past.  I also have a number original units 
> both 
> 274/N and ARC-5 to test and evaluate.  I realize that this is ground 
> that's 
> been plowed many times, but I'm not convinced that these units are ready 
> to 
> put on the shelf as ornaments of a bygone era.  I'm driven by my 
> memories 
> of using these rascals some 50 years ago. Then more out of  necessity
> being 
> a cash strapped kid.  I would also be interested to read  the findings of 
> other folks who have wrenched on these transmitters along with  their
> findings
> 73
>  
> Bob, KE6F
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