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

[MillerKE6F at aol.com]

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