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

[Dennis Monticelli]

Bob,

I applaud what you have done and will be doing in the future to better
understand these fine units.  May I recommend that you look at drift over
time from a cold start in a typical setup. This would enable the user to
estimate the necessary stabilization time before getting into a QSO. A
related test would be the drift due to the extra heating of a transmission
and the cool down from that during receive.

Also, please check the freq pull of a continuously running osc due to a
keyed PA.  Mine pulls nearly 2.5KHz downward when loaded to 50W plate
input.  The degree of pull varies with the power gain of the particular
1625's in use.  Because of the pull I use a dummy load and full key-down
power to zero in on a target signal before responding with the antenna
switched in.  It's the only way to be spot on without causing QRM.

Researching the true root causes of the chirp would be interesting.  Once
the screens and plates are regulated there is still a slight residual chirp
and it is not clear from where the time constant(s) for this effect are
coming.  It is probably heating related, but where is this heating
occurring and how does the heating affect the osc loading?

Keep us posted, Bob.

Dennis AE6C




On Mon, Jan 20, 2014 at 11:57 AM, <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
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>