[ARC5] Audio power output and BFO fixed - 10 meter BC-454

[Ian Wilson]

The 12A6 data is given for 250V on plate and screen, and a total of about
35mA plate+screen current. The grid bias is -12.5V, which means a cathode
resistor of about 360 ohm. However, the cathode resistor in these receivers
is much higher than this. So the typical conditions in the data sheets -
including power output at some specified load resistance - don't apply here.

Ken - perhaps you could measure the voltage across the primary and
the corresponding voltage across the secondary for a sine wave signal
with no load (for a small input signal). That would provide the turns ratio.

73, ian K3IMW

On Mon, Feb 18, 2013 at 6:48 PM, <brianclarke01 at optusnet.com.au> wrote:

>
>
>         Hello Ken,
>
>         Interestinger and interestinger.
>
>         If the output impedance of the 12A6 is 7000 Ohm (RCA design value)
> and the secondary of the OPT is exactly matched to 500 Ohm, the turns
> ratio of the OPT is sq rt (7000 / 500) = 3.74. (This, of course, is
> researchable hypothesis, as I am unaware of the design specs of the
> OPT. All I have seen are wire gauge and resistance specs - probably
> for trouble-shooting.)
>
>         3.74 x 72 Vp-p = 269 V. This is more than the rated output of the
> dynamotor, and doesn't allow for the loss of swing because of cathode
> bias. It is also much higher than the ignition Voltage of the neon
> across the OPT primary. I reckon your output must be highly distorted.
> So, it probably sounds loud because of that distortion. I suspect
> there is something odd here. The likelihood of running the 12A6 plate
> from rail to 0V is negligible, especially if the screen is still at
> rail.
>
>         In some of my Command receivers, the wiring in the chassis is glued
> down with glyptal. The use of single-core wire throughout was another
> piece of Drake's genius - to remove possible sources of
> vibration-induced capacitance change.
>
>         73 de Brian, VK2GCE.
>
> ----- Original Message -----
>  From:"Ken Gordon"
> To:"Arc5 mail list"
> Cc:
> Sent:Tue, 19 Feb 2013 01:33:12 -0000
> Subject:[ARC5] Audio power output and BFO fixed - 10 meter BC-454
>
> I rechecked the audio power output here and tried to be a bit more
> accurate this time.
>
> Peak-to-peak voltage at 400 Hz into 500 ohms on my HP o'scilloscope is
> 72 V.
>
> Peak voltage is thus 36 V
>
> Erms=36/sqrt2=~25.5 V
>
> P=E^2/R=1.296 watts.
>
> Even on the cheap Radio Shack CB speaker with the 8:500 ohm
> transformer I installed in it,
> it will literally rattle the windows.
>
> Secondly, I removed all the broken bits of ferrite from the BFO
> transformer can, then liberally
> brushed Mike's coil dope over every thing that I thought could use it.
>
> I let the stuff harden, then thoroughly cleaned, and then reassembled
> and reinstalled the BFO
> transformer.
>
> Result: no more ringing or frequency jumping from that source, and the
> BFO is much more
> stable. Furthermore, the tone no longer has what appeared to be some
> sort of modulation on
> it.
>
> I have one more thing to do to this thing to stablize it, then I think
> I can button it up and do a
> more thorough testing of it.
>
> Ken W7EKB
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