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

[Kenneth G. Gordon]

On 19 Feb 2013 at 13:48, brianclarke01 at optusnet.com.au wrote:

>  Hello Ken, 

Hello, Brian:

>  Interestinger and interestinger. 

Yes. I certainly thought so too.
 
>  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.

Pretty close, Brian. My AC power supply is outputting exactly 270 VDC. I am 
not using a dynamotor. For this receiver, the dynamotor shock-mounts had 
been completely removed, leaving nothing but 4 clean holes in the deck. The 
dynamotor power connector had been soldered to, but otherwise not 
damaged. I cleaned the solder out of the posts and am using those to 
connect my AC supply to via an old dynamotor base-plate.

> 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.

No. It really isn't too bad. I can send you a .JPG of the 'scope trace if you 
would care to see it. I hesitate to leave the volume up to maximum for very 
long, though. There is some distortion, but it really isn't all that bad. I was 
quite surprised.

> 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. 

It is: 270 VDC, in fact.

>  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. 

As I said, I am firmly convinced the ARC engineers were geniuses.

Ken W7EKB