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

[Kenneth G. Gordon]

On 18 Feb 2013 at 23:54, WA5CAB at cs.com wrote:

> Brian,
> 
> Actually, the turns ratio of the audio output transformer in the R-26
> is well known as it's given in the parts list as 4000 turns primary
> and 500 turns secondary, or 8:1.  The primary is center tapped (you
> have to go to the ARA parts list for the turns count) and the neon
> lamp is across half the primary.

Robert: As I said earlier to Les, this particular BC-454 had no audio output 
transformer when I got it. I replaced the missing one with the audio output 
transformer from a derelict R-23/ARC-5.

> However, apply the rest of your calculations and the situation is even
> worse.  So something is rotten in Denmark.  Ken, you need to recheck
> your secondary voltage measurement.  Try using an audio voltmeter with
> 250 ohm load (speaker transformer disconnected).

OK. I was using a 'scope and measuring the peak-to-peak voltage at the 
input to the speaker. The speaker is a Radio Shack 8 ohm 4" CB speaker 
with an 8:500 ohm matching transformer in it, so ostensibly, the input 
impedance of the combo is 500 ohms...at one or two frequencies at least.

The p-p voltage IS as I measured it. I did it twice to make sure I hadn't made 
a mistake.

The 400 Hz waveform is a bit distorted, but not badly. It still looks like a 
sine-wave pretty much. In fact, the waveform is distorted from a very low 
volume level, around 1 V P-P output, and doesn't really change up to the 72 
V P-P point.

I took photos of the audio wave form as it is at the RF signal generator, and 
even that isn't a perfect sine wave either.

I can turn it up pretty loud before the waveform appears to begin to flat-top. 
Just short of that point is where I measured the 72 V P-P. It hurts my ears at 
that point, and I won't leave it there for more than a few seconds.

I suppose it is possible that the actual impedance at 400 Hz is a lot higher 
than 500 ohms. In fact, I'll bet that is what is wrong: if the actual impedance 
of the speaker/transformer was 1K instead of 500 ohms, wouldn't that result 
in a 4X increase in voltage? Or is the difference a log? I don't remember, and 
haven't bothered to look it up yet.

I'll use a resistor at the output of the audio transformer (once I have a fairly 
good idea of the value needed) and remeasure, but I won't be able to do that 
before Wednesday.

Ken W7EKB