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

[Bob Macklin]

Ken,

Is the modulation waveform from your signal generator distorted. Both of my 
Heathkit signal generators had distorted 400 Hz.

Bob Macklin
K5MYJ
Seattle, Wa.
"Real Radios Glow In The Dark"
----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: <WA5CAB at cs.com>
Cc: "Arc5 mail list" <arc5 at mailman.qth.net>
Sent: Monday, February 18, 2013 11:44 PM
Subject: Re: [ARC5] Audio power output and BFO fixed - 10 meter BC-454


> 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
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