[ARC5] Smart People: Where'd the Harmonic Come From?

[AKLDGUY]

I have some experience with the BC-230 transmitter and know the output tank
circuit well. You cannot simply connect a 50 ohm load via a 100 pF
capacitor while using the same tap on the tank coil that you used for the 5
ohm + 100 pF dummy load. You will need to find another tap setting to suit
the new load. My tap had to be moved further up the coil, and of course
this detuned the tank, forcing a retuning of the "Antenna tuning"
capacitor. The capacitor needed to be moved towards the more fully meshed
condition.

The tap setting must be found by trial and error. It's possible to slide a
finger between the case and the coil box and shift the tap one turn at a
time, so you don't need to remove the coil box. The two settings, tap and
tuning, are heavily independent but eventually you'll find a combination
that works.

My setup was BC-230 --> 150 pF ceramic variable --> several metres of 50
ohm coax --> MFJ tuner --> 50 ohm dummy load.
I was able to get a 1:1 indication on the MFJ by trial and error of tap,
tuning and ceramic variable settings, with a solid 5W indicated.

Neil ZL1ANM


On Monday, April 8, 2019, David Stinson <arc5 at ix.netcom.com> wrote:

> Working on the BC-230 transmitter tube
> substitution project, ver. 2.0
>
> To match the output of the transmitter to my
> 50-Ohm non-reactive load,  I've inserted a 100 pFd
> "doorknob" in series with the dummy load.
>
> http://home.netcom.com/~arc5/SCR-183/BC230TX.jpg
>
> Scoping the output of the transmitter, before the
> 100 pFd cap, we see a nice, clean 500V PTP
> waveform.  Image003 attached.
>
> Scoping the output side of the matching cap, going
> into the dummy load, the waveform is 55V PTP with
> 25V of 2nd harmonic.  Image001 attached.
>
> First I thought of some non-linearity in the
> matching cap, but this happens with every cap
> including a high-quality variable.  I did not try
> another 50-ohm dummy load.
> However, using the original-recommended dummyload
> of 100 pFd in series with 5 Ohms
> displayed a clean waveform of about 700 V PTP,
> with only a minor difference in
> the Ant Tuning control between the 100-pFd-5-Ohm
> load and the 100 pFd-50-Ohm load.
>
> Has anyone an idea of where/why this harmonic
> energy?
> Is it possible my 50-ohm "Mighty Fine Junk"
> dummyload has the non-linearity?
>
> TNX OM DE Dave AB5S
>
>
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