[ARC5] 10 meter BC-454 - more

[Kenneth G. Gordon]

On 15 Feb 2013 at 17:58, Neil wrote:

> > Result of solidifying the coils was to reduce, but not eliminate,
> > the ringing, but it did eliminate the "jumping" of frequency. The
> > jumping was obviously caused by the loose slugs.
> 
> I believe you once said that every plate except one had been removed
> from each stator and rotor,

No.  Not quite.

The original hacker removed all but one of the rotor plates, but left all the 
stator plates in place. At this point, for reasons I will mention below, I think 
this is the cause of the ringing.

This evening, I removed the tuning capacitor (which I am almost certain is 
the root cause of the ringing problem) and very, very carefully examined it.

I could find absolutely nothing physically "wrong" with it, except one very 
minor thing: on each of two of the remaining stator plates, an adjacent pair, 
in the antenna section of the capacitor, there were two fairly large burned 
marks as if there had been an electrical arc there. Possibly a nearby lightning 
strike sometime in its past history. They were quite obvious once I had the 
cap out.

Anyway, holding the capacitor in my hands and turning it over and over, I got 
to thinking that with all those unused stator plates, maybe THEY were 
vibrating, or at least maybe one of them was. Perhaps the one or two that 
got "tempered" by the arc.

So, I very carefully removed all the rest of the un-used stator plates. In the 
process, I dropped one on the concrete floor: it rang at the same pitch, the 
same audio tone, as that ringing noise I hear when I tap the case. (I am also 
a musician.)

"Hmmmm...." I said, "...I wonder..."

> and that these plates were the split ones
> that are tweaked during alignment.

Yes, that is so.

> If so, I believe they are flexing
> due to no longer being braced to adjacent plates by the cross piece at
> their ends.

That is entirely possible and is yet another thing for me to think about.

> IMO, a far better way of raising the operating frequency would have
> been to solder a small capacitor in series with each tuning cap
> section, the value being determined by experiment. This retains the
> structural integrity of the plates, raises the frequency, reduces the
> full mesh to nil mesh tuning range, and minimizes any unwanted changes
> in the tuning cap capacitance.

Oh, yes, I most certainly agree!!! It is very obvious to me that who ever wrote 
these two articles never did actually try their ideas out nor test them. The 
whole thing is "kludgey". In fact, both are.

> Think of the small cap (high reactance) and tuning cap (low reactance)
> as a very large resistor R1 in series with a very low resistor R2.
> Even major changes in R2 have small effect on the total series
> resistance. In other words the small cap has now become the dominant
> frequency-setting component.
> 
> If you have a spare tuning cap from the same model, I'm sure it would
> be worthwhile to change it and try this method.

I probably do have one somewhere around here. After all, I certainly have 
enough parts and hacked receivers.

However, at this point, I am going to re-install this butchered capacitor and 
see what, if anything, has improved....or not....

The things are most certainly very beautifully made.

I might add that in the process of removing the unused stators from the 
antenna section, I dislodged and dropped one of the three glass or plastic 
balls that hold the stators in place...three times...and found it each time: 
twice on the floor amongst all the detritus, and a third time in my lap.

St. Anthony is my friend! :-)

Ken W7EKB