[ARC5] BC-230 / BC-430 - mod for coil set C-381

[AKLDGUY .]

As mentioned in my last post on this topic, I've set up a website where the
modification of the C-381 coil set for 80m is described.

I don't yet have a 6AQ5 plug-in for my BC-230, so the FET (MPF-102) is
still tacked onto the oscillator tube socket. It works well at 15VDC.
The results are, with 270pF polyester capacitor:

DIAL     FREQUENCY (KHz)
0                   3427
5                   3490
10                 3580
15                 3690
20                 3825
25                 3992
30                 4157

Experience has shown that the FET, having far less capacity than tubes
with their long internal leads and interelectrode capacitances, produces
about the same frequency at the low frequency end of the dial as would
be generated by the tube version. Within a few dozen KHz. This is due
to the tuning capacitor plates being fully meshed and obscuring the
difference.

At the high end of the dial (unmeshed), the frequency is quite a bit higher
for the FET version than would be obtained with a tube. In other words
the effect of the tube capacitance shows up much more noticeably at the
high end of the band, and will bring it down lower than the FET version.

So I think the spread obtained with 270pF is about right, and 270 may be
very close to, if not the final figure we use.

If anyone is following this and has tubes fitted, they could try the 270pF
in
the oscillator for starters. It's acceptable to use a ceramic or polyester
capacitor for preliminary tests, but it should be a high voltage unit of
300V
at least.

Later it should be exchanged for a silver mica unit. Silver micas are much
more stable and recommended for oscillator service. They may be called
simply "mica" capacitors in catalogs. Silver mica is a name based on the
historical method of manufacturing them.

We will establish the value for the PA amplifier capacitor later.
My website explaining the modification:

www.zl1anm.com

Comments and suggestions about the site are welcome.

73 de Neil ZL1ANM

On Fri, Oct 24, 2014 at 10:55 AM, AKLDGUY . <neilb0627 at gmail.com> wrote:

> On Friday 17 October, a BC-230 transmitter came into my possession,
> complete with C-381 coil set.
>
> I've rapidly brought myself up to speed on the circuit and physical layout
> and have developed a modification for the coil set C-381 that shifts it
> from 2500-3200 Kc/s to cover the 80m band.
>
> This modification has been tried first on the oscillator coil, since it
> would
> be far easier to track changes in the resonant frequency by listening for
> an oscillator than trying to measure the shift in frequency of the PA tank.
> The idea was that if the oscillator could be shifted successfully, the same
> mod could be performed on the PA tank.
>
> In order to test the modification, but not having original tubes or the
> time
> to build and test a 6AQ5 plugin, I decided to connect a FET (MPF-102)
> to the oscillator tube socket and run it at 15VDC. The FET worked well
> with the two unmodified coil sets (C-381 and C-385) in my possession.
> These nominally cover:
> C-381:  2500-3200 KHz
> C-385:  6200-7700 KHz
>
> The frequency at the bottom end of the dial was 2522 KHz (C-381) and
> 6217 KHz (C-385). Both are very close to the specified frequencies that
> should be obtained when a tube is fitted for the oscillator.
>
> The frequencies at the top end of the dial were 3482 and 9030 KHz
> respectively. The C-381 is a little higher than spec but the C-385 is a lot
> higher. I put this down to the fact that the FET capacitance is far less
> than
> that of VT tube internal capacitances, creating a worsening discrepancy
> as the dial setting goes higher.
>
> Encouraged by the success with the FET, I went ahead and modified the
> C-381 oscillator coil.
>
> The modification does not involve removal of coil turns or capacitor
> plates.
> It consists of soldering a small silver mica capacitor (approx 250 pF) in
> series with the hot end of the oscillator coil (inside the coil can).
>
> The mod worked well, but because I'm using a FET I cannot yet publish
> a definite value for the capacitor. When a definite value is derived (by
> experiment) it should hold for everyone, so that everyone will simply need
> to fit a silver mica capacitor of that value.
>
> What needs to be set in motion is a web page that explains the mod,
> describes how to do it, and shows the high quality 2 MB photos that I've
> taken at each step.
>
> I am currently investigating the setting up of a page that would host that
> kind of data so that others can see what I did and try the mod with tubes
> inserted.
>
> 73 de Neil ZL1ANM
>