Re: Multiplex Decoder questions.



Robert McLean wrote:

> "John Byrns" <jbyrns@xxxxxxx> wrote in message
> news:jbyrns-2610051125450001@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>
> >
> > The H.H.Scott 4310 circuit is similar and can be found here:
> >
> > http://users.rcn.com/jbyrns/pics/4310-p1.jpg
> > http://users.rcn.com/jbyrns/pics/4310-p2.jpg
> >
>
> Thanks. More circuitry to digest.

The 38khz oscillator I dreamed up a few years ago
dropped one of its diodes, and that created some 19kHz that
leaked through to the output, so I rebuilt the whole oscillator.
It did have a 19kHz triode amp ( 1/2 6U8A) with selective filter in
front of it,
and that fed the other pentode 1/2 as a synchronised 19khz oscillator
with a pair of diodes to double the F
into a tuned 38kHz tranny.
one of those diodes went strange as germanium does.
But now I have the same triode amp to amp up the 19kHz piloy being
received, then
the more usual doubler diodes and then a tuned 38khz tranny loading the
pentode.

I will have to make an untuned tranny for the triode input stage because
after the mod the
input stage oscilated with a tuned grid and tuned plate circuit, and the
following
38kHz oscillator wouldn't lock onto the diode pulses from the first
stage, so far more easy oscillation
than I had before, but not synchronised to the input 19khz pilot.

With every mod there is always the unexpected problems of things working
not quite as expected,
especially with oscillator circuits that one wants to synchronise with a
small input signal.
With both stages trying to oscillate, they went a bit LF unstable, and
the oscilations
undulated if the 19khz signal from a station was too low.
So I have to get the thresholds right and range of lock in F and the
stability right.

I have a lot to read about on decoders, but not one that
has diode detectors to give a detected L-R and -L+R signal using 76kHz,
which seems to me to be only possible in the scheme I have been using.
This means filtering the L+R signal out of the composite signal from the
ratio detector
and then adding the 38kHz carrier, so a pair of symetrical AM waves of
opposite
phase can then charge diodes at a 76khz rate making filtering out the
ripple much easier,
but the filtering prevents the phase tracking one must have to get good
separation at HF.
Many decoders do have filtering of signals before the matrixing, so
good separation at 15khz is impossible.

I revised the BA1404 circuit with stereo audio input signals so that the

19kHz isn't a square wave but a sine wave.
This meant adding an LC circuit to the board and a couple or other R&C
parts,
and when i tested it i got the 19khz sine wave but still rather
distorted with what looks like
about 20% 5th harmonic, and nobody on the web who promoted this add on
circuit said that this would occur. So like all the experts one sees
saying how it ought to be done,
one has to expect their advice isn't always full or perfect.
But when i retested the stereo FM generator using an old solid state
Audio Reflex tuner
a I got very nice results, with slightly less noise and more separation,

so the mod didn't upset the BA1404 chip operation.

I may finish up installing a spare 38kHz transformer to act as a drive
element
for the 4 diode balanced demodulator, and settling for the 38khz ripple,

but then I think the shape of that is more benign than the usual saw
tooth one gets with
a normal diode charging a cap with R discharging it, so perhaps
filtering after the diodes
won't be too bad.

I don't think may people understand MPX decoders here. Its just that one
step too hard,
like anything to do with TVs. Such devices as FM radios and TVs and
videos all have
routine techniques which are quite difficult to understand if you don't
deal with them everyday.
One never sees posts about TV sets here.

One decoder circuit I have looks so very simple with a
tube that has deflector plates so the cathode current can be swung to
one or the other of two anodes
at the 38kHz rate.
But the tubes for these have not been made for 40 years, and won't be
again;
maybe there is a way of using a twin triode and a couple of diodes to
act as the beam switcher,
but at the end of the day i doubt the stereo outcome is any more
listenable than a Scott.

The Scott circuits have the refinement of muting and threshold action
on tuning; they are civilised to use, now howls of noise between
stations etc.
So considerable parts of the circuit are devoted to such bells and
whistles.

But there is considerable filtering going on, and I need to study rather
a lot more.
I somehow think that despite John Byne's avid interest in the venerable
Scott
machines, he may not be totally familiar with exactly how the MPX
section works.
There is no spread *** I know with all the typical wave forms at all
parts of the circuit
so its easy to visualize what takes place.


Patrick Turner.






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