[AMRadio] FW: [Central-Electronics] CE 20A Phase shift question


Jim candela jcandela at prodigy.net
Mon Jan 30 22:51:50 EST 2006


something from another group that may be of interest to some of you on this
group. It starts from the bottom as usual. I do primarily use my 20a on AM
:-)

Jim

-----Original Message-----
From: central-electronics-bounces at mailman.qth.net
[mailto:central-electronics-bounces at mailman.qth.net]On Behalf Of Jim
candela
Sent: Monday, January 30, 2006 9:11 PM
To: K3PID
Cc: central-electronics at mailman.qth.net
Subject: RE: [Central-Electronics] CE 20A Phase shift question


Ron,

   Sure sounds like progress at ARS K3PID. I am curious about a few things
with your setup, and a few other comments.

With that VFO, I wonder how much you had to fiddle with it to make the
output tuned circuit track the input tuned circuit? I gave up on mine, and
added a second variable capacitor to tune separately. When operating on 80
meters, turning that 1625 plate tuning capacitor sure pulls the oscillator,
upwards of 15 Kc. The 1625 is neutralized, or was when there was two of
them...

Your carrier null at -72 db seems rather optimistic, and I'd wager hard to
hold. I did one 20a once where I used discreet resistors (3) along with a 5K
pot to in effect use the pot as a "fine null", while maintaining the circuit
impedances as when stock. The 20a I have on the air now has a stock balanced
modulator pot setup, and I'd swear that my null can degrade 10db by just
looking cross at the pots! Later 20a's have a temp compensation circuit in
there so you can hold the null better while the rig warms up.

My 20a's are modified for QRO, and therefore I lost 15 and 10 meter
operation because of additional capacitance from the tube plate(s) to
ground. Still, with a stock 20a to get on 10 meters the VFO needs to put out
say 29 +/- 9, which is 20 or 38 Mhz. There is a CE module kit for 10 meters
that goes into the BC-458 that has 3 tubes, and a crystal in it. I recall it
putting out 38 Mhz for 29 Mhz transmit frequency. Without the optional VFO
kit, your limited to 160 to 15 meters unless you use a crystal. Keep in mind
that when using a crystal, say 3740 for 80 meter CW, the balanced modulator
doesn't work...making this a old time Novice rig. A crystal that beats
against the 9 Mc should work OK for phone work.

I wonder what power you get from yours? On 160 and 80 meters you should see
the maximum. The 6AG7's run pretty darn hot, and a little 12 volt DC fan on
top of the 20A case can cool things a whole bunch. You could 1/2 wave
rectify the filament voltage and with a capacitor filter get 8-9 volts for
the fan. Running down there makes it almost silent. The 6AG7's run class
AB1, and with enough drive, they go Ab2 and draw grid current. If you can
hold the bias stable, several more watts RF output are possible. What
happens stock is when the peak RF grid swing exceeds the -10 bias, the bias
just gets more negative. This is self limiting. Still with today's line
voltages (like 125) you might get upwards of 20 watts from that 20a, and
maybe 25 watts with stabilized bias. On 40 meters where the VFO has to
triple, drive is a problem, so output is usually much lower. 20 meters
should provide good output as well.

Keep up the good work! Maybe we can work each other on 20 meters when your
ready?

Regards,
Jim
WD5JKO



-----Original Message-----
From: central-electronics-bounces at mailman.qth.net
[mailto:central-electronics-bounces at mailman.qth.net]On Behalf Of K3PID
Sent: Monday, January 30, 2006 8:04 PM
To: central-electronics at mailman.qth.net
Subject: Re: FW: [Central-Electronics] CE 20A Phase shift question


Jim, Thanks for the tips!!! I had completely overlooked the 9 MHz getting
into the audio. I added both of your mods!

I could only hold the circle from about 250 Hz to just under 2500 Hz, after
the mod, the circle remains pretty true ( your eyes start to play tricks
after you stare at it for awhile) through just over 4000 Hz ( maybe higher
:>) ).  I can suppress the carrier to about -72db and I can get the unwanted
sideband a little lower.  I just finished converting a BC-458 to the
multiband VFO and while I haven't tried it on 15 or 10 yet it seems to be
working fine on 160 ( my favorite band) and 80, 40 & 20.

I haven't had this much fun since I first got my ticket in 1960......

Next... feed the 20A into the T-368 for some real punch!!


73 & Tnx
K3PID
Ron H.




----- Original Message -----
From: "Jim Candela" <jcandela at prodigy.net>
To: <central-electronics at mailman.qth.net>
Sent: Sunday, January 29, 2006 12:44 PM
Subject: Fwd: FW: [Central-Electronics] CE 20A Phase shift question


>
>
> Ron,
>
>   Hats off to you for remembering how to use Lissajous
> Figures! I forgot about that stuff, and you are
> correct that two signals 90 degrees apart fed into
> scope in X:Y mode should make a nice circle. I did set
> up my 20a a few times with the scope, and with
> perfectly compensated 10X probes (OK, best I could do)
> I was able to get pretty close to correct setup by
> varying the two audio pots within the 20a. Figuring
> out 90 degrees phase shift was pretty tough, but 90
> +/-5 was doable. Heck next time I'll try Lissajous
> figures! Setting the RF phase shifter with a scope
> (looking for 90 degrees shift) is more challenging,
> and a 100 Mhz high quality scope with two matching
> probes helps. Still, doing it the CE way is best as
> described in the original alignment instructions.
>
>    The fuzz in your circle was probably the 9000 Khz
> from the crystal oscillator. Those bypass capacitors
> C11, and C12 don't filter out the 9 MC very well, and
> they do it differently which is I think due to
> different circuit layout between the two audio
> channels. I bet the circle intensity modulation maxima
> and minima you saw was due to power supply ripple
> beating with the tone injected from your audio
> generator.
>
> I do use the RCA jack in the back for a scope input
> (Trapezoid pattern on AM), and the 9 Mc signal is
> pretty high. I filter it out with a simple R-C of 1.2K
> series, and 2700 pf shunt at the jack.
>
>    So over what range of frequencies can you maintain
> a circle? My experience is that the 20a phase shifter
> does a decent job from 300-3000 hz, and beyond that
> the sideband rejection falters. There is a low pass
> filter in the audio (RC at gain control input), but
> the drop off is not all that much up to 5 Kc. I chose
> to fiddle with the phase shifter so it behaves better
> up to 5 Kc. What I did was to bridge R15 (400 ohms)
> with 1.8K, and then make a series R-C consisting of
> 390K and 180pf. The RC goes between PS-1 pins 2 and 9.
> After the mod, go back and set up the pots again, and
> see if things are close to 90 degrees phase shifted up
> to 5 kc.
>
> I just got off 20 meters USB with my 20A.....
>
> Regards,
> Jim
> WD5JKO
>
> >
> > -----Original Message-----
> > From: central-electronics-bounces at mailman.qth.net
> >
> [mailto:central-electronics-bounces at mailman.qth.net]On
> > Behalf Of K3PID
> > Sent: Sunday, January 29, 2006 11:12 AM
> > To: central-electronics at mailman.qth.net
> > Subject: [Central-Electronics] CE 20A Phase shift
> > question
> >
> >
> > Setup:
> >
> > CE 20A to dummy load
> > 1 KHz audio input tone
> > scope accross the output of the phase shift network
> > ( 90 deg shift)
> >
> > Observation:
> >
> > The X-Y scope display shows a very nice circle which
> > varies in diameter with
> > the amplitude of the input and also varies in
> > diameter (although less
> > variation) with the input frequency. The inside of
> > the circle always looks
> > to be a well defined circle but the thickness of the
> > line has some variation
> > around the circumference.  Thin and sharp at 6:00
> > thru 10:00 and a little
> > thicker from 12:00 to 2:00.
> >
> > The two signals look great in the time domain just
> > like two sine waves out
> > of phase.
> >
> > Question:
> >
> > Is there something going on that needs correction?
> >
> > K3PID
> > Ron H.
> >
> >
> >

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