[Central-Electronics] Your thoughts on the 20A .....

[Jim candela]

Theo,

   I own a few 20a's, and they kind of grow on you over time. My perspective
may be different than someone else's since my work with them has been
unique....I transform them to higher power than stock where the goal is 6 db
(4x) stock.

Along the way I have learned a few items worth sharing:

Most of the resistors stand the test of time, and few if any have drifted
far enough to justify replacement

All the tubular bypass capacitors, and electrolytics must be replaced if
still original.

These rigs prefer 115 vac line voltage, and they run pretty darn hot at 125
vac. A little fan helps.

Many of these were built as kits, and it is probable that the one you find
at a swap shop or Ebay NEVER did function correctly due to miss-wires, or
confusion on the alignment procedure.

It takes some getting used to a tuning eye tube as the main indication of
transmitter performance. That said, there is a lot of information in the
display.

SSB performance:

   Getting the carrier nulled, is somewhat hit and miss for a deep null (>
40db), but possible. The carrier null pots are pretty coarse, and often
dirty from age.

   Getting the adjacent sideband rejection down to < 40 db from the desired
sideband is possible. Sometimes it seems impossible to get it good for both
LSB and USB, so in that case the alignment can favor whichever sideband you
use on a given band. Experience with the alignment procedure, and some
determination will result in a better setup that has less compromises.

   With the 20a, the audio phase shifter is good for roughly 200-3000 hz,
and the audio amplifiers have response far beyond 3000 hz. This means you
can be a little broad, and at the upper audio frequencies, the phase shifter
degrades, and therefore your adjacent sideband rejection suffers. So lets
say you have audio to 4500 hz, and you are running LSB. Your LSB will extend
4.5 Khz below your carrier, and you will have a USB that kicks in at 3 Khz
above the carrier, and extends to 4.5 khz above. This makes your signal
about 9 Khz wide! It does however sound beautiful when listening to the LSB
with a receiver that has a wide filter (4.5 Khz). The fix here is to either
roll off the highs, or beef up the phase shifter to behave up to 4.5 khz. I
have some mods that do the latter pretty well.

Power Output:

   With 115 vac, figure about 15 watts PEP output on 160, 80, and 20 meters.
40 meters is tougher because of VFO being tripled, and maybe 10 watts PEP
there. I have no experience with these rigs on 15 and 10 meters.

AM performance:

   This thing runs true AM with both sidebands, or LSB or USB with carrier
insertion. The downside with AM on a 20A is that when it over modulates at
the negative peak, it turns into DSB with reduced carrier. This sounds raspy
on an AM receiver, but fine with either a product detector, or synchronous
detector in the receiver. I have devised some limiting circuitry that really
helps out a 20a on AM.

CW performance:

Never tried it.

PM performance:

  It works, but is part AM, and PM at the same time. I have found several
wiring errors with 20A's where PM mode was dead because someone wired in a 1
meg resistor instead of a 15 ohm resistor (color codes are similar). If the
truth were known, maybe Central Electronics made that mistake......???

VFO:

  This rig uses a modified Command transmitter (BC-458) to beat with a 9 Mhz
crystal internally in the 20a. The VFO output is straight, doubled, tripled,
or with the 10m option it is heterodyned to 38 Mhz. These VFO's are pretty
darn stable by 1950's standards, but they do drift, and age causes issues
with these units as well. That said, the drift is usually manageable, except
on 40 meters where the VFO output is tripled. The voltage dropping resistor
in the back of the VFO does it's job, but with two unintended consequences:

1.) The heat contributes to drift
2.) The exposed high voltage (300v, 105v) is exposed, and WILL eventually
cause a shock.

FWIW, my station currently uses a QRO 20a on 160-20meters on SSB and AM.
This is my only transmitter setup, and I love to use it!

http://pages.prodigy.net/jcandela/CE20AQRO/

Open 20aqro3.doc

Regards,
Jim

-----Original Message-----
From: central-electronics-bounces at mailman.qth.net
[mailto:central-electronics-bounces at mailman.qth.net]On Behalf Of K4MO
Sent: Friday, June 16, 2006 4:40 PM
To: central-electronics at mailman.qth.net
Subject: [Central-Electronics] Your thoughts on the 20A .....


I have always wanted to own a 20A, so I am thinking about indulging that
lust and shopping for one. I want to be able to really *use* it, not just
set it on a shelf and talk about it.

I will mate it with a 75A3.

Frankly, I have never even seen one in operation. How do they perform? How
stable is the "matching" VFO? Isn't it some kind of modified military piece?
As boat anchor go, are they fairly reliable? Do they need a bunch of mods to
get decent audio?

Thanks in advance & 73,
Theo K4MO
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