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

[K4MO]

Wow!

Thanks Jim and everyone for the great information. There may not be lots of
traffic on this list, but there sure is a lot of knowledge and willing
assistance.

Sounds like a 20A will be fun. Once I get my spare receiver sold I will get
serious about shopping around for one. I don't really have the space for
restoration work right now, so I will be shopping for a working or restored
rig. Maybe I can find someone who will swap one for my NC183.

Thanks again & 73!

Theo K4MO

----- Original Message ----- 
From: "Jim candela" <jcandela at prodigy.net>
To: "K4MO" <K4MO at carolina.rr.com>; <central-electronics at mailman.qth.net>
Sent: Friday, June 16, 2006 10:19 PM
Subject: RE: [Central-Electronics] Your thoughts on the 20A .....


>
>
> 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
> _______________________________________________
> Central-Electronics mailing list
> Central-Electronics at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/central-electronics
> --
> No virus found in this incoming message.
> Checked by AVG Free Edition.
> Version: 7.1.394 / Virus Database: 268.9.0/367 - Release Date: 6/16/2006
>
> --
> No virus found in this outgoing message.
> Checked by AVG Free Edition.
> Version: 7.1.394 / Virus Database: 268.9.0/367 - Release Date: 6/16/2006