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

Fri Jun 16 22:38:37 EDT 2006

Very nice post.  Congratulations on your success with the 20A's.   I've 
promised myself to not acquire any more boat anchors until I get more 
of the ones I have working better otherwise I'd be out looking for a 
20A right now based on your experience.

I wonder if the phase shifter is similar to the 100V phase shifter.   
Do you happen to know?   If so, I would be very interested in  your 
mods for extending the frequency range of the phase shifter.   The 100V 
has a limiter to keep those high's and lows out of the phase shifter 
but you should hear how great a 100V sounds with the limiter bypassed.  
   Maybe I could do that and not be 9 kHz wide.

Jon, K6JEK

On Jun 16, 2006, at 7:19 PM, Jim candela wrote:

>
>
> 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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