Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Central Electronics 20A modulation transformer needed




 
The AM Forum
September 17, 2019, 09:27:07 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Central Electronics 20A modulation transformer needed  (Read 11968 times)
0 Members and 1 Guest are viewing this topic.
wd8das
Member

Offline Offline

Posts: 146


« on: April 03, 2011, 11:19:47 PM »

On the bench is a really nice Central Electronics 20A exciter with an open modulation transformer.  It is a small metal cylinder with the transformer potted inside in black tar.  The schematic shows a p/n of 27A0-79-3.  The marking on the transformer is 27-A0-79-3.  So far repairs have failed, and I've not been able to find any specs on the transformer to help me figure out a substitute.

I believe that the same transformers were used in the earlier models of CE exciters, and since each rig has two identical transformers it seems like there might be a chance to find one somewhere.  Anyone have one, or know of a source?

Thanks...

Steve  WD8DAS

sbjohnston@aol.com
http://www.wd8das.net/
--------------------------------------------------------------------
Radio is your best entertainment value.
--------------------------------------------------------------------


Logged
Ralph W3GL
Contributing
Member
*
Offline Offline

Posts: 748



« Reply #1 on: April 04, 2011, 12:06:25 AM »


    Steve,

    There are two transformers in there, only one is bad...   Do measurements
    on the good one and use your results (resistance /turns ratio, etc) to find
    your replacement.

    Basic problem, should not be too difficult to at least get the specs.  Finding
    a close match might be harder...
Logged

73,  Ralph  W3GL 

"Just because the microphone in front of you amplifies your voice around the world is no reason to think we have any more wisdom than we had when our voices could reach from one end of the bar to the other"     Ed Morrow
WD5JKO
Member

Offline Offline

Posts: 1765


WD5JKO


« Reply #2 on: April 04, 2011, 12:07:28 AM »


Steve,

  I posted some info on these transformers below from the CE Email Reflector.
Sooo, you only need 1 good transformer to run AM! For SSB you need two, and they need to be a matched set (or so it seems).

  In the reflector archives you will find many posts by me between 2003 and 2006 concerning the 20A and my QRO modifications.

Jim
WD5JKO


http://mailman.qth.net/pipermail/central-electronics/

May 2005 article by me:
http://mailman.qth.net/pipermail/central-electronics/2004-May/000320.html

Hi Group,

   I been pretty silent lately, but I have been tinkering on various QRO 20A
strategies looking for the best approach, and hopefully the simplest
approach at the same time. This is still ongoing. Currently using a single
neutralized EL-34 for 160-40 meter operation at about 7 - 10 watts AM
carrier output on my latest approach, QRO 20A #3. Gearing up to try a
7984....

   I am writing this email about the audio modulation transformers TR-2, and
TR-3. My current rig with the EL-34 works perfect on AM (10 watts carrier
100% modulated or 40 watts pep). AM operation only uses TR-2 (the one that
also drives the rear RCA jack). When I switch to SSB, I can get a excellent
carrier null, and the single tone (1 Khz) input makes a pure DC carrier 1
Khz offset so long as the power stays below 5 watts. Above that I lose the
adjacent SB rejection, and see significant ripple on the carrier wave that
increases with the RF output.

   Looking at the audio feeding the balanced modulator (scope), I see that the
output from TR-3 becomes increasingly asymmetrical as the SB rejection
becomes increasingly compromised. I replaced the balanced modulator diodes
to see if they were acting strange, but no change. I looked at the 12AT7 (b)
where pin 6 (plate1) feeds TR-2, and that was clean. Looking at pin 1
(plate2) it distorts at higher levels feeding TR-3. If I look at the grids
of same tube, the signals remain clean at all signal levels corresponding
from zero to maximum pep rf output. The distortion, and hence sb rejection
problem seems to occur at the plate at pin 1 of 12AT7 (b). Changing the
12AT7 (b) did not help. Initial thinking here is that I have a bad TR-3. But
wait until I present the data from 10 transformers.

The data:

>From 20A QRO #1 (good transformers):

TR-2  1.36K ohm, 10.82 henry
TR-3  1.29K ohm, 11.00 henry


>From 20A QRO #2 (good transformers):

TR-2  1.57K ohm, > 20 henry (meter cannot measure above 20h)
TR-3  1.37K ohm, 19.4 henry


>From 20A QRO #3 (suspect TR-3 as bad):

TR-2  1.65K ohm, 12.7 henry
TR-3  1.23K ohm, 12.3 henry *** BAD?Huh


>From 20A with meter on front (unknown transformers):

TR-2  1.26K ohm, 10.3 henry
TR-3  1.25K ohm, 11.2 henry


>From 20A bought from Kimberly (1 open transformer):

TR-2  1.3K ohm, 13.4 henry
TR-3  open primary

Transformer from Bob W1CNY:

TR-?  1.3K ohm, 15.5 henry

**Measuring the primary inductance with my Beckman hand held meter


Based on the data, I'd say that TR-3 in 20A QRO #3 is fine. Listening to
the rig with my station monitor and using voice it sounds very peculiar,
something like when a ham has a RF flashover during voice peaks. Maybe the
transformer is OK, but at higher signal levels it arc's turn to turn
internally??


   The data suggests quite a bit of variability in both ohms, and primary
inductance. The transformers do seem to be matched sets however. Does
anybody know about this, or have experience in this area of the 20A? Some of
the transformers (bare aluminum cover) have part number 27A0-79-3 on them,
and others (gray colored) have no identifying part number.


My next move is to replace the suspect TR-3 with TR-2 from from the
"Kimberly" 20A donor since it is close in primary inductance.


EDIT: The problem turned out to be that the two 9 Mhz RF coils in the 90 degree RF phase shifter were too far apart. This one was a kit, and there was an extra hole that the builder mistakenly used. Moving the one coil, and re-aligning, the bad behavior went away.


Regards,
Jim Candela
WD5JKO
Logged
WD5JKO
Member

Offline Offline

Posts: 1765


WD5JKO


« Reply #3 on: April 04, 2011, 01:54:08 AM »



  I looked at a 20A transformer, and the DC resistance of the secondary winding is about 16 ohms. Taking an audio generator at 1 Khz, I see a 10:1 step down ratio from the 12At7 driver to the balanced modulator.

   So to get the 10:1 turns ratio, look for a 100:1 impedance ratio. One such transformer is the Edcor, 8000 ohm / 8 ohm:

http://www.edcorusa.com/products/483-xse10-8-8k.aspx

  My bet is the original transformer was 20000 / 200 ohms. The Edcor has the right turns ratio, and it will scale up impedance wise to a point. Not sure how high, but I bet it will work.

  I looked through the Hammond stuff, and found this one:

http://www.hammondmfg.com/jpeg/125ASE_B.jpg

Here we have a 17.7:1 Tr.

Good Luck!

Jim
WD5JKO
Logged
WQ9E
Member

Offline Offline

Posts: 3225



« Reply #4 on: April 04, 2011, 07:06:46 AM »

Steve,

15 years ago I was repairing a newly acquired CE-20A and also found one transformer open.  I was in a hurry to replace it for an upcoming vintage gear "contest" so I replaced them both with a pair of 12.6 volt transformers from Radio Shack.  I planned to replace these with proper audio transformers later but they worked so well they are still in place.

This particular CE-20A is paired with the matching VFO, CE-600A, CE monitor scope, and a RME-4350A with CE slicer.  It worked and sounded fine with the original cheap RS replacements so they are still in place. 

Also check the ER article about using a large capacitor to isolate the phasing networks from DC to prevent intermittent crackling sounds. 
Logged

Rodger WQ9E
WD5JKO
Member

Offline Offline

Posts: 1765


WD5JKO


« Reply #5 on: April 04, 2011, 08:44:30 AM »

I was in a hurry to replace it for an upcoming vintage gear "contest" so I replaced them both with a pair of 12.6 volt transformers from Radio Shack.

  Yup, 10:1 turns ratio!

Jim
WD5JKO
Logged
KM1H
Contributing
Member
*
Offline Offline

Posts: 3519



« Reply #6 on: April 04, 2011, 03:45:22 PM »

I wonder if CE just potted the transformers made popular by the W2EWL QST rig? If so those still show up at hamfests
Logged
WD5JKO
Member

Offline Offline

Posts: 1765


WD5JKO


« Reply #7 on: April 04, 2011, 04:33:46 PM »

I wonder if CE just potted the transformers made popular by the W2EWL QST rig? If so those still show up at hamfests

Carl, I think you are correct. Here is a link to that rig:

http://pages.prodigy.net/jcandela/W2EWL/W2EWL_Special.PDF

  The parts list says 20,000 ohm to 200 ohms, or 10:1 turns ratio.Also Tony Vitale did not run the driver tube plate current through these transformers like CE did. Instead he used a 22K 1 W plate load resistor, and capacity isolated the DC current from the transformer primary.

Jim
WD5JKO
Logged
KM1H
Contributing
Member
*
Offline Offline

Posts: 3519



« Reply #8 on: April 04, 2011, 09:08:18 PM »

Thats what I thought I remembered Jim but its been forever since I built that little rig. I can still see those little tan transformers.

That site will be scrapped 6/11 so hopefully it gets moved somewhere

http://pages.prodigy.net/cgi-bin/index.cgi?pwpurl=http://pages.prodigy.net/jcandela/W2EWL/W2EWL_Special.PDF

Carl
Logged
w1vtp
Member

Offline Offline

Posts: 2612



« Reply #9 on: April 05, 2011, 08:12:38 AM »

Thats what I thought I remembered Jim but its been forever since I built that little rig. I can still see those little tan transformers.

That site will be scrapped 6/11 so hopefully it gets moved somewhere

http://pages.prodigy.net/cgi-bin/index.cgi?pwpurl=http://pages.prodigy.net/jcandela/W2EWL/W2EWL_Special.PDF

Carl
Already gone!
Logged
KM1H
Contributing
Member
*
Offline Offline

Posts: 3519



« Reply #10 on: April 05, 2011, 10:21:10 AM »

Click on Jim's link, the article and the warning will both appear.
Logged
WD5JKO
Member

Offline Offline

Posts: 1765


WD5JKO


« Reply #11 on: April 05, 2011, 05:36:00 PM »

Click on Jim's link, the article and the warning will both appear.

   This is infuriating to me on how ATT is removing a service (prodigy personal web space) I've had for years, and no rate cut either after this web space evaporates. I called them, and complained to some guy in India. Never got through to him that the problem was with their service, and not with my computer. All I want is for the pop-ups to be less sinister, or to go away until the service is unplugged June 1st.

15 years ago I was repairing a newly acquired CE-20A and also found one transformer open.  I was in a hurry to replace it for an upcoming vintage gear "contest" so I replaced them both with a pair of 12.6 volt transformers from Radio Shack.  I planned to replace these with proper audio transformers later but they worked so well they are still in place.


   Roger,

It seems that Rat Shack still carries a version of this transformer:

http://www.radioshack.com/product/index.jsp?productId=2102494#inTheBox

12.6V 300mA PCB-Mount Miniature Transformer

Model: 273-1385  | Catalog #: 273-1385

I wonder if AM is better with these transformers versus the stock XFMR? With stock on AM, the balanced modulator diodes load the transformer on only a half cycle basis. So you might see 80% upward modulation at 100% downward modulation.

This asymmetry can be compensated by taking a 1N34a diode in series with about 220 ohms, and connected to an RCA connector. The connector plugs into the back of the 20A where there is an output to see the audio for scope trapezoid pattern. If the diode is phased properly, the modulation tends to even out such that the average level can be raised without becoming DSB with reduced carrier.

Note: must remove plug for SSB!

Maybe the RS transformers improve the AM situation without the need for loading compensation?

Jim
WD5JKO

Logged
WQ9E
Member

Offline Offline

Posts: 3225



« Reply #12 on: April 05, 2011, 06:34:11 PM »

Jim,

I am not sure but I did get another 20A as part of a trade deal last year.  Assuming the stock transformers are good I will make some tests.  As busy as I am currently it will probably be early summer before this happens.

Rodger
Logged

Rodger WQ9E
w1vtp
Member

Offline Offline

Posts: 2612



« Reply #13 on: April 05, 2011, 07:55:23 PM »

Picked this 20A up at NearFester a while back.  It probably need work but I'm looking forward to putting it on my project bench post-retirement

Nice catch with the companion VFO - I think

Al


* CE 20A WITH VFO.jpg (256.33 KB, 2000x1325 - viewed 537 times.)
Logged
W9AMR
Member

Offline Offline

Posts: 1


« Reply #14 on: January 26, 2012, 03:18:47 AM »

iI beleive the Transformer in the QT1 unit is the same. Steal it if you're not going to use VOX.  Although you might have found one by now.
Logged
ke7trp
Contributing
Member
*
Offline Offline

Posts: 3659



« Reply #15 on: January 26, 2012, 12:28:56 PM »

Good info on the rat shack transformers!  Gotta file that away. 

Here is my CE20A setup.  VFO, CE20A, scope and slicer with Q multiplier AND the Plug in preamp on the back.

I used this into the GK500 with an SX28A as a reciever with the slicer.  Talk about an "active" radio experience.. LOL.  The 28A is drifting one way, the Slicer is drifiting the other way!

Neat little stations. I never use mine. I might sell it off.

C


* ce20A.jpg (219.65 KB, 781x1306 - viewed 447 times.)
Logged
WQ9E
Member

Offline Offline

Posts: 3225



« Reply #16 on: January 26, 2012, 12:46:43 PM »

Nice setup Clark.

Vintage SSB operation is very high involvement compared to AM.  Once you get the AM rig setup, everything is OK unless something blows up.  But with vintage SSB using a phasing type exciter you get to have fun keeping the carrier balanced, the VFO on frequency, and the receiver (often with both a drifting VFO and drifting BFO) somewhat tuned in.

Gonset uses a receiver type single crystal filter in the GSB-100 transmitter as a carrier notch filter which provides for excellent carrier suppression even with a little drift in the carrier balance controls.   

The way a lot of phasing rigs are designed if you optimize it for one sideband you can achieve alternate sideband suppression similar to a good filter rig but a compromise setting allowing for use of either sideband generally results in noticeably less unwanted sideband suppression.

These little phasing rigs are capable of producing very nice quality AM.
Logged

Rodger WQ9E
WD5JKO
Member

Offline Offline

Posts: 1765


WD5JKO


« Reply #17 on: March 29, 2012, 09:38:36 AM »


    Last week my 20A with the 7591 finals crapped out pretty good. First the frequency started shifting, and then the audio went to heck on SSB, and on AM no audio. Well I went in circles for a while. Turns out I had two problems.


1.) One of the modulation transformers opened up. Fortunately I had a spare. I put it in and also did the para-feed trick, and used a 22K 1W plate load resistor. To get the thing to bias at 1/2 plate voltage I tried a 12AV7 instead of a 12AT7, and that got the plate voltage to 150V.

The change was rather simple. After adding the two 22K 1W resistors from each modulator plate to the B+, I took the two RED modulation transformer wires off the B+ terminals and keeping them together, I then bypassed that junction to ground with a 1 uf 275V AC rated polypropylene capacitor.

The lower gain of the 12AV7 was offset with a higher audio pot setting. One thing I've seen now on multiple tube rigs is a form of waveform distortion when the audio gain pot is at the center. At the center the wiper is at the highest impedance, and depending on the load impedance, this can cause some distortion that goes away at higher or lower pot settings like 1/4 turn or 3/4 turn.  

Years ago on the Central Electronics email reflector I sent R-C additions to the audio phase shifter where I did get good response from about 200HZ TO 4 khz. After the change to para-feed it improved to about 150 hz to 6 Khz before the sideband suppression dropped to about 30 db. Over the range of 200 to 4500 Hz the sideband rejection is very good.  Apparently there was phase shift on those transformers that was reduced when the DC magnetization current was eliminated. For sure, on AM, I can pass a triangle waveform with less ramp non-linearity than before. A sine wave is near perfect all the way to 100% modulation with a 12.5 watt carrier level.

2.) The frequency shift seems like a heater cathode short on the 9 Mhz oscillator tube. I use a 6MU8, and after replacing it the problem seemed to go away. I was quite upset over this since I had the Huff & Puff (Cumbria Design) addition to the VFO, and a crystal oven for the 9 Mhz crystal. Prior to this issue I was enjoying a very stable 20A that was ready to talk on SSB after a mere 5 minutes warm-up. Time will tell if I am back to that again.

I got everything back on the air two hours late for the Wednesday evening 7:30 PM CST 3870 Boat Anchor net...

back in the saddle again with the 20A..:-)

Jim
WD5JKO
Logged
ke7trp
Contributing
Member
*
Offline Offline

Posts: 3659



« Reply #18 on: March 29, 2012, 12:26:08 PM »

Good work. Sounds like your rig is very modified. My central station is lined up on display. It worked well and sounded good.  But the Sideband supression was not enough for the low Drive amplifiers in the shack.

C
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.064 seconds with 19 queries.