[Central-Electronics] 20A restoration: Tuning a PS-1

Sat Feb 20 15:37:17 EST 2021

Jim, thanks and I did find your ideas on the amfone.net forum a few weeks back. They are “on my list” after I get some basic functions working. Meanwhile, you helped me locate replacement modulation transformers, the Radio Shack 12V filament ones, which now seem to be working.  Hugh, K5HPG also gave me the same link. Thanks guys.

After that I ran into a problem with the L1 coil – the slug would not tune.  It was completely locked in place no matter what I tried. The RF signals were unequal and out of phase regardless of L2 tuning or L1-L2 spacing. I used a different capacitor to get L1 to resonate at 9 MHz but I didn’t have a trimmer for fine adjustment. So Nick Tusa, K5EF came to the rescue with replacements for L1 and L3 (also stuck) out of a 20A that he uses for parts.  Saved… I now have a nice CW signal using a VFO with 16 W output on 80m. The RF phase shift is a perfect 90 degrees and I can suppress the carrier.

Turning to the audio stages, I next found the audio phasing problem and I’ve isolated it to the PS-1. The resistors can be measured for DC resistance without removing wires, but the caps cannot. So I had been wondering what magic the CE techs used to tune this thing, and I think I found it in that article in GE Ham News. Thanks again to Nick for pointing me to this article. This requires removing connections inside the PS-1, however, and I had wondered if anyone had a better way. Your suggestion to try one side of the PS-1 and then the other is a good one. I’ll give it a try Jim.

73,
Bill AB1XB

From: Jim Candela <jcandela at prodigy.net> 
Sent: Saturday, February 20, 2021 10:15 AM
To: 'David Olean' <k1whs at metrocast.net>; comnoz2 at comcast.net; Bill Blackwell <bblackwell2009 at gmail.com>; Jim Candela <jcandela11 at outlook.com>
Cc: central-electronics at mailman.qth.net
Subject: Re: [Central-Electronics] 20A restoration: Tuning a PS-1

Another good reference, see Reply 17:

http://amfone.net/Amforum/index.php?topic=27305.0

Jim

Wd5JKO

On Saturday, February 20, 2021, 9:07:43 AM CST, Jim Candela <jcandela at prodigy.net <mailto:jcandela at prodigy.net> > wrote: 

Somehow I hit send while typing...see below:
    On Saturday, February 20, 2021, 9:01:53 AM CST, Jim Candela <jcandela at prodigy.net <mailto:jcandela at prodigy.net> > wrote:  

  Bill, etc, all;
Since your PS1 is likely broken, I'd try to isolate which section is miss-behaving. As I recall, one side has a 45 degree "lead", and the other has a 45 degree "lag". Therefore I would check each side individually to narrow the problem in half.
The link below has a lot of good info, particularily Reply 6 from me:
http://amfone.net/Amforum/index.php?topic=37143.0

Many easy to implement enhancements...
73,JimWd5JKO
    On Friday, February 19, 2021, 10:32:38 PM CST, Bill Blackwell <bblackwell2009 at gmail.com <mailto:bblackwell2009 at gmail.com> > wrote:  

A couple of folks wanted more info so I am copying the group.

First, the issue of GE Ham News describing PS-1 alignment is here: http://n4trb.com/AmateurRadio/GE_HamNews/ge_ham_news.htm
Scroll down to Volume 5, Number 6 for Nov-Dec 1950, and download the PDF. It's the article on Page 1, "SSB Jr."

To those who know the 20A, the schematic will be familiar in the modulator stages and the PS-1, which is identical, including labeling and values of caps and resistors. PS-1 alignment begins on Page 6 for those who built it from components. The Designer's Corner following the article is by the same author and designer, Don Norgaard, W2KUJ. We are very grateful to Brian Page N4TRB, for providing these wonderful issues of GE Ham News from the past.

Dave: Yes, I measured the phase shift by using a dual trace scope and wave generator. I applied an audio signal from the WG to the mic input of 10 mV peak (7.1 mV RMS) to stay below distortion level. The probes were connected to pins 2 and 7 (grids) of the 12AT7(b) which are the PS-1 output. Starting at 300 Hz I could see both signals in quadrature (90 deg). Changing to 1 kHz and beyond, I watched the traces go farther apart as I went up in frequency. They shouldn't do that. At 1.5 kHz they were actually 180 degrees shifted.

If you switch the scope to XY mode you should see a perfect circle when the phase shift is 90 and amplitudes are equal. If unequal, an ellipse. If not 90, the ellipse or circle will be slanted. Voltages of input pins to PS-1 should be in a 2:7 ratio so adjust R15A to get that. I tested all audio frequencies with speech level pot at 50% and 100% (fully clockwise) and results were unchanged.

One effect of the signals not being at 90 degrees is that one sideband will not be suppressed enough. For the 20A, 40 dB suppression is the goal.

I am new to the list and haven't searched the archive to see if this material is anywhere else, so please excuse if it is!

73,
Bill
AB1XB

-----Original Message-----
From: David Olean <k1whs at metrocast.net <mailto:k1whs at metrocast.net> > 
Sent: Friday, February 19, 2021 5:44 PM
To: bblackwell2009 at gmail.com <mailto:bblackwell2009 at gmail.com> 
Subject: Re: [Central-Electronics] 20A restoration: Tuning a PS-1

Bill

I would be interested in what you find. I have a 20A that I restored, but I did not check the phase shift network. The SSB sounds good and I did not investigate further. How did you measure the phase shift?  Did you use a dual trace scope and an audio oscillator?  I will drag mine back out and check it.  Any details?

Dave K1WHS

On 2/19/2021 4:51 PM, Bill Blackwell wrote:
> Good afternoon.
>
> Has anyone tried to tune a PS-1 phase shift network?
>
> The 20A that I am restoring has about a 150-degree audio phase shift 
> measured at the PS-1 output pins, at 1000 Hz. At higher frequencies it 
> gets worse. At lower frequencies (around 390 Hz), it's a perfect 90 degrees.
>
> The PS-1 input voltage divider is correct at very near a 2:7 voltage ratio.
> Phase shift is constant over any speech level input. So I suspect one 
> or more of the capacitors in the PS-1 has drifted out of tune or gone 
> bad in the 50+ years it has sat on the shelf.
>
> I found, thanks to N4TRB, the issue of GE Ham News where Don Norgaard, 
> designer of the "SSB Jr" (precursor of the CE exciters), describes how 
> to tune the PS-1 in some detail. Is this the best way to go about it? 
> I hesitate to start clipping component leads in this little box to 
> start testing and tuning. The fact that lower frequencies are OK while 
> higher ones are not, could indicate which RC pairs I should start with first.
>
> Is there a better way? Any help or insights appreciated.
>
> Thanks, 73!
>
> Bill
> AB1XB
>
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