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Author Topic: Swap meet find - Central Electronics 20A  (Read 2314 times)
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W6BRY
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« on: July 04, 2022, 12:43:00 AM »

Happy Independence Day!!

A few weeks ago I was at a boat anchor swap in Minden NV and found a CE 20A for cheap and brought it home. I have been working on it for a while and got it running pretty good but still need to figure some things out on it so I figured I would ask here as there seems to be quite a few posts on these old rigs.

Mine is in pretty good shape and cleaned up well. I was able to find a few crystals in my collection that got me on 75M but I really wanted to use it on 40 so I started hunting for a CE 458 VFO. Then a friend of mine sent me a link to an ebay auction for a 458 at a reasonable price so I clicked the BIN button and got that on it's way to a new home.

While waiting for that to arrive, I started on the alignment but the relay coil opened up, so I had to wait another week to get one from Nick Tusa so I could finally start working on it again this week.

Once I received the VFO I found it is missing parts and needed a full rebuild. I ordered a 4K 20W power resistor and started working on a cable but then I realized I could possibly drive this with my DDS Signal Generator so I made a cable to connect to the crystal plug on the front and boom I am now able to dial in whatever frequency I want and it's working great and very stable.

So at this point I have run through the alignment and I am getting decent power on 75M (18W) and decent audio but I am having trouble adjusting the 2 pots for the phasing. I just recently found a writeup by Nick on how to adjust the 2 pots but it sure doesn't seem to make much of a difference when I tweak the pots. I am able to null the carrier and the audio sounds decent, so I wonder if I am chasing my tail now?

I attached a photo but I am not sure how to insert it into the message.


* 20220703_144000.jpg (718.91 KB, 2030x1522 - viewed 216 times.)
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WD5JKO
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WD5JKO


« Reply #1 on: July 04, 2022, 09:08:43 AM »

Brian,

   The 20a rig can be intoxicating, and so frustrating at times.

Always consider, that if yours was built as a kit, then it may have never performed to specification. Translation is that miss wires, and unsoldered connections are possible.

Since you can null the carrier, and have modulation, then it appears that a lot of stuff is working. That means you are offsetting the DDS properly, or does it? For 40m, the original BC458 output was tripled, whereas on 80m, and 20m, it runs at the fundamental frequency. You also need 8v p-p as a minimum, often a challenge for a DDS VFO unless you have an amplifier.

The Carrier Null pots are ideally near the center when the carrier is nulled. This is not always possible, but when one or both pots are at an extreme, bad things happen when you modulate; audio peaks exceed some threshold, and side band suppression suffers as the RF PEP output is increased. I had one 20A where the RF phase shifter coils were not spaced apart properly (there is an extra hole in the chassis). The result was as I described above.

Your issue is likely that the two audio channels are not correct in amplitude, or phase, or perhaps that one channel is missing entirely. The other channel that is used on AM may be OK.

I like to use a dual trace scope, and two identical 10X compensated probes to look at the audio at the output of the modulation transformers. The amplitudes should be balanced, and the phase close to 90 degrees. If your scope has an X:Y mode, then ideally you will display a perfect circle when injecting a 1.2 Khz tone.

BTW, off topic...the 6AL5 bias rectifier..If it fails to light up (dirty socket pins), then there will be a Chernobyl effect as the 6AG7's superheat while fighting the power supply....Bad day!

Hope this helps.
Jim
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W6BRY
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« Reply #2 on: July 04, 2022, 11:40:30 AM »

Hi Jim,

I was hoping you would see this and reply. I appreciate your help on this.

I am assuming my unit is factory built because the tube sockets are riveted in place. I have always heard that is the case with Johnson Viking gear so if that is not a clue on these please let me know.

One thing I am confused about is that switching from SB1 to SB2 makes no difference on my SA display, as I expected to see the sideband move from one side to the other.

When I null the carrier I noticed the bottom pot is jumpy and the null is very close to full CW rotation. I have a huge supply of high quality AB pots so I may try to replace that one or at least try cleaning it again.

I do have a dual channel scope and the DDS SG I am driving the 20A with has a good tone gen on a separate channel so I will try looking at those today and report back.

Thanks for the help!!

Brian W6BRY
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WD5JKO
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WD5JKO


« Reply #3 on: July 04, 2022, 03:03:25 PM »


Brian,

   Some info about those carrier null pots. They are bypassed for RF, and therefore they can be wire wound pots. On one 20A, I use a pair of Bourns Knob-Pots. These are ten turn, and have what looks like a clock hour and minute hand inside...very nice. On another I used  500 ohm pots, and on each side used fixed resistors that were offset from each other such that the total resistance was 1K, and the pot was centered when the carrier was nulled. The balanced modulator carrier null drifts over temperature. Later 20a's have some temperature compensation at L1, and L2 coils. Both of my 20a's drift some between cold and hot chassis. Changing the diodes from 1N34A to 1N5711 helps a bunch in several ways.

   I have successfully used a scope on the 9Mhz phase shifter, but to do so, extreme care is needed. First off, loading at A and B (pot wipers) has to be minimal. I used two 10X identical, and compensated probes with a short ground clip. To minimize loading, I soldered a 3pf capacitor on each pot wiper (A and B), and with very short leads. Then I can clip the probe on each 3pf cap, and the loading is just a few picofarads. The goal is to adjust L1 and L2 to get equal amplitude, and 90 degree phase shift. This is somewhat subjective, but possible. Using X:Y mode is useful here as well. The scope should be a good one too. Re-balance the carrier as you go. A compromise is sometimes necessary, and perhaps a 60-40% or 40-60% balance is the best you can get.
Removing the probes will cause a shift, but not a lot.

Jim
Wd5JKO
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K6JEK
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RF in the shack


« Reply #4 on: July 05, 2022, 02:08:21 AM »

Brian,

Those completely crazy results on the spectrum analyzer means something is way out of whack. This happened to me when at Nick's suggestion, I replaced the PS-1 with a PS-2. That's in some presentation of his. This is a very simple modification then voila, 10 dB improved suppression. But the order of the two legs, which leads, which lags, is reversed between the PS-1 and PS-2 . The outputs have to be swapped. When they weren't, I got results like you're getting.

So that's a long shot. Someone swapped in a PS-2 from a 100V but didn't do the part Nick doesn't mention (and didn't remember when I talked with him).

More likely one or the other of the phase shifters is bonkers. Although final alignment is much easier with a spectrum analyzer, I'd suggest looking at the signals with a scope to see if they are even in the ballpark.

You're talking to the right guy. Jim knows these things inside out and backwards.
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W6BRY
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« Reply #5 on: July 06, 2022, 11:20:32 AM »

Brian,

Those completely crazy results on the spectrum analyzer means something is way out of whack. This happened to me when at Nick's suggestion, I replaced the PS-1 with a PS-2. That's in some presentation of his. This is a very simple modification then voila, 10 dB improved suppression. But the order of the two legs, which leads, which lags, is reversed between the PS-1 and PS-2 . The outputs have to be swapped. When they weren't, I got results like you're getting.

So that's a long shot. Someone swapped in a PS-2 from a 100V but didn't do the part Nick doesn't mention (and didn't remember when I talked with him).

More likely one or the other of the phase shifters is bonkers. Although final alignment is much easier with a spectrum analyzer, I'd suggest looking at the signals with a scope to see if they are even in the ballpark.

You're talking to the right guy. Jim knows these things inside out and backwards.


Hi Jon,

I wish it was that simple but I have confirmed I have the PS-1 installed in there.  I do have a 200V here that I need to finish restoring so I could pull the PS-2 out of it and make the mods to run that... so I will keep that in mind for when I have finally given up with getting this working as is.

Yesterday I had a little free time so the first thing I did was install a bulkhead RCA jack on the rear panel just above the terminal strip and connect a piece of RG-174 to terminal #18 on Switch-A which is shown on the schematic. My transmitter did not have this circuit so I thought I would see what I get from that output. On my first tests I got an odd looking sine wave that would flip horizontally when I switch from SB-1 to SB-2 and back, so I was excited to see something that looks like it's supposed to be there Smiley  Unfortunately it turned into a mess again so I need to go back and start from the beginning on this.

As of today I need to pack it up and put it on the "come back to this later" shelf. We have our 3 granddaughters visiting this week so I will report back when I am able to get back into the CE restoration mode.

Thanks to all,

Brian W6BRY
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K6JEK
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RF in the shack


« Reply #6 on: July 06, 2022, 12:35:54 PM »

Brian,

Those completely crazy results on the spectrum analyzer means something is way out of whack. This happened to me when at Nick's suggestion, I replaced the PS-1 with a PS-2. That's in some presentation of his. This is a very simple modification then voila, 10 dB improved suppression. But the order of the two legs, which leads, which lags, is reversed between the PS-1 and PS-2 . The outputs have to be swapped. When they weren't, I got results like you're getting.

So that's a long shot. Someone swapped in a PS-2 from a 100V but didn't do the part Nick doesn't mention (and didn't remember when I talked with him).

More likely one or the other of the phase shifters is bonkers. Although final alignment is much easier with a spectrum analyzer, I'd suggest looking at the signals with a scope to see if they are even in the ballpark.

You're talking to the right guy. Jim knows these things inside out and backwards.


Hi Jon,

I wish it was that simple but I have confirmed I have the PS-1 installed in there.  I do have a 200V here that I need to finish restoring so I could pull the PS-2 out of it and make the mods to run that... so I will keep that in mind for when I have finally given up with getting this working as is.

Yesterday I had a little free time so the first thing I did was install a bulkhead RCA jack on the rear panel just above the terminal strip and connect a piece of RG-174 to terminal #18 on Switch-A which is shown on the schematic. My transmitter did not have this circuit so I thought I would see what I get from that output. On my first tests I got an odd looking sine wave that would flip horizontally when I switch from SB-1 to SB-2 and back, so I was excited to see something that looks like it's supposed to be there Smiley  Unfortunately it turned into a mess again so I need to go back and start from the beginning on this.

As of today I need to pack it up and put it on the "come back to this later" shelf. We have our 3 granddaughters visiting this week so I will report back when I am able to get back into the CE restoration mode.

Thanks to all,

Brian W6BRY

That RCA jack is audio output for the MM-2 modulation monitor. Early models like mine did not have it. Ones that do are called a "G" model, if I recall correctly.

I really wouldn't mess with the PS-2 mod that I did until you have this thing working well and are running out of things to do. The 20A is fine without it. I was just curious if I could get it working. The PS-2 design was done as an engineering challenge for students at Stanford and, as I understand it, the 20A was the test bed. But they never put it in the 20A, only the 100V and 200V. I bet you knew this story already.

I have quite a few of those back-on-the-shelf projects. My wife is a quilter. In those circles, they call them UFOs, unfinished objects.

Have fun with the three granddaughters. I have one of those and a grandson arriving here in just a couple of hours. They keep me hopping.

Jon
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