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 Post subject: Central Electronics 20 A Multiphase Exciter
PostPosted: Nov Mon 23, 2020 3:13 am 
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Location: Costa Mesa, California
I have started restoration of a 20 A that sat on a shelf for a few years. The face is quite clean and beautiful to look at. The inside was another story--having probably seen some moisture at one time. So far I have changed the power cord to a three wire. The old cord was shot--stiff and ready to crack. I saved the electrolytic capacitors by re-using the case and cardboard covers. The plastic-coated paper caps had to go along with the domino paper caps. It needed several new tubes. I cleaned up the chassis and removed the dust and cobwebs. Then I fired it up and it lights up and works--but needs a full alignment and I have to do something about the B+. Because my mains runs around 122 VAC, the B+ is 405 VDC. The manual calls for 300 VDC. I did use 47 Mfd capacitors for the filter where the old filter caps were 40 Mfd, but I don't know how much of a difference that made. I think my solution will be a 13 Ohm power resistor in the AC main line after the fuse and a 400 ohm power resistor after the rectifier. That will bring down the B+ to about 350 VDC in Standby. The finals bias is probably okay--a little higher than called out for cut-off, but right on for transmit.

I never considered restoring this unit before because of the drawbacks with the design. It is a phasing SSB exciter that needs a linear amplified to be other than a low power transmitter. It also needs a separate VFO or single frequency crystals to operate. The problem with VFO operation is that the injection frequency to mix with the 9 MC internal crystal oscillator requires a VFO that generates oddball frequencies. For 14,200 KHz you would need a VFO delivering either 5,200 KHz or 23,200 KHz. Obviously at the time these exciters were sold, that VFO did not exist, so the solution was to acquire something like a BC-458 and a kit to completely re-make the BC-458 into a VFO. I do have the BC-458, but have decided not to destroy it by turning it into a VFO for the CE 20A. That leaves either using the exciter as a single crystal transmitter--with some oddball crystals--or creating my own VFO. I am exploring that route now. I have a Collins 70E-type PTO and I am thinking if I use a mixer and a crystal controlled oscillator I may create my own VFO.

In any case, here are the photos.

Norm


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Chassis.jpg
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Underneath.jpg
Underneath.jpg [ 319.89 KiB | Viewed 648 times ]
Injection frequencies.jpg
Injection frequencies.jpg [ 158.39 KiB | Viewed 648 times ]

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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Nov Mon 23, 2020 3:55 am 
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They are fun little SSB units Norm.

For the alignment, although you can use a scope as suggested in the manual that would be my third choice because so many issues with this transmitter will create a pretty similar scope display. I use a spectrum analyzer for alignment of phasing rigs which makes it very simple. A good second choice is any quality selectable sideband receiver which provides for very easy nulling of the unwanted sideband and carrier.

For future reference, the tiny modulation transformers used in this rig suffer from the same problem as the tiny driver transformers in a lot of vintage AM rigs. The wire gauge is extremely fine and the acidic insulating paper corrodes it over time. This was a fault with with the first 20A I restored and I substituted a pair of Radio Shack filament transformers so that I could complete troubleshooting. The RS transformers fit within the old transformer covers and I ended up using them permanently.

Another common failure is the relay that is in the plate circuit of the relay control tube. These often die from an open winding. These telephone type relays used to be common but aren't as easily found now. I believe that you can still buy replacements from the ham who specializes in CE parts but there were some sensitive relays available from other sources that will work.

The modified BC-458 VFO shows up fairly regularly on ebay, the "deluxe" version included a crystal mixer to cover all of the classic HF ham bands. But making your own is also a good approach.

Not all 6AG7 tubes like to be operated in the horizontal position as I found out when aligning my first one. One final developed an internal short which caused the 5U4G plates to start glowing. This CE 20A restoration was early in my experience and I found after I pulled the plug that a previous owner had used a 20 amp fuse in place of the correct 2 amp slow blow size. I was fortunate not to lose the power transformer and I learned my lesson to always check to see that the proper size fuse is in place when initially going through a new piece of gear. I suspect a previous owner used the 20 amp because he had one handy, he probably used pennies to repair his old blown screw in house fuses also :)

Rodger WQ9E


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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Nov Mon 23, 2020 2:17 pm 
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G morning Norm. That is a beautiful looking front panel. Good luck with your restoration and quest for a VFO.

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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Nov Mon 23, 2020 2:54 pm 
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Neat transmitter Norm... show us how it progresses. There is certainly no shortage of knobs on that panel!

I have a few of their Sideband Slicers in use. They made some unique products.

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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Nov Mon 23, 2020 7:10 pm 
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Norm-
If you would like to use crystals rather than a vfo for some experimentation in the 75 meter band, we have seven crystals available between 5053KC and 5500kc. Five are in FT-243 cases, one in larger case with FT-243 pins and one in larger case with larger pins. Sorry, I don't see any others in the injection frequency ranges posted above.
Please PM me if any of these would be useful.
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Chuck K7MCG
Radio Recycling Chairman, BEARS(K7NWS)


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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Nov Mon 23, 2020 9:33 pm 
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I believe also the Lakeshore band hopper VFO is an option and they show up from time-to-time. Those CE VFOs also show up periodically.

My VFO attempt is an Arduino-controlled AD9850 with a buffer amp & filter. Have not started testing.

OP, I have a parts unit, w/o relay FYI.

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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Nov Tue 24, 2020 3:07 am 
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Hi Norm,
I restored mine about a year ago. Fun little rig and it is amazing what you can do with the 15 watts or so ut. Besides turning it up is really very simple once you get used to it. I use a variac to keep the boatanchor outlets at 110.

The solution I found for a relatively inexpensive and stable VFO is a digital signal generator like this. https://www.ebay.com/itm/FeelTech-FY6600-60M-15MHz-Signal-Generator-Dual-Channel-Function-Precision/164532582094?hash=item264ee7f6ce:g:otsAAOSwfDRfvA5A It works out fine and you just leave it running. Since you are never listening to the frequencies you use for transmit, you never hear it. There is a little calculation to get your frequency, but once you get it set you are good to go.

I have talked with other stations who have replaced the transformers with matching audio transformers or even small matched filament transformers.

I am building up a 2x 6146 amp to run with it.

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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Nov Tue 24, 2020 3:24 am 
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Quote:
Because my mains runs around 122 VAC, the B+ is 405 VDC. The manual calls for 300 VDC.

That much voltage variance is rather extreme to be caused by line voltage that's a little high. I would look for some other problem such as not measuring the voltage under the conditions they used or the possibility that someone replaced the power transformer.

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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Nov Tue 24, 2020 3:55 am 
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Norm,

That 300 volt specified B+ is with the CE-20A transmitting at full carrier input. If you are taking that reading no load then consider it normal.

In standby, the load is very light and B+ will be well over the loaded values specified on the schematic.

Rodger WQ9E


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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Nov Tue 24, 2020 4:09 am 
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Thanks all.

Rodger, I was planning on adding the power resistors to get down in the no-load 350 VDC to 360 VDC range. The 13 Ohm AC line resistor brings me down around 113 VAC on the primary side. The 400 Ohm B+ resistor drops me down a little more there, but both of these can be reversed and I will see what the voltage is under load later and adjust. Carl, I have one of the cheap signal function generators already and now will give it a try. They can deliver much high RF voltage than my HP 8656B, although I still like the idea of having a dial and a knob instead of menus. It looks like my next act will be to see about the modulation transformers under the cans.

Norm

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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Dec Sat 05, 2020 1:41 am 
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I have a bad audio modulation transformer—as Rodger and others suggested was possible. I assume I would need to replace both and filament transformers have been suggested. My question is 115/6.3? What amp rating? What about other transformers, as I haven’t yet seen any that fit inside the cans. Can I use an audio transformer and what current or power rating should I need for these transformers?

Norm

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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Dec Sat 05, 2020 2:59 am 
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Norm,

I am pretty sure mine were 12.6 volt secondary transformers. The smallest current rating filament transformer you find will be overkill for that circuit so go with the smallest that you can find.

Rodger WQ9E


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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Dec Sat 05, 2020 4:40 am 
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Thank you, Rodger. I found a couple Hammond 166E12. They look like they will exactly fit the cans. Now I have to clean up the one I started and get the other one sitting on the wood stove to melt the tar. Good thing it is winter up here in the mountains of Southern California.

Norm

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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Dec Sat 19, 2020 12:11 am 
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So I took a detour and picked up a BC 458. That occupied my time while I waited for the Hammond transformers. I have it working okay, cleaned up, three-wire power cord, new capacitors, a new tube and showing the correct frequencies on my spectrum analyzer.

Questions:

Is there a simple test to determine the phasing orientation for the transformers—as they don’t have the color coded wires of the original? I have a filament supply I can use for low voltage feed. Do I just flip them around in different configurations and check voltages at the secondaries? They are perfect fits for the cans and a little filing allows them to sit inside with their mounting tabs captured by the can screws and the cans down tight to the chassis deck.

Second question:

The transformers are center tapped 12 volt filament types so I could run them either as 6 volt or 12 volt. I had planned on 12 volt but if anyone has further suggestions, I am open. I figured I would leave myself the option by not cutting the center tap wire off.

Norm

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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Dec Sat 19, 2020 6:39 pm 
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The transformers that I am replacing, TR2 and TR3, are the modulation transformers, so it is important I get the new transformers correctly connected to match the original scheme. The old transformers have 4 color coded leads and they are identical to each other. One could assume the schematic might give a clue to the orientation, but unfortunately, it has been drawn such that the layout for TR2 and TR3 are depicted opposite. Because TR3 is still functioning and in the radio, I was able to map out its layout before removal and identify the faded colors of the leads. Side note: there are extra holes for mounting the transformers. Had they been changed once already? They are actually wired backward in the radio from the way it is shown on the schematic as far as the primaries are concerned. Probably not an issue.

My new transformers have unmarked leads, but using a filament supply to provide low voltage AC for testing, I did a simple configuration (primaries in series, secondaries in series one way then the other) to determine the phasing relationship between the secondaries and primaries. The transformers have wires exiting such that they are consistent with expectations. I marked one wire for the primary and one for the secondary to show orientation. Once they are installed inside their cans, it won’t be obvious without the markings. Taking the old working transformer, I connected it to the supply. The secondary is lower voltage than the replacement indicating that it would be the equivalent of a 9 volt filament transformer. There is a Hammond 10 volt transformer with the correct physical size, but I am going ahead with my 12 volt (not 12.6) replacement. By connecting the original transformer to the supply with one secondary lead connected back to the primary and measuring the potential across the other secondary lead and the primary, then flipping it, I determined the orientation of the original and matched it to my replacements.

Now I just need to melt some tar and clean up the mess. Soon I will have a working exciter and a VFO to connect to it and be ready for alignment (or the next problem).

Norm

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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Dec Mon 21, 2020 12:24 am 
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Alignment started out well. I got about half way through, to the point where I was adjusting the traps, when the relay stopped working. By the time I had the exciter upside down, checking voltages, my B+ had dropped by a third and the bias was at transmit levels. I gave up and went to bed. In the morning, I returned to the beginning, checking some tubes, then bringing the exciter up with the rectifier out and testing the bias, finally inserting the rectifier. All voltages we’re back to normal, except the relay still wouldn’t work. I finally shorted the back of the key jack—I had been activating the relay with the key—and the relay clicked. My funky cheap key has a mini-plug and I have used an adaptor to get it to a standard 1/4” key plug. The adaptor / mini-plug contact was sporadic. At that point, I was very glad I hadn’t started fixing parts in the radio that didn’t need fixing. The relay tube sensitivity resistor (cathode) was a 3.3K where the schematic calls for 2.7K—that sort of thing. The 3.3K should work okay as my B+ is 365 where the manual calls for 300.

I have reached a stopping point, though. My output throughout has been low—maybe 5 watts at best. First off, I am not sure of my wattmeter accuracy at the low end and second, the cathode follower / driver is pretty weak. I ordered a new one when I started the restore, but what the EBay seller sent was in weaker shape than the one in my transmitter. I have ordered a couple new tubes. These are 12BH7’s which are $20 tubes at the low end. Apparently the audio nuts use these tubes and the prices get really astronomical. Once I get a new driver, and if the output is still low, I will start back through the stages to see where the transmitter needs help. I was worried that the VFO might not be putting out enough voltage, but it performs at least as well as a crystal when I switch to XTAL operation with a rock plugged into the front panel.

Norm


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Radio 1.jpg
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Radio 2.jpg
Radio 2.jpg [ 289.54 KiB | Viewed 321 times ]

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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Dec Mon 21, 2020 12:52 am 
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Not sure what mode you are checking PO in.
Is your carrier level set correctly?

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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Dec Mon 21, 2020 1:15 am 
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I am checking the power out in CW / Manual with full carrier. I have been using the RF voltmeter to peak the circuits and a wattmeter to see if I am getting some output. I clip my spectrum analyzer to the outside of a test lead connected to the antenna terminal. Once I have a good cathode follower / driver tube, I can can start back through the exciter looking at signal levels. I also need to do the audio checks. The 12BH7 is a two section triode--one half used for the cathode follower. My tube checks very weak on one half--which I suspect is the half used for the follower. When I initially tested the tubes, many of them were very weak, including the finals, and they were replaced.

Norm

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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Dec Mon 21, 2020 1:31 am 
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Oh well, I am the king of missing the obvious so I thought maybe I might have a quick fix for you.
But you are ahead of me!

There is an AWA SSB net in New England Mon evenings and there are about 4 stations running them. Great little rigs. I am running it barefoot have made contacts on 160 very easily.
Good luck

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 Post subject: Re: Central Electronics 20 A Multiphase Exciter
PostPosted: Dec Mon 21, 2020 1:45 am 
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Norm,

Rather than pay audiophile pricing, a 12AU7 should work fine in place of the now expensive 12BH7 and the higher mu 12AT7 would likely also work if it is all that you have on hand.

I found that the most picky tube in the set is the 6U8 9 Mhz. oscillator so try different tubes there if you have a spare. Remember that what your tube tester tells you may not be true at RF so try different tubes in this spot.

Be careful because some 6AG7 tubes are prone to sagging elements as they heat up if mounted in some orientations. I found this out the hard way while working on my first 20A.

Rodger WQ9E


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