Central Electronics 10B Exciter Restoration

Just as I got it

This past spring I picked up a Central Electronics 10B exciter. This is a low power radio transmitter that was part of the on the first generation of single-sideband voice on Amateur Radio bands in the 1950’s. You can think of these transmitters as full-carrier double-sideband AM transmitters that use phasing circuits to cancel the carrier and the unwanted sideband. In fact, if you ever written software for an SDR (like one of Tony Parks’ softrock kits) its pretty much the same complex multiplication using a quadrature signal, just with analog parts rather than software. What’s old is new again!

I got this one for a reasonable price at the Antique Wireless Association’s spring meet auction, but it came with a disclaimer: it wasn’t plugged in or even inspected.

I am too young to remember tubes being a common sight in my childhood, but I’ve learned some flea-market skills from those older and wiser than me. I noticed the following:

  1. No obvious extra holes that I could see–but I didn’t have a picture in front of me.
  2. No empty sockets inside the lid. Tubes had the same amount of dirt as the chassis.
  3. The tube names were scratched into the top of the chassis
  4. There were colored markers (pieces of faded electrical tape?) on the tuning controls.
  5. A little paint loss on the edges, but all the labels looked good.
  6. Original unpolarized cord

All this was enough to tell me that it was probably built as a kit rather than factory assembled. Sometimes you find an incomplete kit where the builder got in over his head, but this one had obviously been used. A radio that was once working has a good chance of working again. It also means that the wiring job could be of varying quality. Of course there’s always a chance of an open transformer or other failure, but that was a chance I was willing to take.

Under the hood. The box in the middle with the tube sticking out sideways is the option QT-1 VOX unit. (For those who don’t know, VOX is for people who like the whole world to know dinner is ready or who believe that DX stations might be able to locate a favorite stuffed animal.) The rectangular box on the left is the PS-1 phasing unit, which you need for voice operation. The 6BA7 Mixer (or crystal oscillator) and the 6AG7 amplifier are in the bottom right with the 40m plug-in coils.

When I got the thing home and opened it up (only two screws on the back) it was clear that it had the original capacitors in it. I found the schematic online, along with the change bulletins. There were some small changes to the audio circuit that don’t appear to be implemented, but that’s not something I really care about. I tested the tubes–all good with signs of use, which is a good sign. The critical parts–HV transformer, modulation transformers, relay coil, all checked all had continuity, and there was no obvious signs of excessive heat. All good!

Recapping

Recapping…the old capacitors are shown below, except for the dual-40uF filter capacitor which was electrically disconnected but left screwed to the chassis.

Before applying power, I replaced the electrolytic and paper caps. I didn’t want to remove the HV filter cap can (pair of 40uF electrolyics) for cosmetic reasons, so I placed two terminal strips on it’s anchor bolts where I could wire a pair of 47uF caps underneath. Then I just disconnected the positive terminals of the old caps and moved them to the new strips. This keeps the look the same when you open the top lid and didn’t require moving any wires more than about 1 cm. The rest was pretty straightforward, using close modern values near the original caps’ values. Because modern capacitors tend to be much smaller for the same voltage rating, I occasionally had to find a different connection point, but since most of these go to ground finding something is usually not a problem. In one place I opted to use a 450V cap where only 150V was required because I needed a cap big enough to span the original connection points. Thankfully none of these were too deeply buried by the original builder.

Close up of the two 47uF caps underneath the original dual-40uF filter cap can mounting bracket (C43/44). The oversize 450V cap (C76)is on the left, but was a drop in replacement because of its physical size.

Once this was done, I brought it up on a variac. After waiting for popping and smoking that never arrived, I keyed it with a 40m crystal. The relay clicked and there was power on the wattmeter! When in “CW” mode this transmitter is a straight MOPA, and the “mixer tuning” and “amplifier tuning” work like any other two-stage 1950’s novice transmitter. There’s no meter on this thing (not even a cheap iron vane like on the Adventurer), so you just tune it up for maximum on the wattmeter. I was able to get about 4W on 40m, but there was also a swamping resistor inside the PA coil. This resistor is recommended if you have a high impedance load (like an amplifier) to prevent self-oscillation. I was able to get it up to about 6W without the resistor. The tone was good and clean, so the filter caps were doing their job.

With the RF stages working the next order of business was to see if LO and mixer stages would do their thing. In the AM/PM/SB2/SB1 modes the plug-in crystal is mixed with an internal 9MHz crystal oscillator. Thus a 7.0-7.2 MHz crystal will put you on 160 meters. A 5Mhz range crystal will map you to either 75M or 20 meters. In theory 40m could be reached with a 160M crystal, but the manual warns you not to do this because of the 11Mhz image not being well filtered, though I suspect its doable with some extra filtering if one is really determined. Many people used an external VFO like a modified BC-458. Higher bands were possible but required higher injection frequencies than could be accomplished with the crystals then available.

Coils…

The homebrew 40m coils that came with it on the left, my 160m coils on the right, using some old Birnbach #24 “double-nylon-coated” wire.

I only had the 40m coils that came with it, so I needed some 160m coils to check out the mixer with the crystals I had on hand. The original coil data suggests using B&W 1″ coils…which is apparently still available from the manufacturer at astronomical prices. Ham’s are cheap animals, however. Even the $3.75 for a pair of band coils in 1957 for the original owner was too steep, which is why he made the ones on the left side of the picture. Not feeling like shelling out $100 for a B&W #3016 (seriously), I made some replacements that should be in the ballpark for 160m. A check with a 160m crystal in CW mode showed that they worked fine: about 9W out–better efficiency down there, I guess. It’s surely not because of my high-Q construction methods!

That was unexpected…

Things were going far too well. I followed the manual’s instructions and was able to null the carrier about 60dB using the S-meter on a K3. At this point I turned up the Speech Level knob to listen for excessive hum internal to the radio in the audio stages. I saw the power meter go up to full scale without any change in the s-meter on the K3, so I immediately thought I had a parasitic oscillation in the PA stage. I had just wound the coils and thought I had done something something terribly wrong.

I turned off the receiver and started things up again in a slow and controlled fashion. This time, with the receiver off, I could hear a faint, high-pitched whine coming from the chassis. It was an oscillation in an audio stage that was putting out a signal that fell well outside my receiver’s passband. I turned on the reciever, tuned up 8 or 9 khz, and there it was at 60dBc! I found that the oscillation would only start if the audio gain was lower than about 1 o’clock. (That’s odd, isn’t higher gain more prone to oscillation?) NE1S gave me some pointers on the AWA reflector and the culprit turned out to be a cold solder joint on the shielded wire going to the VOX circuit. I suspect the reason it the oscillation died at high gain is because the driver tube saw a lower impedance when the gain was turned up and apparently this was enough to swamp the oscillation.

I connected a modern 600-ohm hand-mic to the Hi-Z input–not the greatest arrangement, but enough to check if the AF stages were working. I could heard my voice, and the SB1/SB2 switch shows something close to the sideband rejection I would expect on the local receiver. I’ll get out the analyzer to see if it can be improved (40dB is about all that can be expected with this design).

Future work

This transmitter has a couple line-to-chassis ceramic caps on the AC input that I will either replace with line rated caps or just remove altogether. It came with an unpolarized two-wire cord, which means you have a 50% chance of switching the neutral instead of the hot side of the outlet; it needs a polarized plug for saftety in today’s world. I’d also like to get some crystals for a couple of the boat-anchor watering hole frequencies on 160m and 75 meters, so I intend to grind/etch some crystals to make that happen. Those that will be used on SSB will be more precision work than I’m used to, and I might make a special holder to allow a few 10’s of Hz of VXO operation. If anyone knows of some good 1940’s/50’s articles on homebrew variable crystal holders, I’d love to hear it…

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