EICO 723s
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| Jun 23rd 2023, 02:23 | |
K0WUQTotal Topics: 0 Total Posts: 0 |
No one ever seems to comment on these rugged little 60W transmitters, so here goes: I bought two of these via eBay; in both cases I did a Buy Now for less than the asking price and had my transmitter in about 3 to 4 days. The reason I bought a second one is a sad but simple fact: You can buy a thing on eBay from someone who knows absolutely nothing about the quality of what they're offering. And, in the case of the first one I got, the power transformer was on its deathbed. It succumbed fully while I was testing it out. So, this one goes to a new home on the Spare Parts shelf. However, I was blessed on my second try. The reason I was attracted to these eBay offerings was nearly a lifetime of success with EICO kit products. My old Model 460 'scope is still running after better than 50 years, and I went through a bunch of their tiny solid-state kit offerings during that time. Anyway, this 723 that I got running is a fine example of their design philosophy, which is basically Keep It Simple. And, they did. This thing will supposedly tune up and run on 10, 15, 20, 40 and 80 meters, using only an 80-meter or 40-meter crystal! Of course, they are not perfect. Basically, this is a very compact, self-contained crystal-controlled transmitter for CW use (with optional external plate modulation for AM). The thing that turns most 723s into doorstops is the twin set of 40 uF electolytics, which go bad and overload the 600-0-600 transformer winding. This destruction doesn't take long to accomplish. So, the usual first step after acquiring a 723 is to see if the original filter caps are there and replace them with modern units. When I looked at this one, I was disappointed to find the original types - however, I soon noticed that they had been wired in as replacements. So, I figured it was a good risk to proceed, and got her fired up with no apparent problem. (I did take a few days of running just the filaments to give the getters in the tubes a chance to sweep up the stray atoms bouncing around in there after 50+ years of sedentary coolness.) The poorest feature of the 723 by far is the tuning meter, which is a VERY shaky, bouncy moving-vane type. Man, that takes a LOT of getting used to! It has two scales, 0-150 mA for Plate Tuning and 0-5 mA for so-called Grid Tuning (which seems to really function more as an output tuning meter for the oscillator section). I will shed no tears at all someday when I replace this with a good d'Arsonval meter, assuming I can find one with some kind of suitable scale. For me, though, the biggest difficulty is the amplifier tube, type 6DQ6B, because of my interest in modulation at the control grid. I downloaded a spec sheet on this tube, and found to my horror that the tube has NO region of linear operation - not even close! This is pretty bad news when you're trying to amplify a modulated signal with minimal distortion. The oscillator is a miniature tube, the almost ubiquitous 6CL6. A strangely-named GZ34 was originally used as the HV rectifier; however on this second unit, that had been replaced with a plug-in solid-state impostor, which works just fine (and reduces the load on the transformer, since the 5VAC filament winding is not loaded). An interesting design feature is a rear-panel octal socket called the Auxiliary Socket, which simplifies plate modulation (AM) and supposedly helps with emergency power for the unit. I needed a place to bring in audio for my Displacement Modulation at the grid, so I removed the 6.3VAC heater voltage leads from the socket and soldered them together and covered with heat-shrink tubing. This freed up pins 5 and 8 on the socket. Then, I ran a lead from pin 8 directly to the amplifier cathode. Finally, I made up an assemblage of a junk-box RF choke and a 30uF electrolytic, then soldered this between pin 5 and the tube grid circuit so that the RFC is almost directly attached to the grid and the positive end of the cap is at the Auxiliary socket (my reasoning being that the grid will always be negatively biased relative to the cathode). At the octal plug, I installed a 9200 ohm resistor between pins 5 and 8, to give the high-impedance audio driver a reasonable load. Another nicety of the Auxiliary Socket is that when the unit is switched to the Transmit function, 120VAC magically appears between pins 2 and 7. They explain that this is to power an 'Antenna Change-over Relay' (in other words, an automatic Send/Recv switch). And, it really, really works! Yesterday, I set her up on my multi-lightbulb dummy load and (I guess) pretty much perfected the art of tuning for CW operation (maximum efficiency tuning). Tonight, I started experimenting with tuning for Displacement Modulation, which is vastly different (as I knew it would be from my previous experiments). I did succeed in getting the modulation propagated, as shown clearly on my 'scope, though I didn't quite succeed in distortion-free audio detection. More experimentation required. This shows that a 'standard' type of transmitter is capable of being set up for the modulation, easily and cheaply. This unit is at least double the output power of the little homebuilt amplifier, and is much easier to work with (except for the exceptionally silly meter), so I should be able to actually use the modulation on-air 'any day now' - ha. Larry Cottrill, K0WUQ |
| Jun 27th 2023, 23:47 | |
K0WUQTotal Topics: 0 Total Posts: 0 |
Today I made yet another simple modification. I put almost 200 ohms of resistance (takes about 6 watts) in the cathode circuit of the amplifier tube. The reason for this is that plate tuning for Displacement Modulation is pretty far from the Peak Efficiency tuning point, so the plate current runs above the 120 mA that EICO considers to be the running limit for the tube. Resistance in the cathode circuit creates 'cathode biasing', causing the control grid to be relatively more negative and thus, choking down on the plate current. With this in place, I was able to stay down around the 120 mA point with good-sounding modulation achieved. Tuning for good modulation sort of requires three hands, because the grid, the plate, the loading capacitor and my 'random wire' type antenna tuner all affect the outcome. This is the one serious flaw in using this form of modulation - the Goldilocks Syndrome, where it's only satisfactory when everything's 'just right'. But, "It works if you work at it." Once you hit the sweet spot, my little 10 watt Heathkit audio amp gives full modulation with the volume knob turned only halfway up, showing the minimal power required to modulate in this way. To be perfectly honest, the modulation is not perfectly distortion free - but I think that comes from the non-linearity of the tube used as the final amp. There might be another beam power pentode of the same power rating that could be substituted (maybe with a little re-wiring of the tube socket), which would have essentially linear characteristic curves. If that could be done, almost perfect modulation with zero sidebands would be achieved. A also bought (eBay) a 3-inch D'Arsonval 5mA meter to replace the original jittery one, but haven't seen it on the doorstep yet. That will spoil the authentic look of the front panel, but the convenience of a decent meter will be more than welcome. Larry K0WUQ |
| Jul 11th 2023, 00:07 | |
K0WUQTotal Topics: 0 Total Posts: 0 |
Well, the filter capacitors in my newer 723 bit the dust a couple of days ago. I had just powered up, and was tuning up when I began hearing some snapping and popping in my headphones (i.e. coming through my monitor receiver). I immediately thought of those original style tubular electrolytics breaking down (they are red cardboard tubes resembling roadstand fireworks), and shut her down quickly. Fortunately, I already had some replacements on order - I am as smart as the next guy and ordered four of them, enough to re-fit both my 723s. Mouser didn't have 40uF 450V in stock, so I ordered 47uF 500V ones, certainly close enough to work. Today this order arrived a couple of hours after the regular mail, and I wasted no time getting them installed in both transmitters. They are half the physical size of the original ugly ones. I was pretty much convinced that the transformer in the older 723 was shot, and when I powered her up, I was not disappointed - all I got was smoke from the transformer. When I powered up the newer one, things were much better, which means I responded properly in immediately shutting down as soon as I sensed there was trouble. While I was in this unit, I also fixed up the 120VAC output that is used to pull the antenna Send/Receive relay. That had mysteriously stopped working the other day - this turned out to be a cold solder joint from the factory! So, it was easily fixed. Altogether, a pretty blessed day at the workbench. All I need for the older transmitter is an impossible-to-get power transformer that will fit in the limited space designed for it. Meanwhile, the second unit is working fine and I'm back to "cookin' with gas" running with that one. Larry K0WUQ |
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