Mechanical Engineering in our Radios, Rigs and RADIGS
Being hams, we often think about the electronic hardware in our rigs. Does it have an IRF510 in the Output or a Real RF Device like a RD06HHF1? Is it being "Run" with a digital VFO or one of those drifty, shifty Analog boxes.
BUT we often forget that some of these beloved radios rely heavily on our brother Mechanical Engineers to make it all play. You only need to look at James Millen who did all of the ME on the HRO receivers. Or pop the hood on that Collins R390A and savor the mechanical tuning mechanisms. Perhaps you overlooked the same in your KWM-2 or Drake TR-4. The ganged tuning of various networks is a marvel. Hey don't forget the SBE-33/34 and the "Geneva" Turret tuning arrangement.
But there is a special place in my shack for those really "cool guys" at Ten Tec. If you perhaps have some earlier Ten Tec radios like the 505, 509 or 540 that string driven dial system was pure Mechanical Engineering as was their PTO's. Can you imagine -- you could rebuild your PTO right at your kitchen table -- pretty clever.
But at times some of the Mechanical Engineering was hidden behind the panel and we simply used it without giving it a second thought. The Model 540 has a band switch located in the lower left hand corner of the radio. Actually it is THREE band switches with two visible on the front panel. When you put the main band switch on 10 meters you have an additional 4 position switch that is engaged to give you four band segments. But there is a third hidden band switch that is engaged as you change bands. Here is that Mechanical Engineering stuff at work
Before you gulp -- this is a Model 540 that was upgraded by me to a Model 544 and was the subject of a QRP Quarterly article in 2013.
When we think about the Model 540 and the principal band switch you must think about what that switch is really doing. It has many jobs/functions.
The way the frequency scheme of the Model 540 works is to have a singular fixed VFO range operating at 5 to 5.5 Megahertz --yes you guessed it a 9 MHz IF. But to work the various bands this basic VFO signal is mixed with various crystal frequencies to produce the LO Signals that results in the 9 MHz IF --it is a single conversion radio.
Now we have all seen the 9 MHz IF and 5 MHz VFO that gives you two bands -- add the IF and VFO and it is 20 Meters. Subtract the VFO from the IF and you have 80 Meters. But you do have to pay attention to any possibility of sideband inversion.
But Ten Tec used for all bands except 20 Meters a VFO signal that is crystal mixed to give an injection frequency above the incoming signal on 80/40 Meters and then for 20 Meters and above the LO injection is below the incoming with no crystal mixing on 20 Meters.
So this mysterious hidden band switch is THE mechanism that "on's" the crystal mixing for the various bands.
Another Mechanical Engineering problem -- packaging. The Ten Tec Model 540 and Model 544 were very "desk top" friendly having a very small compact footprint. Thus a very long shaft could engage many band switch segments to engage multi-function circuits. But you are faced with the trade off of long band switch size and compact footprint.
Another function of that band switch was to switch in various LOW PASS Filter Networks following the final amplifier "brick". Along the way other circuits are switched into play with changing of the band switch.
Did I mention yet another Mechanical Engineering problem -- circuit shielding and isolation. That crystal oscillator and mixer circuit were shielded--yes in a shielded box there is also a MC1496 DBM that mixes the Crystal Frequency with the VFO to produce the proper injection frequency. Another wafer on that third band switch adds some filtering to the output of the driver stage ahead of the final amplifier brick. Believe it or not -- that wafer is mounted on the back side of the shielded box, right adjacent to the driver board.
Oh more ME stuff -- all of the major circuit boards have through board pin connections that on the bottom end plug into chassis mounted sockets and actually protrude through the top side -- this give you access to literally hundreds of test points. Smart Mechanical Engineering!
Back to our third hidden band switch. So if this switch is not on the same shaft as the main band switch how is it engaged. Enter a piece of brilliant Mechanical Engineering which I call "a slider crank mechanism". Two mechanical linkages affixed to the main tuning shaft essentially clock the third band switch so that when you place the main band switch on 40 Meters the Crystal Oscillator puts in line a 11 MHz crystal so the output when mixed with the 5 MHz VFO output a 16 MHz injection signal results. When that injection signal is mixed with the incoming signal at 7 MHz the result is 9 MHz at the IF.
BTW if you look at the Model 540 BFO, a single BFO frequency will result in the "Normal" sidebands for all of the ham bands and with a bit of capacitor padding will shift the BFO frequency to the "Reverse" sideband. Ten Tec does not call them USB and LSB. I keep digressing
Here is a photo of that slider crank mechanism. Essentially what you have is two pieces of metal coat hanger with a loop formed at each end. These loops fit around a drum like spool assembly so that as you clock the band switch the coat hanger material is free to move around the spool; but the push pull action moves the third band switch in unison with the main band switch. If you don't know that the spool assembly comes apart and you somehow get the metal, bars out of the spool -- they simply cannot be fed back in place without removing the cap
The reason I mention this is that I have a project on the bench where the "slider crank mechanism" was in shambles, the dial cord was chewed up and it was obvious some non-engineer maybe one of those BTE's tried to "fix" something without knowing what they were doing. Pretty obvious who they voted for in the last election.
The metal bars were pretty bent up and I did a bit of straightening. The interesting engineering aspects of the nylon assemblies that fit on the two shafts they are molded so that the fit only one way on the half moon shafts of the two band switches and the bonus --there are really small set screws that further lock the nylon pieces to the two shafts.
Good thing I have acquired many small Allen wrenches. BTW the same Allen wrench that works with the main tuning knob will work with these set screws on the nylon assemblies. This is a long term project and you will be amazed at the final product.
Pete N6QW