The design presented some real challenges because of the much smaller size - and the biggest problem was not the electronics, but the battery! To fit into the new and smaller case, the battery could have only about half the volume of the old HT-200 battery, and this was a real problem. Marketing turned out to drive many decisions.
Originally, the radio you know as the HT-220 was to be called the HT-180. Reason? The old HT-200 put out 2 watts of transmit power (VHF). 200 ---- 2 watts - get it? But in order to get reasonable battery life from the new and smaller design, the transmitter had to be limited to 1.8 watts. Hence HT-180. The marketeers, of course, decided that HT-180 sounded like a step down rather than a step up, and since this was to be a shiny new product, pride of the fleet, it could not be permitted! To make customers feel that they were getting the same high quality Motorola product (which they were), but with some improvements, the number had to be higher! The choice of -220 was pretty random, but it had to be catchy, and higher than -200.
Even with slightly reduced power, the battery was still a problem. As part of its specs, Motorola advertised a given number of hours of battery life, based on a 10-10-80 duty cycle (10% transmit, 10% receive, 80% standby with closed squelch). The new radio had to have the same or better battery life, of course, but the new battery wouldnt support it. The solution was to quietly change the ground rules, and the HT-220 battery life was based on a 5-5-90 duty cycle.
Why an oddball number for the receiver IF?
I dont think I ever knew why it wasnt 10.7, the standard frequency for all other FM radios in the world.
Why they used the spkr as a mike in the earliest models? The audio was super bad.
The HT-220 as I knew it used the speaker for a mic. The audio never sounded all that bad to me as long as you talked INTO it, rather than PAST it. Remember Broderick Crawford in Highway Patrol? In the opening scene, he was always leaning against the door of his police car, squinting into the sun, holding a microphone on a coil cord somewhere up near his ear, and saying dramatic things PAST his mic. Everyone thought this was the cool way to hold a mic, and it gave us fits. Anyway, I can tell you that a rush to market was NOT the driver in that choice. The design of a separate mic circuit would have been an absolutely trivial exercise, and Motorola would not have risked any kind of adverse customer reaction to save these few pennies or few days. Motorola Communications and Electronics Div rightly considered itself to be an absolutely top-quality house. There were many schlock houses pushing cheap products at the time, but only the comparable division of GE was in our class. It would have been absolutely unacceptable for Motorola to allow any taint of schlock to get out of the house and into the customers hands.
Why 39 ohms for the speaker resistance?
The value of speaker resistance (39 ohms) was based purely on technical parameters. From the point of view of the guy designing the audio stages, several of the relevant design parameters were already fixed. Given these parameters, 39 ohms is the right value for the speaker. An 8-ohm speaker, for instance, would have resulted in compromises somewhere in the circuit or the performance. The fixed parameters included:
1. Amount of audio power desired -- too little and you cant hear the radio, too much and it eats into your battery life.
2. Battery voltage
3. Desire to use a capacitor to couple the audio amplifier to the speaker, rather than an output transformer (which would have occupied too much space).
Given the power desired and the voltage available, you know how much load resistance the output amp must see (P=V**2/R). Once you have decided that you are not going to use a transformer, then the audio amp is going to see the resistance of the speaker directly, not a transformed version of it. Voila! Well, it aint quite that simple, but thats basically it.
"Theyre almost impossible to fix without the EXACT parts from Motorola."
Re use of Motorola parts -- There are, in fact, probably several answers. First, Motorola had a big and growing semiconductor manufacturing division in those days, and there was certainly a desire keep the business in-house. An in-house source of supply also allows you to get devices with specific parameters, or tighter control of a critical parameter. It would also enable the designer to specify a new part which didnt even exist in the commercial world at that time. Remember, in the mid-60s transistors were still a pretty new thing, and not many types existed compared to todays long lists.
Motorolas semiconductor products division ran production lines for many commerically-numbered transistors. Each commercial number (2N918, for example) had published and industry-accepted specifications, and if you used a 2N918, these were the specs you had to work with. Sometimes the published specs included a wide range of variability in a given spec, and the designer had to cope with that wide range. I dont know this for a fact, but my guess would be that as 2N918s (or whatever) were coming off the production line and going into final acceptance test, the test techs could select out parts which met the 2N918 specs, but also had some particular spec which was within a tighter tolerance band. Leakage current, for instance. These selected parts would be pulled, given an in-house number, and sent to the HT-220 production floor. This eliminated the variability that the designers would have had to cope with if they had been forced to use standard commercial 2N918s. It also means that if you were repairing that radio a few years down the road, and you substituted a 2N918 for that transistor, it might or might not work.
How many people were on the HT-220 design team?
The actual design team -- RF mechanical and electrical designers, as opposed to technicians, stockroom personnel, upper managers, etc -- probably amounted to 10 people. Of course, the support cast was many times this size. The development of a new product like this is a VERY large undertaking for a company like Motorola.
Did the design team make special units for themselves?
We didnt really make special units, although a lot of hacking was done on the preproduction units for various purposes. As I recall, we went through at least six formal preprod cycles before the radio went into actual production. For the first preprod cycle, only one or two would be built. As we got into the fifth and sixth cycles, closer to the final configuration, the number increased to perhaps 10-20 units. Most of them went through very detailed and formal production and operational evaluations and tests, but when those had been completed, Motorola encouraged us to take the units out and give em a spin in the real world. The preprods, of course, never went to customers, so we were allowed to do most anything we wanted with them. The hams, of course, immediately padded the tuned ciruits down to 2 meters, and then put hundreds of hours of amateur use on them. A number of us would tuck them in a pocket and take them skiing in upper Michigan. This not only confirmed that they worked well in cold weather, and while covered with snow (I wasnt a great skier), and after being dropped from chairlifts, but it also gave us designers some real-world insight into how a radio really gets used, what happens when you go over a hill and lose line of sight, how fast a battery dies if you talk too much, and so forth. We did uncover some design flaws this way, but the real value was to get us closer to our customers view of our products.
Another tidbit of possible interest. As the design of the VHF unit was being transitioned from preprod into production, Motorola actually went to preproduction with a version of the HT-220 called the HT-100. This was simply an HT-220 with a jumper wire where the transmitter final amplifier transistor was normally installed -- in other words, the driver transistor (and output tuning) was connected directly to the antenna. It put out 100 mw of transmit power, which was felt to be adequate for many short-range applications. The advantage was that the case could be much smaller. The entire bottom half of the HT-220 case (where the battery lives) was removed. The lower half of the HT-220 front cover, the part that covered the battery compartment, was removed, but otherwise it was identical to a standard cover. The back cover was also shortened - actually was replaced with a cover which looked identical except that it was thicker (maybe 3/8 inch?). The extra thickness was for a specially made ni-cad battery. It was a nice little unit.
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