The following is from Jerry Zacker, a member of the Motorola HT220 program.
I recently retired from Motorola after 35 years of service which started back in February, 1966. My first assignment was to work on the VHF transmitter of the HT220. Back then, the 220 was only intended to be a 1 watt, 2 frequency, private-line radio at best. Due to some competition by General Electric, who was also coming out with a 1 watt unit, we upped the power to 1.8 watts on the VHF models. We also added the UHF models to the HT220 series. The initial acceptance of the 220 was so great, we went on to develop high power models and multi-frequency units. That kept us pretty busy for many years.
I helped develop the first multi-frequency VHF model for the Pacific Air Command Air Force, called the PACAF. It was used by guards patrolling Air Force bases and also had a VOX feature. That was the very first multi-frequency model. Then we went on to develop that model for commercial use.
I then worked on the HT100 VHF transmitter but the HT100 never became a big-seller. At 100 mW of power, it had limited range for a rather pricey unit. It was probably the smallest two-way portable radio of its time, however.
I spent some time on the IDC circuit, one of the first ICs ever used in a portable radio, along with the 455 amp in the receiver. When we first planned the 220, we tried to use the receiver speaker as the transmitter microphone. However, we could never get consistent frequency response from the speakers. Some sounded good, others didnt. We eventually abandoned the idea and put in a small mic. Costly solution, but we had little choice.
The printed circuit boards were the first double-sided plated-through hole design to be used. We had a lot of problems getting those boards to be reliable. Many trips to the manufacturer to work-out processing problems. Eventually the quality issues were solved. But, occasional interconnect failures still showed up as field or factory problems. Those issues kept a lot of us busy and on the road.
Why an oddball number for the receiver IF?
I dont have an answer to your question about the 10.7 MHz IF. The receiver Designer is long gone from Motorola. He became an Engineering Professor at Purdue University back in the 1970s. The IF frequency we used was most likely chosen to minimize receiver spurs and image interference. We would get a lot of guidance from Field Engineers on issues that dealt with system interference problems out in the real world of FM communications.
The HT220 was developed back at the Augusta Boulevard facility in Chicago, Illinois. Motorola has since sold that facility after the Portable Products Division was moved to Ft. Lauderdale, Florida back in July of 1971.
The old schematic I gave you was one of the working copies we used while the receiver was being designed. Youll probably note a couple of revision dates in the lower right-hand corner. It eventually developed into the final version that was first sold as a final product. That typically is how we documented the designs during development and revision levels.
As far as how we finalized the design and kept the circuit the same, product development was quite different back then compared to todays methodology. In the past, each product line had its own unique design and circuit layout. The first portable radio (HT200) was completely different from the next generation of portable, the HT220. The same for the MT500 and MX300. The reason was, there were so many new innovations taking place in electronics back in those days. The HT220 was the first to use ICs. The MX300 used plug-in hybrid electronic circuits. Every new radio we designed was using some new state-of-the-art electronic device that rendered the previous designs technically obsolete. In todays electronic designs, most products are developed with a lot of similar circuitry that changes very little from model to model.
Also, remember that we had minimal competition back then. We could take 3 to 4 years to develop a product line. Todays markets want new designs in a year or less. That leaves very little time to make drastic circuit changes.
The manuals today are very different from those of the HT220. Remember, the 220 was a discrete component, very repairable radio. Unlike the custom IC based designs you see today. A lot of modem electronics is basically throw-away design when it breaks. Therefore, product manuals dont need to go into a lot of detailed repair or tuning info simply because you cant do that with todays products. Troubleshooting down to the component level is becoming a lost art. Even in the automotive industry.
Thats enough rambling for now. I hope this history is helpful to you and some of your other 220 enthusiasts. I may have repeated some of the info already on your web-site. It looks like you, and perhaps others, have done an excellent job of capturing the spirit of the HT220 and are keeping it alive. Thanks for a great job and I hope you continue to enjoy the product and get some use from the goodies.
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