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Here is an authoritative article on the much-discussed Cosmo Compo radio, written by the man that invented it. This receiver, which is already being sold in metropolitan department stores, has been hailed by some non-technical writers as a means of emancipating the radio owner from the repairman. Read what the inventor, himself a former radio serviceman, has to say on that subject.

This Radio Fixes Itself
Roberto Brenta, Radio-Craft, December 1947

These Compo cans may be changed in a fraction of a minute, curing any trouble due to the enclosed component. As one of the cans contains the complete audio-circuit and another all the filler components, it is obvious that a radio can be repaired in a few minutes. The drawing at left is the artist's conception of the contents of one (which one?) of the Compo cans above.

Before anyone can honestly call himself a radio repairman he must have a thorough knowledge of radio electronics. He must be familiar with all types of radio receivers. He must have the proper tools and equipment to give proper service.

For many years many complex problems have faced the radio repairman from the time he puts a radio receiver on his bench till he returns it to its owner in playing condition. It is not uncommon for him to find himself on the short end of a deal. When all the time, energy and materials that go into the repairing of a radio are considered, there is always the chance that he will not make any profit and may lose money. That may sound strange to the layman, but is recognized by the repairman as an everyday possibility. For example, the customer doesn't realize that the breakdown that occurs several days after his set has been fixed has absolutely nothing to do with the original trouble. He insists that the repairman did a poor job and is responsible for putting the set right without further cost. In some cases the cost of additional parts that are needed plus the added time spent in repair exceeds the price of the original estimate.

Rear view of the plug-in receiver.

A serviceman's dream

This among other things prompted me to try to find a suitable answer to the problem. During my years studying engineering in the Navy as a radio technician, I had little time to work on the idea. We worked night and day building and repairing radios of all types. Sometimes we had plenty of tools and materials--at others we lacked almost all the proper parts and replacements. Most serious of all, we were understaffed because of the lack of trained technicians. When a component was unobtainable we either had to improvise or scrap an entire unit or set.

I began to think again about my old plan to simplify radio receivers. Seeing all the waste proved to me that the time was ripe for a radio that would eliminate the complexities of hundreds of parts and wires and reduce the repairman's problems by expediting the work.

When the war in Europe ended I was stationed in Guam in the Pacific. With. the cessation of hostilities in the West our job lessened daily. I began to get a few hours for myself each day and used the time for experiments. I wanted to find a way to reduce unnecessary waste. I wanted to find a way to utilize the cast-off parts and units that were deteriorating from lack of use. It struck me that if I took several parts, or better still, a complete circuit and tied the parts together in a single replaceable unit I would have the answer I was looking for. From the beginning I toyed with the idea of segregating into sections a conventional 5-tube circuit.

The first experimental set gave me quite a bit of trouble because of the long grid leads. This was eliminated by re-arranging the socket positions. I realized then that with further engineering effort such a radio was a practical possibility.

Another big problem was the size of the filters, resisters and condensers. It was impossible to squeeze those components into a unit of practical size. I then considered the use of miniature components, which was the answer to the problem of reducing the size to fit the containers.

There was always something to slow up my progress. After solving the difficulties of putting r.f. components into cans, I ran into trouble with audio quality, This was solved by special attention to matching the output transformer, output tube and speaker, and by using a 50B5 output tube.

The Compo receiver

In brief, I have succeeded in developing a compact AM receiver in which all the elements of a superhet circuit have been made plug-in. Oscillator and i.f. units are pretuned and there is sufficient tolerances to make interchangability of units quite practical. All the normal under-chassis resistors and capacitors are included in their respective, leaving nothing underneath but the filament wiring. Volume control will be plug-in eventually.

Schematic is standard, though mechanical features are novel.

The present model consists of 5 vacuum tubes: 12BE6, 12BA6,12AT6, 50B5, 35W4. In addition there are 6 component tubes which are color-coded and numbered. The oscillator section includes a specially designed oscillator coil with the coupling from the coil to the grid and the grid resistor. The first and second intermediate frequency transformers are in two separate cans or containers. Also in these containers are the automatic volume control line and the first audio grid input. Then there is the power supply consisting of a filter and filter resistor.

Top--An old style set with its maze of under-chassis wiring and dangling components. Lower right--Present Compo receiver. Lower left--Stamped-wiring Compo receiver of the future.

Included in the Compo cans are the following parts: Oscillator is in a green can numbered 114; the first i.f. is in a blue can numbered 115; the second i.f. is in a black can numbered 116; the a.v.c. is housed in a yellow can numbered 117; the audio output is in a brown can numbered 118; the filter is in a red can numbered 119. There is also a 35W4 rectifier, an 12BE6 converter mixer, 12BA6 i.f. amplifier, 50B5 power output and a 12AT6 detector and first audio.

A test has been going on in the laboratory since June 9th. A Compo receiver has been in operation 24 hours a day without interruption under normal operating conditions. So far, the set has been working perfectly and without any signs of difficulty.

This is a glass-enclosed demonstration model.

A plug-in amplifier

Using the same principles, a 40-watt experimental amplifier has also been developed. In this case, I have worked out the amplifier so that all the condensers and resistors are wired on a plug-in fibre panel. If anything goes wrong all that has to be done is to remove the entire unit and replace it with a completely new working part. The part that has gone bad can then be repaired and used as a spare.

The Compo system could well bs extended to include not only receivers and public address systems, but to cover many types of electronic devices. It would be particularly valuable in commercial and industrial equipment where the time lost in repairing a piece of equipment is more valuable than the price of the parts. A stock of replacement units might in such cases save costly delays, or even the cost of complete standby equipment.


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