Crosley Model 173
Written by Bryce Ringwood   

The Crosley Model 173 was manufactured in 1933. It is a 6-Valve superhet using 78 (RF), 78(Mixer), 6B7(IF and Detector), 78 (1st Audio) and 43 as AF amp. Rectifier is a 25Z5. It covers two bands, approximately 500-1500kHz on Medium Wave and 1500 – 3100kHz on Short Wave.

After nearly 80 years – quite a number of things had stopped working! Fortunately, the valves were fairly easy to find and replace, but this was only the beginning. A small choke coil had fallen apart, and was fairly easily repaired. These were the obvious things. The set was then slowly brought up to working voltage, but it remained silent. The cathode resistor of the 43 was then checked and found to be open circuit. Replacing this did not help. Putting a modern loudspeaker across the output transformer brought forth a small amount of sound. A check on the speaker field coil revealed it to be open circuit.

The field coil needed a mere 15000 turns of hair-fine wire, so I duly rewound it. (The hard part is the unwinding and counting the turns).*  The speaker then came to life, but really didn't sound too great compared to a modern speaker. In addition, the speaker cone was very brittle - a candidate for re-coning. At this point, the set was working, but neither myself nor the owner were too enthralled by the sound quality, so I said I would have a think about it and we would give it another try early in January.

In December, I had scrounged a Krohn-Hite function generator from a colleague (and got it working) – hoped this would let me see the quality of the audio produced, if nothing else.

In January the set was returned to me, and I couldn't believe how horrid the audio had become. The set had also become very “dead”. Clearly, the alignment hadn't survived the journey home and back, so this time I put a small dab of paint on all the trimmers after carefully re-aligning it.

In the mean time, I had managed to get a permanent magnet speaker the same size as the original. It even had space on the back for mounting the output transformer! I also decided to return the volume control to that of the original circuit diagram. Crosley had obviously made a modification involving a volume control having quite a high resistance., since it had a 3MOhm resistance in series. I replaced it with the 470k log volume control. I checked the volume control I had removed and to my surprise it measured more than 30MOhms. Obviously, it was the main culprit (when I measured it in circuit, it was about 3MOhm). How it changed the volume level, I will never know. As a final touch, I added another large value electrolytic in parallel with the cathode by-pass of the 43, to give a more pleasing bass response. The only remaining problem was a tendency for the RF stage to go into self-oscillation on a very short aerial – cured by placing a 27k resistor in parallel with the 1st RF coil on medium-wave. (Many radio receivers have this problem - the Eddystones I have used all had it.)

By this time, it was sounding pretty good, but I'd repaired the Krohn-Hite and wanted to SEE how the output looked. It looked like a sine-wave should – so I called the owner to collect, and this time we were both happy with the result – it worked on just 600mm of aerial wire.

When re-wiring a radio of this age, nothing looks worse than modern vinyl covered wiring amongst the cotton covered rubber cables used in the original. It's possible to buy a woven sheath which can be put round the PVC cable to hide it. It also makes the wire about the same diameter as the original. It is possible to get the original type of cable from some American suppliers. The other problem is modern capacitors, which are about a tenth of the size of the original paper ones, and can look a bit silly. If you don't want to drill out the original paper tube and hide a modern cap inside, then the next best thing is to choose ones with a huge working voltage.

By the way - just look at that cabinet! Not only does the radio sound new - it looks it too. Congratulations to the owner on a splendid job.

* Don't feel any sympathy towards me for winding all those turns - I have a machine that does the counting and rewinding in a little more than a minute. Well the winding, anyway. I have to pull the old wire off, while it does the counting. It might be nice to get the original speaker re-coned to make the set all-original.

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