One of the more common failures in the Zenith "Lunchbox" radios (Royal 750/755) is a cracked or broken volume control. The carbon track is very fragile, and the way they mount it in the radio and solder one lug straight to the chassis makes it extremely prone to damage. NOS replacements are few and far between and donars are hard to find because everybody else's has the same problem. I had two of them, myself. So I went looking for a current replacement.
This is what it looks like:
royal755_volumepot.jpg [ 190.91 KiB | Viewed 491 times ]
This is the resistance trace and the phenolic board it sits on, and you can see this one has a fair bit of the trace missing:
royal755_pot_trace.jpg [ 243.59 KiB | Viewed 491 times ]
If it's just cracked you can sometimes carefully jump the crack with silver epoxy, but this one is mostly gone. Another common failure mode is that the solder tab at the low end (on the right in this ) breaks off completely and takes some of the phenolic and trace with it. Thats the end soldered to the chassis and when the solder tabs holding the entire assembly to the chassis gives up, the next good bump of the volume control knob breaks it off.
It's a 5K pot with a switch, and a 3/16" shaft with a flat for indexing purposes. I found this one at Mouser that looked promising:http://www.mouser.com/ProductDetail/Alp ... ffxKlY8%3d
This is what it looks like:
royal755_shaft.jpg [ 230.05 KiB | Viewed 489 times ]
I temporarily wired it into the radio, and it worked dandy. And the threads fixed the existing holes perfectly.
Of course the shaft of the replacement is 1/4" spilt, and splined. I had hoped that the resistance element board would be the right size to put into the original housing with the original wiper and switch. It's not as crazy as it sounds, many of the larger pots have the same dimensions, but no such luck here. There are several obvious solutions but I figured I would graft the old shaft into the new control. I decided to use 3/16" ID brass tubing from my scrap box as a joiner, so I had to turn down the shaft of the new control somehow. I know people with small lathes AND I have a drill press but that doesn't satisfy my need for instant gratification, so I decided to use my hand drill. Then I quickly found that the number of splines on the shaft is not divisible by 3! Any attempt to chuck it up resulted in it being eccentric, and not in the good way like we are. So I found some tubing about the right size, with a 1/4" OD and a 7/32" ID. This could be forced over the shaft but I split it lengthwise and opened up the kerf so I wouldn't have to bend anything getting it on.
brass_sleeve.jpg [ 246.28 KiB | Viewed 491 times ]
Then I put it in the drill and it spun nice and true:
royal755_spinningpot.jpg [ 153.27 KiB | Viewed 491 times ]
This is a true ACTION PHOTO, you can see it isn't blurred up and down, just around. This picture is just a simulation:
royal755_machining.jpg [ 176.37 KiB | Viewed 491 times ]
I turned the shaft down with a flat needle file. The shaft is *very soft*, and this took very little time, in fact, I overdid it on the first one and went past 3/16". This one came out just right. It's so soft you need to be a little careful to keep the file exactly square because it's easy to put more pressure on one side and put in a tapered groove. Once this was done, I cut the shaft off, and this is the completed part:
royal755_replacement_shaft.jpg [ 246.14 KiB | Viewed 489 times ]
Here's all the parts for the whole thing:
royal755_replacement_part.jpg [ 173.56 KiB | Viewed 491 times ]
I think you can figure out what to do from here. It could certainly be soldered, but I used J-B Weld. The rewiring was straightforward. The open switch on the back of the control is now semi-exposed but it's covered by the cardboard backing that goes over the back and I don't foresee a problem with that. I see looking at the link that the cost of the new control went up from $1.95 to $2.10 since August when I did this, but I think anybody should be able to swing the extra 15 cents. I took one of the replacements apart and it seems MUCH more durable inside, with a nice solid-looking trace that will certainly outlast most of the the parts.