That radio looks to be in good shape. It should take very little to get it working, since it doesn't seem to have any electrolytic caps. A little solvent to thin the grease on the tuning worm, and some cleanup to break the corrosion that has frozen the tuning cap in place. May have similar problems on the pots. That exposed wirewound will certinaly be noisy, but a little pot cleaner will do wonders for it. Won't hurt to use it on the tube pins also, since ancient corrosion will have built up.
That would have been a battery powered headphone unit. These days you would want to build a well-filtered power supply for it, 300V batteries are hard to come by and expensive! (Yes, it would have been a stack of 45V batteries or so, but still very expensive today, and wouldn't last all that long.) Beware the filament voltage - it was probably 2V nominal (I havne't looked up the tubes to be sure) and would quite possibly be runing low if supplied by a dry cell, which was common in the early days. There was often a series pot to adjust to the most effective filament voltage. This was often less than the nominal tube voltage.
You would probably have to run out 20-30 feet of wire for an antenna on this thing, especially if you live in a highrise building made of steel. A good ground might also help reception considerably. If you have nearby stations you could probably pick them up fine just using a 3 foot cliplead on the antenna terminal.
Freeing up the tuning cap: you may be able to get it lose with a little NON-silicone pot cleaner. Spray some on the grease and bearing points on the tuning knob, and some at each of the bearing points on the main condenser shaft. Rocking things back and forth might start to break things lose. You might end up having to use somethinng like some Kroil to get things broken lose. Then you need to rinse it out with some thinner or other thinn solvent, and re-lube wth some decent lube. Be careful with lube on the condenser bearings, the shaft and pltes are grounded through those contact points. A little lube is good for preventing corrosion and smoothing operation, but too much can cause grounding problems.
Thanks, Herr Bell, for posting that schematic. This stuff animates me on so many levels: I'm an EE, I'm a guitar player who plays the kind of music that requires massively heavy tube amps (both uses of 'heavy' apply), and finally I've studied many an audio amplifier schematic in my day and I love to apply my EE skills to tinkering with my amps.
In doing so I've gained a lot of respect the classic tube designers of the early part of the last century. They were true engineers who did so much with so little. Just look at that schematic. It's clean, it's efficient, it is indeed a thing of beauty. To me it is down right artistic. (And on a technical note, one thing that jumped out at me is the use of filamentary tubes - where the filiment and the cathode are one. Being from 1927 these tubes pre-date the subsequently developed indirectly-heated tubes seen since the 30s, and which are the building blocks of all guiatar amp designs. This makes sense since most of those designs are basically lifted from a standard audio amp design of the 40s, brought to you by Mullard, through Leo Fender).
But that was my second impression. The first thing that jumped out at me is the price: $82.50?! In 1927? Are you kidding me?! That had to be on par with most mortgage payments back then, likely more. They couldn't have sold many of these things.
Anyway, these days tubes survive in only three markets: guitar amps, high-end audio and some high voltage radio transmission applications. Little money can be made there, but I love this classic stuff. Studying it has a redeeming quality for EEs today as well. It helps to know where we all came from. I recommend everyone do it. Please keep it coming and thanks again.
It is indeed a Tuned Radio Frequency design. One quite interesting point, the intemnediate stage transformers are aligned on different axis' - this was done so that the stages would not couple with each other - the magnetic fields from the coils point in different directions. If they did couple, then the stage would oscillate. It is a lovely piece of engineering - thanks for sharing it with us.
As we unveil EE Times’ 2015 Silicon 60 list, journalist & Silicon 60 researcher Peter Clarke hosts a conversation on startups in the electronics industry. Panelists Dan Armbrust (investment firm Silicon Catalyst), Andrew Kau (venture capital firm Walden International), and Stan Boland (successful serial entrepreneur, former CEO of Neul, Icera) join in the live debate.