I picked up this Knight Space Spanner several years ago in non-working condition. I have an entire shack full of non-working project radios. Actually, more than I have working radios. It’s a simple tube regenerative AM receiver with a selectable band for broadcast or shortwave. Think of Knight radio as the do it yourself branch of Allied Radio back in the day. They produced a wide array of radio kits for the consumer market. This model was produced from 1956-1964. It’s not great as far as receivers go. Even back in the day there were much better offerings available, but I’m guessing this was aimed more at the general radio listener than hams.
The regen is a funny beast with a history that goes way back to the early 1900’s. Tuning in a station is a balancing act between volume, regeneration, antenna coupling, and dial setting. Any mis-adjustment of any combination of these switches results in useless noise or screaming oscillations. The screaming oscillations are the worst! It might be wise to offer up a solid state electrical device as a sacrifice or recite ancient incantations prior to resurrecting one of these demons. But when proper tuned, a station does sound surprisingly nice. I’ve found it quite pleasant to listen to local oldies radio on the AM broadcast dial.
What it lacks in features it makes up in simplicity. Not a lot going on inside this small box. The schematic is simple to follow and the circuit is all point to point wiring. I’m sure it would’ve been an entry level project when new. Back then you had to be tough, though. There’s no transformer isolated HV power supply. All the tubes are lit in a series string straight across the wall line voltage. Insert the non-polarized plug in the socket backwards and you’ll get AC voltage on the chassis. I guess folks were just supposed to know better than to do that more than once! What I’ll refer to as a self-correcting error.
I’m preparing to rebuild a Hallicrafters SX-28 this year. That’s way above my current skill level and I need more practice. This Space Spanner seemed like the ideal candidate. It already didn’t work so I figured I couldn’t make it any more broken! I had to go at it in small spurts due to my busy work schedule. Overall, it’s been quite rewarding to make it play again.
It didn’t have any tubes so I started with ordering them from the web. Tried it out, but no love. I then removed the aged electrolytics. There were only three, and plenty of room to work. I also found a suspect ceracap. I don’t know anything about them, but replaced anyway. There’s a cap straight across the neutral to ground. I believe these are called a “death cap”. That was replaced with a proper XY AC-rated safety cap. Powered it up slow on a variac, hooked up an antenna, no smoke, and…no love! Not really even much noise.
Well, it took me a long time to get back to the radio again. This time I printed off the schematic and started tracing it line by line. I finally came across a mis-wired ceramic cap. I did nothing more than move it from one point on the terminal tie strip to the spot right next to it. This time it worked straight away! How awesome a feeling to bring it back from the dead! I wonder if it ever worked, or who mis-wired it. I also found an extra wire tack soldered that was removed during the repairs. I wonder what it was for. Repairing old radios is often a mystery.
I always seem to learn something new after each repair. Each case is unique and I rarely turn up a unicorn that work perfectly straight away, have never been touched by human hands since construction. I guess I’m about up to 1960’s skill level. With any luck I’ll progress to the 21st century by the time I retire! Maybe not, old radios have more soul than modern stuff. I’m glad to keep another piece of history from the landfill.