Monday, September 17, 2012

Project Clean UP Part 1: Twin Tube 80

This is Chapter 1: The Power Supply!

This is part 1 of a series,  a complete list articles in this series is available under "Ham Radio Master links" on the sidebar.

Recently I've been wanting to clear various projests out of my project queue. Some of them are things I've been working on, and needed specialized parts to complete it. Others were not working because of technical issues. Some are because I'm afraid of drilling holes into some things and potentially ruining them :-)  This last week I've been working on a radio that has vexed me for for ten literal years. The radio is the NoGA Twin Tube 80 schematics and instructions available at that link. I built it as it appeared in the NoGA Compendium available here. The kit itself is no longer avaible, but the parts to build it are out there. Mike Branca W3IRZ (sk) helped me get the kit working when I was having issues with it at my house. My goal was to put it on the air for the NoGA CW net on Tuesday nites. Well, Mike gave me a lot of great advice about building radios on that trip to his shack, and at the end of the day I had a working radio. On the drive home, something affected the æther stored in the radio, and while after plugging the radio in, I got no smoke out, neither did I get any RF! Alas ! Every trick I tried played out, and I set the little tube transmitter aside for other projects. Somehow, I knew the problem was in the power supply. I suspected the large value capacitors in the rectifier circuit.
Fast forward to the present.
Every time I would look at this radio I would get sad. Sad for the loss of a friend (W3IRZ sk in 2003), and sad that I couldn't seem able to figure this thing out. I decided it was worth a try. Time to stop being sad, and get back to slingin' dits and servin' dahs.
Since I've always suspected the powersupply in this radio, I decided to just hammer away at the power supply until it was working. The way this particular rig works is that you have a 12 v ac line for the tube filaments, a 150 v dc line for the plate, and a ground that you ground through the plug. I got out my caps, diodes, resistor, plug, and began the project for the third time. After soldering everthing together, making sure to follow instructions, I took the leap and plugged it in. No smoke! I tested the 12v ac circuit. I got 13v, that's ok, the transformer output is  actually rated at 12.6v ac, so no big surprise. The 150v dc line, no dice.
Hmmmmm, this is where I left off ten years ago. I went to bed dejected and tired. I'd stayed up until 3:00am for nothing. I hadn't even learned anything yet. Course all that was going to change the next night.

I do my best to junk out old stuff so I can have strange parts when I need them, you can't buy 220 microfarad 250volt rated capacitors at radio shack. You have to go online and look them up on Mouser or Digi-Key. Fortunately, one of my recent junking exercises produced the caps I needed. This is recycling! The next night I replaced the caps and diode (just in case) double checked my wiring, plugged it in, and nothing! Still no B+ voltage. Dejected I tuned around the band hoping to find something interesting. I began thinking of what the issue could be and applied a little problem solving.

 Grabbing the VOM, I hooked the ground wire up to the black probe and began poking things with the red. Positive terminal of 470 micro farad cap, nothing, positive terminal at 220 microfarad cap, nothing. Bottom side of 1N4001 diode, nothing. Top side of 1N4001 diode, I switch to AC because I'm connected to the ground and the black (hot) wire directly, nothing. Wait, whut?
I have 13v ac at the transformer, between the white (neutral) and black (hot) wire I have 120v ac, but between green and black I got nothing?
Then it clicked. The black wire wasn't hot, the white wire was! Two snips and a dab or two of solder later, and I have the white wire hooked up as if it were hot, apply juice, check voltages, and we are good to go:

How's that for live action?
It's also why every shack needs a 'chicken stick' of some sort. Link takes you to a site all about stories of chicken sticks.

So, I have power at my supply, but It's coming from a funny wire. Before buttoning everything up, I needed to figure out Why the White Wire is Hot.
That's Chapter 2!

Continue on to Chapter 2.

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