Monday, March 13, 2017

TV 3A/U Navy tube tester restoration, still investigating disk tool issuing writing cartridges


Still working through the microcode and hardware to understand each and every cycle I should see on the Alto when it is trying to boot a disk - both successfully with natively written cartridges and failing with the ones written by my tool.


I stopped by an electronics swap meet this weekend and picked up a US Navy tube tester - the TV 3A/U which was built by Hickok. It measures the gain of the tube, not just the static emission strength like most tube testers, which is important to truly verify that a tube is working properly.

The pilot lamp goes on and one of the two tubes inside has it filaments glowing (the lower voltage 5Y3 rectifier tube) but the other 083 tube did not light at all. I may have a failed tube, but need to check voltages and condition to be sure that something more sinister doesn't exist, either to have caused the failure or perhaps inside the power transformer.

Another flaw is that the meter movement does not budge - it should deflect for the line voltage measurement and also to full scale when the tester is set to the ohmmeter function. The meter has a capacitor across it, which may have failed in a short, or I may have a more pernicious problem given that the line voltage check is a very direct connection.

I checked all the AC voltages coming off the transformer and it is good! Further, there is filament voltage on the 83 rectifier tube but it is stone cold dead. I looked for replacement tubes on ebay, found these ran around $30 each but a kit to create a solid state rectifier that plugs in as a replacement was only $12. It should arrive toward the end of the week.

Next up is diagnosis of the lack of meter movement. Separated meter from circuit - wide open circuit. Something burned out inside. Ouch! These are unobtainium parts. My plan of attack is to first open it up and see if I can find the open circuit somewhere external to the meter winding where it might be fixed. Secondly, I would need to find a close enough alternative and swap the faceplates.

I have removed the meter but have to figure out how to open it nondestructively and then inspect it carefully to find the cause of the open circuit.


  1. Have any luck with your meter? I am working on a TV-3A/U with the same problem, open meter.

  2. Hi Jeremy

    I worked out the specs for the meter, then found a physically compatible one from a different type of system (an old VOM).

    I can move the faceplate over to the new meter, but I still have to do some delicate work to get the shunt and inline resistance correct. Only when those match will the meter deflect the right amount to match the original.

    I got distracted with other projects, but have this sitting in project boxes waiting for the next time it bubbles up to the top of my work queue.