Tuesday, April 18, 2017

Building curve tracer into case


With the circuit board itself working flawlessly, it is time to build out an enclosure with tube sockets and rotary selector switches, mount the board inside, and wire everything up. I hadn't selected an enclosure yet. I do want to move forward on this.

I initially thought to gut a Century tube checker I own, but the panel has dozens each of octal, 7 pin mini and 9 pin mini sockets, whereas I only need a few slots. Further, it has a large meter and other parts installed which are unnecessary, and lacks the rotary switch openings. Still, I decided to hack it up, as the extra socket openings could eventually hold compactron, novar, nuvistor, acorn and other socket types.

I have five rotary switches with 11 settings, not enough to route each connection type (anode, screen, grid, cathode, and one side of the filament) to any of 12 possible pins (most sockets have 7, 8 or 9 pins but there is a compactron type that has all 12). Thus, I will have to connect in the opposite way, using 12 rotary switches to select among six voltages - anode, screen, grid, cathode, filament A and filament B.

The existing sockets are pop riveted in the panel, thus I need to drill out any that I remove. I can then use screw and nut, or new pop rivets if I can find my gun and supplies. First step was to gut the transformer, unused controls and wiring, Next was to trim back the rats nest of intertube wiring.

I have ordered another batch of rotary switches, enough for 10 pin sockets which will cover everything except the compactron (12 pin type). These are almost exclusively used in tube based color televisions, which is not an area where I expect to receive tubes for testing. Thus, I will ignore the compactron type entirely.

There are a wide range of tube socket types, of which I will support as many as I can easily and inexpensively obtain sockets for. The three workhorse sockets are the octal, 7 pin mini and 9 pin mini. I have plenty of these already on the Century unit.

Loctal sockets are similar to octal but have a center pin that snaps into place in the housing. These were intended for use in automobiles and other high vibration environments. I happen to have two loctal sockets on the unit, otherwise I wouldn't have bothered. Again, mobile radio tubes are not what I expect to be testing. Hifi, computers and test equipment will be the primary source of tubes for testing.

There is a 10 pin socket that is like the 9 pin mini but has an extra connection in the center hole. I happen to have one of these sockets, so it will go in. There is a novar type, which is nine pins but thinner and similar to the compactron style. I think I have one of these as well.

Older radios used 4, 5, 6, 7 and 8 pin sockets, UX4, UX5 etc, that I might stick in if I had them cheap and ready to go, but these are mostly from 30s and older era radios and clearly not going to be needed often.

I have the first six rotary switches (one from the original tester and the five I bought for the project) installed in the unit. Next up is to choose locations for the next three switches, which will arrive in two days. That will give me switches for the nine pins of the sockets already in place (7 pin mini, 9 pin mini, octal and loctal).

I began wiring up the first six switches and the five sockets I am using. It is tedious work, building a loop for each pin across all five sockets, including a ferrite bead to soak up any RF oscillation that might try to build, and hooking each loop to a different switch. I can only build the loops for pins 1 to 6 until the new switches arrive and are mounted.

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