Heath TC-2 Vacuum Tube Checker

This Web page last updated date Nov 12 2013.

This Web page is about the Heath TC-2 and related tube testers or "tube checkers". It's part of our Web page support for old computer repair and restoration. We support a lot about old computing and other retro-technology, see my Web site home page.

For more Heath and Heath/Zenith items, and the current status of the Heath company, check my Heath - Zenith computer page. Details on the Z-100 series computer systems are on another page.

If you have any corrections to this information I'd appreciate hearing them: please include appropriate references so I can confirm that info. I am not responsible for errors or omissions in the above info, use it at your own risk. - Herb Johnson

Heathkit TC-2 Vacuum Tube Checker

In April 2009, I went to a hamfest where, long story short, a guy gave me a somewhat chewed up Heathkit TC-2 tube tester. (Either that, or I paid $10 for it - practically free.) Looks to me like it sat on some wet basement floor for a few years. But opening it up, it looked like the damage was only cosmetic. You can see the paper covering is water-damaged and a little moldy. But the components were not damaged.

Got it home and opened it up. Physical damage was to the plywood cabinet at the bottom of the cover and the base, where the metal feet are. Here's an image of the removed base section. I decided to replace the base wood section, with another piece of plywood. Here's the inside of the base with new section in place and the old inserted for show. The top edge of the new section is rough, because I chopped out a groove to hold the edge of the tester's metal chassis. This matches the groove around the other edges of the base, which are obscured by the vinyl cover material (possibly old Contact paper).

When time permits, I'll replace the lost paper. Here's what the outside corner looks like at this point. Probably I will try to photocopy or inkjet the original covering image onto some thin material. I also need to get a pair of hinges which allow the cover to be removed, and two more metal "feet" - the old ones rusted out of course.

Technical

The operation of the tester is straightforward. As you can see by the schematic, this is an "emissions" tester. A roll chart, or supplementary chart, shows for each vacuum tube which connections to make and what load to set, and the filament voltage. Filament voltage is set by selecting a secondary tap of the correct voltage. The vacuum tube is set up as a diode, all grids connected to the plate.

At a given voltage (fixed at about 30 VDC) and a given load (set by a 200 ohm rheostat), the tube should draw a given current based on cathode emission, which the meter measures. As the tube ages, the cathode emits less, and eventually the filament breaks, emission is insufficient, or shorts or opens occur between elements. Shorts and opens can be tested by disconnecting the grids (reduces current unless open) or by shorting them to ground (stops current) respectively.

Here's a photo of the tester in operation. The only components in doubt, are the old-style selenium rectifier and the .1 mFd 600VDC condenser. The resistors generally read "high" by several percent, the filament voltage low by the same amount when set to "line=50%". And the "line setting" which sets primary voltage, is still a little high - even though there is an additional resistor in series with the AC line (not in the schematic, it's a common upgrade). What I'd like to do is measure known "new" 100% tubes, to see what the maximum reading is compared to full-scale. [I've been told since that even new tubes vary a lot.]

But as of early August 2009, it's good enough to check tubes for general operation.

Further repairs

Something I could use would be a pair of hinges! Mine are rusted through, and I can't find duplicates. I'm afraid I'll have to hack a pair of conventional hinges to get what I want. I can do that but they may not look like the originals. Anyone have a clue about getting a pair of these? Or matching Contact paper to match the old coverings?

Related tube testers and docs 2013

At a 2013 hamfest, I picked up a TC-3 manual.

Also in 2013, I acquired a Heath IT-21 tube tester. this one is a vertical desktop model, not a portable in a Contact-papered box like the TC-2 above. Other than some nuvistor sockets, I think it's much the same as the TC-2. The front panel of the IT-21 shows the additional sockets. It's fairly clean but shows some points of rust and wear. I've not opened it up, I'll probably power it up on a Variac and put it through some tests. Here's the IT-21 schematic.

Thanks

Thanks to Mike Loewen, who provided a schematic as copied from his manual. It happens I was at his Vintage Computer Workshop, where I saw this tester in use. Also thanks to Bill Degnan, who was at that workshop, and who later asked for help with some vacuum tube equipment, after I acquired the tester but before I worked on it. I got it in shape to help him out.

Thanks to Paul Third, who wrote to me: "Hi Herb, I saw your web page on your TC-2 tube checker and the questons you had on your TC-2. The "old style" diode is a selenium rectifier as they called it back in the day. .. I replaced the selenium rectifier with a 1N4007, any 1N4000 series diode will work, and it's been working fine in the fifteen years I've had it. I did this with no schematic, so I had to trace the entire circuit and draw it out, so thank you for providing me with a printed schematic rather than the hand drawn one that I have been using in the last fifteen years. On the bottom of the [rolled] tube chart, on the right side there is a [date] like this: 1-1-64, just above the word STOP. I hope this helps you, and it's nice to know there is another TC-2 owner out there. Enjoy your TC-2."

IT-21 schematic courtesy of Nostalgic Kits Central.

Herb Johnson

Copyright © 2013 Herb Johnson