SECO Model 107B Tube & Grid Circuit Tester

I ran across this tidbit that may help in adjustments.

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"The Seco 107B which I have, has an aging adjustment for the internal vacuum tube which is reachable through the compactron pin straightener with a regular screw driver."

Part II.

To start off with, this instrument has (4) DC power supplies and (2) sets of AC supplies. The DC supplies all use silicon half wave rectifiers with fairly small filter caps so the current demands must be small. There are (4) fairly high AC voltages available for the emission tests. There are a range of filament voltages available (tester made during the series string TV era). The only info I have on the tester is what is presented in the back of the tube chart including a small and hard to read schematic diagram.

I started out trying to get the tester in calibration in regards to getting the line test indication and filament voltages about right for average 6.3 volt and 12.6 volt tubes (300mA to 600mA heaters). This couldn't be done after getting it powered up and running. I followed the given procedure by setting the incoming line voltage to 117V and setting the line rheostat at mid-point. Actually, the mid-point of the rheostat was tapped so it wasn't hard to find. The instructions said that R1 was to be readjusted if the line calibration was off. It is mounted under one of the pin straighteners on the emission test panel. Adjusting it to get "100" on the meter  didn't really help much (for long) and it was obvious the pot needed cleaning to get a stable reading. Probably some of the other trim pots needed cleaning too.

In addition, there is a (4) section trim pot assembly mounted on the epoxy pc board. After a lot of checking I found out that they are R2, R3, and R4. One section is not used. R3 affects the line test calibration as well but is to be readjusted only on changing the tube (12AU7). The 12AU7 tube comprises a DC amplifier circuit so these trim pots allow the amplifier to function as intended with what tubes are commonly available (within limits as I found out). Possibly, the tube was selected at the factory. Earlier on, I had checked the 12AU7 tube pins and socket for corrosion and they were clean and shiny.

Eventually, I tried a number of good 12AU7/12AU7A tubes out of my supply in place of the original Telefunken 12AU7 tube. At first, I was not willing to try readjusting R3 and the results were dismal. All gave worse results than the original tube. Trying to get this circuit to work was like trying to repair a tube type Tektronics scope with ordinary everyday replacement parts. At one point, I pulled out whatever tube I had in there and used the tester for a while without the line test or GCT/Short test functions working.

Then, I ended up putting the Telefunken 12AU7 tube back in but this time I tried adjusting R3. I left R1 in its previous position. There was some improvement overall but not enough. Having checked this tube out before hand I can can say it is basically a good tube but the two sections did not appear to be close to one another-at least not in the gain tests I was able to do. I went back to some Sylvania 12AU7A tubes that I had checked out and tried before. One of them showed very close results between the two sections and so I put this tube in. I adjusted R3 to get the proper line test reading of 100 with 117V line voltage coming in. The results since then have been much better over a wider variety of situations and with higher line voltage. The line voltage currently is in the 122-123V range. back during the summer it could be as low as 114-115 Volts with every one's air conditioning running. When the heating season starts in earnest it will probably go back down again.

Part I.

Recently, I got interested in the SECO GCT (Grid Circuit Testers) made for checking tubes for grid emission. Grid emission can be a problem in circuits with R-C coupling using large grid resistors (2 Meg Ohm and higher). I was thinking of getting one as an add-on to my other tube testers. I looked at the GCT-5 and GCT-8 models first. They appear to be the most common but there are other models also. These testers use a "Magic Eye" tube as the indicator and a 12AU7 tube as a DC amplifier.

Well, I ended up buying a nice looking SECO 107B Tube and Grid Circuit Tester in one unit. The 107 is part of the SECO series of combined testers. It has a mutual conductance test section primarily for small signal amplifier tubes; emission test section for power output and rectifier tubes; and a combined GCT and Shorts test section. All indications are on the panel meter for this tester.

The unit I received appears to have had little if any use. The panels look good and the case is in good shape too. It probably was put away some decades ago. The seller did not plug it in-far as I know. It may not have been stored in the best of conditions as there was some musty "electronic" odor about it and the plated hardware and controls have some corrosion on them. Otherwise, it looks very good.

I used the first few days it was here to give the insides of the (3) panels and case the "sunshine treatment" outdoors and that has helped to reduce the odor to acceptable levels. Next, I got out the old caps I wanted to replace before powering it up. These are mounted on (2) P.C. boards. One of which is a nicely done epoxy fiberglass board and the other a common brown phenolic board. The caps in there still looked like new but of course they weren't. The (3) Planet 10uF 150V and (1) 50uF 50V electrolytics were in poor to bad shape while the (2) large IMP .1uF 400V tubulars (paper-mylar) removed checked out excellent. I got new ones for them all. Care was needed for replacing the caps on the phenolic board. One of the pads lifted a bit. 

After checking out the new parts first and then getting them installed I got it back together and was ready for the first test. The 107B has an unusual plunger type on-off switch and I wonder if it is part circuit breaker too. Well, it fired up OK and appeared to work but showed signs of line test setting problems. A 12AU7 dual triode tube is used for the line-test and the GCT-Short test functions. The line test reading is the starting point for the GCT and Short indications on the meter so I was concerned about those tests too. The meter deflects downward if there are any shorts or problems with grid emission. In general, the line test was off quite a bit in regards to actual filament voltages delivered to the sockets and not consistent. It varied all over the place-more so the longer the tester was on. Actually, this turned out to be a big headache to figure out. Also, learning the various quirks of this otherwise interesting tester has been a trying experience too. 

More to come.

73, Rod  WB6FBF