U-Test-M Tube Tester Model 2400 Schematics or Manual

[Jecarroll7]

I own several tube testers but I'm currently restoring a U-Test-M Model 2400 and looking for a schematic, calibration instructions or repair manuals for any of the U-Test-M Mfg Co. tube testers or the U-Check-E'm Tube Testers.

Not a great tester but it just looks cool in the shop.

This unit is similar to mine without the base. https://www.ebay.com/itm/Vintage-U-T...oAAOSwMpxc~~i9

Any help would be appreciated.

[nosaj]

Probably not any calibration instructions. As crazy as the backside looks, if you can ignore the overwhelming rats nest of wires. Focus on the PT to find out what voltages you have.
Though it might look cool it's not likely to get much use.
I basically only use my tester for shorts test on unknown or questionable tubes that I don't want to test in an amp.
nosaj

845 for a tube tester is for fools. I paid a 120 for a jackson 648 which covers everything and closes up and sits under a bench out of the way. But then I'm not into ooohs and ahhs and rarely let people into my workspace.

[Jecarroll7]

Yea, I didn't pay anywhere near that either. Its somewhat working now, tested a 12BE6 right in the range as long as I held the test switch in the right place. Biggest issue right now is cleaning the switches, last maintenance stamp was 1974 and those switches have seen some use. From there I thought I would move to testing filament voltages etc. There are a few guys trying to restore these so I hope we can find the info.

[nosaj]

Did you check inside for a schematic? Jackson put theirs inside the tester.
nosaj

[Jecarroll7]

I've heard in some units the schematic was glued to an inside panel, my model 2400 didn't have one.

[blanchae]

Just got through troubleshooting a U-Test-M Model 2500 tube tester with Ron D who was restoring it. We met after Ron posted a plea for help on Facebook Marketplace after running into a brick wall troubleshooting for the past 3 months. The problem was a bad solder joint on the daughter board and Selector A knob was off by one position. Instead of selecting position 2, it was selecting position 1. It wasn't as easy as it sounds to solve - it took about a week of banging our heads against the wall.

[blanchae]

Schematic of CMTX-A1 daughter board

[blanchae]

Components diode D1, C1, R3 and 18V Zener D2 make up the regulated power supply. Roughly 30 Vac come in on Pin 11 (Orange wire on edge connector). D1 rectifies the positive cycle, C1 filters it. I changed C1 to a 220 ufd 63V electrolytic from the existing cap because I had one handy and the original cap (no markings) looked sketchy. It's a power filter so any value above 100 ufd should work. Resistor R3 and Zener D2 work as a shunt voltage regulator. We measured 18.6 Vdc where it's marked Vcc on the following schematic.

[blanchae]

We found a bad solder joint at R2 where it connected to the board edge connector. It looked fine but when we measured it there was 2 Vdc difference from the trace to the solder joint.

[blanchae]

After fixing the bad solder joint, we checked the tester for the standard checks - bad or burnt components, broken wires, and found nothing wrong. We had a known good 12AX7 tube and placed it in the tester, set the switch positions to 2H7 (selector's A, B and C positions) and no response on the meter. I was wondering how do we even know that the switches were physically set to the correct position so Ron said go ahead and turn one. I turned selector A from position 2 to position 1 and the tester worked!

Looking at the back of Selector A or B switches, starting at the common contact, position 1 is one contact counter clockwise, then position 2 and so on. Selector A had the knob off by one position. We verified with the second half of the 12AX7 which calls for 7H7 and switched it to 6H7 and it worked again. We the used our reference tube 6BA6, again offsetting Selector A by one position from what the chart called for and it worked! We were puzzled because the selector switches had a flat spot on the knob shaft and this shouldn't have happened. Removed the knob, re-installed making sure it was pointing to the right location and everything worked! I guess there is just enough play on the flat spot to move one position over.

Rudimentary calibration (our best guess)

1. Verify that there is approximately 18.6 Vdc present at Zener D2. It is 1973 technology so if it is close, it is okay.

2. With no vacuum tube present, measure the voltage between TP1 and TP2 adjust the meter Null Adjustment potentiometer P5 for 0.00 Vdc.

3. Place known working tube or reference tube. We had a 6BA6 for reference that we knew measured 95%. We adjusted P4 Meter Gain and it didn't quite have enough juice to move the meter to 95%, I returned P4 to the middle of its play and then adjusted P3 Tube Calibration. By adjusting both pots I was able to get the meter to move to 95%. Checked the known 12AX7 tubes and both triodes showed 97%. In the next couple of days, Ron is going to check known bad tubes to verify that it can detect bad tubes also.

Notes:

* Q1 and Q2 are JFETs 2N5457, I tried continuity in circuitry testing with a digital ohm meter and it didn't matter where I measured, I measured roughly 400 to 500 ohms. Theory said that the Drain-Source junction should measure high Z but that wasn't the case. It was very frustrating as we couldn't figure out if they were working or not. We didn't want to remove them to test at this point. JFETs are transconductance devices and from what I understand a voltage on the Gate changes the resistance on the Drain-Source junction.

* Q3 is a NPN transistor with the Collector-Base tied together to make a diode. Why? The only reasons that I can think of is to lower the diode junction forward voltage Vf to match the JFETs? Or maybe temperature characteristic matching with the JFETs? I remember back in the 1970s that semiconductor thermal run-away was a big thing.

* It appears that all of the heater voltages to the sockets are pre-wired and not controlled by the selector switches. The selector switches control the grid, anode and cathode.

* Lastly dogs are faster in Greenland because the trees are farther apart.

[blanchae]

For Reference:

With a known 12AX7 installed and selector positions set to 2H7, we measured the following on the board's edge connector:

Pin 1: Grounded to chassis
Pin 2: 6.5 Vdc
Pin 3: 1.2Vdc
Pin 4: 0.94 Vdc
Pin 5: 1.5 Vdc
Pin 6: 6.3 Vdc
Pin 7: 0.0 Vdc
Pin 8: 29 Vac
Pin 9: 16.9 Vdc
Pin 10: 1.0 Vdc
Pin 11: 30 Vac
Pin 12: 5.7 Vdc (adjusted by potentiometer P1)