Need Help with Seco Model 107 Tube Tester

[Garry777]

In the last few days I received a Vintage Seco 107 Tube Tester.

I was able to test a 6AU6 [per page 1 of the Seco "Operating Instructions"] however the "magic eye" cannot be adjusted with the pot control, and it is always closed [that is full illumination].

I check the B+ voltage on the 10 mfd cap on the [right half- meter side] of the tester and that reads 150 vdc with only about 1.5 vac ripple.

I checked the B+ voltage on the 10 mfd cap on the [left side -socket side] of the tester and it reads about 113 vdc [is that too low ?].

I tested the Seco's 12AU7 on my other tube tester and it seems fine. By the way, the Seco's 12AU7 is a Telefunken [probably from the late 50's]

I have not tested the "Magic Eye" tube yet [6AF6G] because it looks like more work to get it out of the tester, but will do that soon.

Does anyone know what might be wrong ?

 

[Garry777]

I have tested the 6AF6G "magic-eye" tube and it's good.

 

[Tom Bavis]

I have a SECO 107, but it's always worked fine - I've never looked inside. The schematic is not the greatest, but it looks like there are a number of high-value resistors that sometimes fail open.

 

[Garry777]

I've taken the following voltage readings in the affected circuit area:
[the magic-eye control pot was set to mid-scale]
The magic eye is always fully closed [full illumination].
All tube tester controls were set to standard default locations.


12AU7 Cath pin 8 ... 0.132 volts dc
12AU7 Grid pin 7 ... -1.14 volts dc
12AU7 Plate pin 6 ... 9.06 vols dc
12AU7 Cath pin 3 ... 9.06 volts dc
12AU7 Grid pin 2 ... 0.13 volts dc
12AU7 Plate pin 1 ... 102 vols dc
6AF6 Control Grid pin 3 .... 102 volts dc

 

[Garry777]

Thanks Tom Bavis,

I found a couple of [really old style] 100K ohm resistors that are out of spec.

One was in the divider that sets the 12au7 grid bias on pin 2 . That one was "open" to my Fluke 77 which means it's >32 Meg ohms.

Second 100K was 142K ... I'm going to replace them both and see what happens.

 

[Garry777]

Replacing the two 100K Ohm resistors fixed the Magic-Eye problem.