Hickok I-177B tester help!

[TV7]

Gday from Australia, this is my first time posting here.

I purchased an exceptional I-177B tester not long ago and am getting an imbalance in the readings. For instance a 6SN7 was tested which I know has identical mu between the triode sections, however the tester shows the second section as around 400 micro ohms less than the first. This happens with all tubes tested this way.

I used the 949 adapter box to test both sections independently whilst keeping the switch settings the same and the results are perfect.

I calibrated the unit the other day, and before doing this replaced the type 83 and 5y3 tubes with NOS JAN tubes.

Calibration was almost perfect, with only the right POT (bias) needing the smallest adjustment. This amazes me seeing as the unit is around 50+ years old and all original components inside.

So the only thing I can think is causing the reading imbalance is maybe a slightly higher resistance path in the switch settings, I could use the adapter box to get around this however it takes much more effort to test many tubes.

Would anyone have a clue as to what's happening? I would appreciate any help available.

As can be seen from the photo the tester is in excellent condition, a real find.

Thanks.

[radiostar]

Do you have problem with measurement only with double triodes or also with single tube?

[TV7]

Single tubes are ok, like the VT137/1626.

This seems to be because on one side of the tube socket, signal voltages are not what they should be.

Switch setting 10/4 gives testing on one half of say a 6SN7, and 4/1 tests the other half.

On setting 10/4 all voltages are correct, anode current is supplied when pressing the AMP test button and the grid signal is a nice 5 volts AC. Single tubes like the 1626 use this rail.

However the switch setting for 1/4 has a much lower grid voltage when measured between the grid and cathode, around 3.3 Volts AC. This has the effect of making the reading about 400 mus higher.

I have spent 20+ hours trying to find a possible HR somewhere in the many wiper switches, have pulled the unit apart and traced cables and it almost seems like the problem is not the signal rail but the cathode for PIN 1 on the G octal.
This would have to common up with the signal voltage somewhere but it's quite possibly the hardest fault (if it is even a fault) I have yet to find the cause of.

I know this particular 6SN7 is matched, I even made connections up to test both sides on switch setting 10/4 and they were perfect.

Thanks for your reply, I'll keep scratching my head.