Stark 9-66A meter reverse issue

[stereohifi]

Hi,

i'm fixing , cleaning and repair a Stark 9-66A tube tester. Unit comes on, pilot lights, fuse lamp slightly also.
I can measure Line voltage fine to mid point where line test on meter indicates.

I can also check a tube for test like a 6L6 with the English scale, indicating good tube.
BUT my problem is when i want to measure micro mhos scales, the reading flips the meter in reverse??

I do use the inversing switch to read micro mhos reading, it shows good tube, same 6L6 used as a calibration one from Jesses Recordings.
A bit weird.

I've been checking for wiring all follows as per schematics and i can trace wires and parts like resistors, all are measuring fine as per Stark 9-66A parts list.

I wander if micro-mhos switch does feed in reverse testing voltages to the meter section?
I don't see any canibalism wiring position, anyway i can't compare to another Stark 9-66A side by side...
Otherwise unit is quite clean and rectifiers tubes 83, and 5Y3GT being swap with tested ones, no changes,

Deoxit all switches and contacts.

Ideas??
Thanks


Did some measurements, both rectifier tubes from cathode to anodes voltages:
83: 209Vdc per anodes.
5Y3gt: 170vdc par anodes.

Found R2 to be a 3,3Kohms, on schematics it is a 1,2Kohms, replaced it.

Voltages across C3, 175vdc, across R6: 91vdc.

All passive components measures fine resistors, pots.


So to resume, tube tester comes on, pilot light ok, fuse lamp lightly brite, a normal condition.

I can mesure line voltage P7 and set it to mid point on the meter.

Meter moves freely, full deflection voltage needed is 100mVdc

I can measure a 6L6 tube, filament heats up to 6,3vac on English scale goes in the good green part of the meter.

BUT switch to micromhos readings are negative voltages to the meter, i flip the reverse switch and get a very good + 7500 micromohs reading.

I've been chasing wiring, all seems in the right place, no extra long, or very short strecth wires to their connections on selectors or switchs.

Looks like to this was built faulty like that since it's construction ? 1967 is stamp on power transformer....

Now from micromhos switch, i have to reverse ingeneering wiring on how this readings becomes negative on the meter circuit.

Someone with a fine working Stark 9-66 could post pictures of wiring maybe??

I do have both schematics of 9-66 and 9-66A


Thanks.






Top

 

[stereohifi]

After more troubleshooting, i've found R17 a 41 ohms custom resistor( made of very fine 36awg or small wire) to have this small broken wire just below his soldering point.!
Had to use frontal LED camping light to see it and a lens to resolder to the contact.
So a small surgical job -kind of- to have this R17 back in circuit.
So it was a weird problem and you always have to test each custom made part.
Now it read micromhos allright within 5% of my other units.