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hugeshows hugeshows is offline
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Default Restoring a TV-7/U Tube Tester

Oh Yes, the manuals I keep talking about are he

https://www.dropbox.com/s/k984kab5v9...%2011-5083.PDF
https://www.dropbox.com/s/ykudra7u1r...625-274-12.pdf
https://www.dropbox.com/s/zt0mgdjoay...625-274-35.pdf

Thanks again to the kind RAT member who sent these to me, they are quite helpful and the service manual is actually searchable in Acrobat, which is nice. I've seen lots that aren't.

So, on to bias calibration steps...

To set up this test, you set the meter to HS5-3460. Once again, I want to remind you that this step is for the TV7/U (my tester) and when you look at the above manuals, you will see that there are different measurements and steps not to mention additional pots and resistors in various versions of this unit. So, moving forward with the TV7/U....

Get your DMM set to measure from pins 5 & 6 of the octal socket (uppermost two holes). Warm up the tester for at least an hour in the box, and perform the line adjust procedure until the meter is centered at 60 when line adjust is pressed. Let it sit for an hour, re-adjust it, and proceed to measure voltages according to the table in the manual. On the left, I show what voltage readings you should expect when the BIAS knob is turned to a certain value. You see what you expect versus what I got:

BIAS Spec. Volts Tester Before Tester After Final Tweak
22 3.00 +-.2 2.515 3.093 3.005
50 13.4 +-1.0 10.91 13.60 13.22
75 25.0 +-1.4 21.42 26.26 25.66
100 40.0 +-2.0 32.08 40.1 39.08

As you can see, before adjusting the band on variable resistor R130, the readings were completely off. Incidentally, they were taken before I replaced R123 and R125 in my last post. Now with those replaced, and having adjusted clamp A on R130 a few times, I arrived at what I feel is reasonable. Since most tubes I will be working with are tested with the BIAS knob at lower settings, the readings at 22 and 50 are more important to me than at the higher end of the scale. I realized that part of the difficulty in finding the best adjustment was because the dial was moving slightly on the knob shaft, introducing a sizable error. The final reading was taken after I had cemented that dial in place. It appears that this early knob design is prone to having some wiggle between the numeric ring and the knob itself, and I strongly recommend you fix that issue before attempting to squeeze that last little bit of calibration out of your tester. I added some CA glue where the aluminum meets the brass as seen in this picture. I put a decent amount, but was careful not to let it leak out the other side from over-application. Here's the underside of the knob:

https://www.dropbox.com/s/rvjdscejo8...Knob_Glued.jpg

Not a great shot, but you see where I added glue I hope. After it dried all play in that number dial was completely gone.

So now that we have a good set of bias readings, next step will be to measure and adjust the plate voltage.

Cheers!

-fokrinthesocket