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 Post subject: Precision 612 calibration question
PostPosted: Dec Tue 05, 2017 11:51 pm 
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Joined: Nov Wed 22, 2017 6:01 pm
Posts: 10
Hi:

I just retrieved my father's Precision Apparatus 612 tube tester from the attic. I would like to be able to use it in restoring some AA5's, and it has not been used in 30 plus years. Following the calibration instructions I found on this site, I opened it up, replaced the .1 mfd cap, and carefully lubed and worked the R5 pot.

I turned it on, warmed it up, and centered the meter needle.

Per the instructions, I placed my DVM probe on the 5Y3 and poked around the battery selector switch until I found 300 plus volts. I couldn't figure out where R13 was, but it is supposed to be in the battery test switch section, so I figured I was close. I then backed off the needle adjust pot until I had exactly 300 volts on the DVM.

Now comes the question. According to the calibration instructions, I am to adjust R5 until the meter needle is centered. Unfortunately, no matter which way or how much I turn R5, the needle does not move. It will move with the needle adjust pot, however, then my voltage is about 315 or so with the needle centered.

Any suggestions?

Thank you.


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 Post subject: Re: Precision 612 calibration question
PostPosted: Dec Wed 06, 2017 8:33 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2053
Location: New Port Richey, FL, USA
I'm not sure which calibration instructions you have, so I might be giving you info you have, but mine says:

Now, set the Line Adjust control for exactly 300 volts on your voltmeter and adjust R-5 until the meter needle on the 612 is centered over the line adjust mark. You will probably find it necessary to work back and forth between the Line Adjustment control and R5 until you have accomplished the following goals:

a) The meter needle must be centered over the "Line Adjust" mark
b) Your voltmeter must indicate 300 volts
c) There must be no hum from the transformer
d) The pilot lamp must not appear to be overly bright
A too-bright lamp and/or humming noise from the transformer means you need to back off on the Line Adjustment and then re-adjust R5 to center the needle. We have now completed the first part of our calibration procedure.

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 Post subject: Re: Precision 612 calibration question
PostPosted: Dec Wed 06, 2017 5:24 pm 
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Joined: Nov Wed 22, 2017 6:01 pm
Posts: 10
Yes, those are the same calibration instructions.

However, adjusting R5 has absolutely no effect on the needle position. The line adjust pot will move the needle easily, but of course, it is varying the voltage. To get the needle centered over the line, the voltage exceeds 300. 300 volts has the needle left of the center line.

My question is what is causing the lack of adjustment at R5?


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 Post subject: Re: Precision 612 calibration question
PostPosted: Dec Thu 07, 2017 6:59 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2053
Location: New Port Richey, FL, USA
My first guess would be that R5 is shorted. Could be something loose or broken inside or possibly has some crap inside that needs cleaned out. I'm not sure if you have the schematic, but one end of R5 goes to a plate of the 5Y3 tube, so when it's off, you can just put your ohms meter on R5 from the wiper to the end by R4 and see if you get a resistance reading or continuity. It's a 6K pot.

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 Post subject: Re: Precision 612 calibration question
PostPosted: Dec Thu 07, 2017 6:02 pm 
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Joined: Nov Wed 22, 2017 6:01 pm
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TPAairman:

Thank you for the reply. I will do that and report back.


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