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 Post subject: Sprague TO-4 calibration
PostPosted: May Tue 20, 2008 4:33 am 

Joined: Jan Thu 01, 1970 1:00 am
Posts: 28327
Location: Detroit, MI USA
Wirewound pot R16 in the 12J5 cathode, shown as 1000 ohms 1.5 watts in the parts list, is actually a 2500 ohm wirewound pot in some units. This gets intermittent and causes meter readings to fluctuate while checking insulation resistance with button "C", even when no capacitor is connected to test.

What does that pot actually calibrate, and what is the correct procedure for adjusting it? I suspect it is for full scale reading at a specific voltage, but maybe it isn't. It can also be used to calibrate the meter scale using a high value precision resistor. When that is done, then the "set" point is no longer at the high end of the scale without readjusting the voltage control. Which way is correct?

I have the TO-4 factory manual, and the calibration instructions for the TO-5 and 6, but nothing for calibrating the 4.

Also, some TO-4's have a tapped filament winding giving 12 volts to the 1629 but only 9 volts to the 12J5, may have been done to minimize some problem they were having with that tube?


Experience is what you gain when the results aren't what you were expecting.

 Post subject: Re: Sprague TO-4 calibration
PostPosted: Sep Mon 13, 2021 5:07 pm 
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Joined: Feb Mon 22, 2021 1:29 am
Posts: 105
Location: Castro Valley CA
If there is a prize for responding to the oldest unanswered post, I am applying for it. After probably 40+ years of great service I thought it was time my TO-4 deserved some attention. While it is actually fairly accurate at reading capacitances, I really only use it for checking electrolytic leakage and insulation resistance (wax paper caps etc.). Sometimes I like to assess the operating condition of a radio before “warming up the soldering iron” and the TO-4 keeps me out of capacitor trouble and the collateral damage capacitors can cause.
After my work on the TO-4 I wanted to check the insulation resistance and electrolytic leakage calibration so to the WWW I go; but I did not really find anything specific. The operator’s manual is quite good so using that I was able to understand what I needed to do to calibrate the TO-4. Attached is the procedure I used to calibrate the Electrolytic Leakage and Insulation Resistance functions of the TO-4.
Please understand safety notes are not meant to offend anyone or question their abilities. We all have different skill levels and I would not want anyone to be injured.
TO-4 Electrolytic Leakage_IR Calibration.pdf [75.81 KiB]
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