Re: Waking a slumbering 475


I can confirm as I have one of the variac, dim bulb, and isolation supplies in the lab the purpose for this is REALLY old tube stuff. It for protecting the power transformer which might be unobtanium. It is also to protect the rectifier tubes in the supplies. If the main filter caps are bad dry or shorted, you can blow the cathode bonding wire off the rectifier tubes (Over current trying to charge the bulk filters). The bulbs are in series with the load and take advantage of the dynamic resistance of the light bulbs cold the bulbs are almost a short. Hot they go high in resistance and limit the current. I can confirm that the 7603 series don’t like dim bulb testers even with 3 amps available with the scope. 6 X 60 Watt bulbs. The scope oscillated trying to start up. But for more modern supplies I bring it up around 115V 120V and watch the current. If it seems to draw excessive current, I can shut the DUT down before damage is done. I know the numbers on a 576 but recapping the power supply in a 576 dropped the power consumption at idle by 10 Watts. This was a VERY early production unit though.


-----Original Message-----
From: <> On Behalf Of Jeff Dutky
Sent: Friday, February 12, 2021 5:40 PM
Subject: Re: [TekScopes] Waking a slumbering 475


I think that the advice to use a variac came from a tradition of servicing tube radios and TVs. I have a variac that belonged to my grandfather, who dabbled in television repair at one point. He loaned it to me, along with a signal generator, a resistance-capacitance bridge, and a tube tester, when I was trying to refurbish a 1949 Admiral TV. It was a long time ago, but I think that he advised me to bring the TV up slowly on the variac to give the tubes time to heat up slowly. I have since gotten the advice to use the variac in on-line communities, and seen it repeated in YouTube videos, the later by people who work on old tube radios and TVs. Those same YouTube videos show the use of incandescent bulbs to indicate and limit high currents.

Sadly I can not find an archive of the sci.electronics newsgroup from 2003, or I would be able to produce the exact correspondence that resulted in the advice to use the variac. I'm a little surprised that I am not able to find such an archive, or my specific messages therein: my name is unique, and it should only occur a very small number of times in relation to that newsgroup and the word "Tektronix"

-- Jeff Dutky

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