Beltron 8081A Caution

[jeyurkon]

I purchased a Beltron 8081A on Ebay. The filament adjustment was way too sensitive going to maximum with just a small turn of the pot. I looked at the circuit and it uses a LM317 voltage regulator as a filament supply. The pot was a 20k and I figured a 4k would be more appropriate. When I replaced the pot I saw that the shaft had been cut confirming my suspicion that it was a replacement of the wrong value. I put in a 3.5k and now it works as I expect taking the full range of the pot to get maximum output.

The previous owner seems to have been a heavy smoker. The 8081A smells stronger than a sports bar. I suspect this killed the original pot.

This is where the warning comes in. Because of the design of the LM317 circuit the output will go to it's maximum if the wiper of the pot gets dirty and loses contact. It puts more than 20volts on your 6.3volt filament with predictable results. Given the low usage I'll put it too I don't expect to have this happen to me. But if you get a used one I'd recommend checking it out thoroughly before hooking up a CRT.

The circuit does have the advantage of keeping the filament voltage constant as the CRT warms up.

The 8080 design with it's rheostat is a safer version even if it's not as modern.

John Y.