Hello,

?? Am I a glutton for punishment, or what?? Having completed my restoration of a 410B, I happened on a fairly rough 410C at the flea market for $15.? Seller said it worked.?? Actually, not quite.? No surprise there.?? But it just needed some TLC for the? switches and pots.

? Not getting good waveforms out of the optical chopper.? The manuals on the Net show one pot to adjust the chop
rate;? my instrument has two - one in series with the +270V, the other in series with the resistor to ground.? I'm
guessing that the upper 40K pot is the "main" adjustment, and the lower one - a 20 ohm wirewound - is the "fine tune" guy.

?? I stumbled on a youtube video by "curiousMark" where he repairs a 410C with a dead neon bulb in the chopper.? He went into very good detail on how it works, took it apart and replaced at least one bulb - probably two.??? Then he
tweaked the capacitor and the resistors to get it oscillating at the required 100Hz.? In experimenting, he noted that the
higher you ran the B+,? the harder it was to get it oscillating properly.

?? So - high B+.? When this instrument was new, the specified voltage was 115VAC.? Nowadays, the wall sockets
around here deliver 121VAC.? So all the B+ voltages are high.? The nominal +270V measures +301V.

? Looking from the outside world, the instrument works fine.? It zeros and calibrates.

Whups - I looked back in the manual at the conditions for the published waveforms.? Basically you set it to the lowest range, and then input a gross overload of DC.? Basically, input .5VDC on the .015V scale.

Once I did that, the waveforms were perfect.? Plugged the probe into my counter and easily adjusted the chop
rate to 100Hz.