Some time ago I made a previous post about a 577 D1 with a collector supply issue being partially stuck on despite the variable voltage control parked all the way at the stop. Ever since then I haven't touched that instrument and it is temporarily in storage for a few weeks due to house remodeling.
In AC mode at 6.5V a horizontal line appears 100mV long which cannot be further reduced by the variable collector supply parked at the stop. Raising the collector voltage aggravates the issue in a magnitude that relates to each greater voltage increment I can set. If I short the emitter and base on the fixture the collector supply then goes to 0V.
I find this problem to be only particular in AC, any other setting such as ramped DC or rectified DC the collector goes to 0V or a spot on the display
I checked for any presence of voltage and current on the test fixture at 6.5V AC, I saw about 40mV AC RMS with a voltmeter and about 0.6 mA of current with a 100 ohm resistor shorted across E and C
I tried to park the variac wiper control as far back as I could, improving the 100mV to 10mV big improvement, unfortunately the wiper was off the winding, it would stick, and voltage control was not smooth, the entire wiper has been reset where it is parked entirely over the the beginning of the winding 100mV present again
I tried modifying the brush of the variac wiper to form more of a point instead of the entire area being flat but with no improvement, I was cautioned for doing this so I reformed the brush back to its flat surface. After this point I left the instrument alone and now it has been in storage for about a week, with about 2 more weeks to remain before I can pull it out again
I have been notified of other 577 users who report their collector supply can reach 0V easily (spot on display) at 6.5V which suggests to me I have unique yet minor problem with this instrument. I found the residual voltage seems to originate from the variac after probing it with a voltmeter, wide open it reads 120V and fully off reads about 11V, I will need to recheck this measurement again after I realized I measured to ground and not to neutral from the wiper. No buzzing, burning smells, or visual signs of burning up from the variac. This is weird and I been thinking about it ever since.
Could jumping a clip lead from Neutral to the wiper with the brush insulated from the windings be useful to diagnose why the ollector will not go to 0V or near it? My theory is if this causes the collector to go to 0V or near it then perhaps I have enough resistance occurring from the neutral terminal to the first windings of the variac causing the collector issue I am experiencing, would you agree?
Thanks and happy new year!