? ? ?My meter has a DC 1000V range, but I have seen readings today go up to 1900V. I think I figured that the sparking during measurement was due to my bouncing fingertips when approaching the?junction to be tested. I switched to alligator probes that I connected before turning power on, and haven't had any sparking or blown fuses since.
? ? ?This evening's measurements:
? ? ?* D611 and D612 get warm, but not hot.
? ? ?* After I lifted the blue T620 primary lead, I measured infinite resistance to ground.
? ? ?* The +500V rail immediately goes to +1100, +1900V, then beyond range, when power is switched on. The voltage stays high for a long time after power is shut off.
QUESTION: ?Is this *immediately*, before V620 has had a chance to warm up? ? It should not read that high.?
? ? ?* When I disconnected the 3kV wire from the end of the diode tube, the +500V rail swung back and forth from +420V to 550V.
This is puzzling, but I now see a problem with my earlier logic about the 3KV leaking to the 500V side. ?It's MINUS 3KV.
I think we need to know that your voltmeter is working properly and that we trust it. ?
At least the T620 resistances you reported earlier are reasonably consistent. ?I expect the low ohms readings to be off. ?Mine were measured with a 2-terminal meter and may just as well be low. ?The point is the resistance values are roughly proportional to the number of turns in the secondary coil of T620 and the 200 Ohms is close to nominal. ? T620 may be
ok. ?
? Try removing V620. ?The 500V rail should read 2x 190V RMS / 0.707 = 537V or so. ?D611 and D612 double the peak voltage on the T601 secondary coil which shows 190V RMS on the schematic. ?If you get much more than this, suspect the meter. ?As pointed out, the voltage should decay with a time constant determined by C611/612 and R611/612, ?so should fall by about 1/3 in 19 seconds. ?The measured resistance of the primary of T621 to ground is correct at about 300 Kohms so we know R611/612 are good. ? The decay time constant are a rough measure of C611/C612.
While you are at it, make sure you have disconnected the -3KV from the CRT filament winding, NOT from the CRT cathode. ?Then measure the CRT filament winding resistance to ground on a high-ohms scale (T601 pins 6, 9). ??
? Resistance of the -3KV line to ground, just as a double-check, should be about 4 MegOhms.
Dan
