Quote:
Originally Posted by old_tv_nut
Update: No joy yet.
I started chasing the brightness drift this afternoon The suspect resistors measure OK cold, and after the brightness has drifted, freeze spray shows no effect on any components on the video board.
Cathode voltages drift by 10-15 volts, and adjusting the brightness control for the same variation shows the same brightness variation as the drift.
I noted that the far end of the brightness control goes to a negative grid bias voltage developed by the horizontal output tube, but changing the HO tube and/or the H driver made no difference.
I backed up and measured the 385 V supply, and it is low (as low as 330) and varies with beam current. If I understand the HV regulator properly, it should keep a constant load on the HV with brightness changes and therefore the 385 supply should not vary drastically. So, the regulator may be wonky or just acting wonky because the B+ is low.
I think my next move is to replace the 5U4's in the power supply, but I don't have any at the moment.
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If the variance of the +385v supply is that great, something is wrong. On my CTC5, upon inspection before I even powered it up, the ground to the transformer center tap was not properly soldered from new! I never had the opportunity to find if it affected the B supply! But a drop to 330 volts can mean a few things eg bad 5U4 tubes, bad fuse holder of a leaky electrolytic. But if it varies with beam current that would suggest the series power supply resistance is too high: bad weak 5U4 tubes seem the most likely candidate.
The voltage sag for a 5U4 at 250mA is about 50volts per plate. The seiers resistor would be 50/0.25 = 200. Two plates in parallel son 1 100ohm 50W resistor per would work.
To test I would simply replace the two 5U4 tubes with a pair of of 2A 1500 PIV diodes.