I'm looking for a recommendation on what to use as
a replacement for tek's 152-0242-00 diode as used in the dc restore circuit of
the 485. Initial symptoms with this scope were no visible beam, even when the
beam finder was depressed, and the cathode supply voltage was running at -2300
volts. The dc restore diodes were suspected so I substituted a spare horizontal
board from another 485 and cathode supply voltage jumped back up to -3000 volts
and I had good intensity control. I replaced the diodes
(CR1660,1666,1643,1646,1652,1651) with MUR120's. Now the original board has
cathode supply potential at -3000 volts but the beam intensity is full on and
neither intensity control, or grid bias or astigmatism controls have any effect
on the beam intensity. The 485 parts listing shows these diodes (ND341) as 225
volt, 200mA units. The forward voltage drop reads 2.3 volts. I suspect these
diodes have stacked junctions to limit the capacitance. That being said the next
thought is to try two series connected 1N4148's as a replacement (now 2pF vs the
MUR120 at 45pF). Does anyone in the group have experience with these or have a
recommendation for replacements?
?
Phil
