"MPalmquist" wrote in
ink.net:

I have a tektronix 485 with power supply that shuts down because of a
fault on the balance node of the controller IC. This is a common fault
that appears often on threads in this NG.

The manual suggests opening pin 3 of comb "U" to isolate the -2950VDC
supply. When I do this, the inverter runs OK, the scope draws about
0.8 amps, and all of the LV power supplies are at the correct
voltages. (The fan doesn't run, however). So I have to find the short
in the HV supply. Dug into the scope and could find no shorted caps or
on the transformer board buried in the middle. The 15k resistor
feeding the HV block was a bit toasty and measured 33k so replaced it.
No change in the performance. Caps on the inverter board check out OK
too. Is there a way to test the HV multiplier block for shorts? Any
suggestions on how to test the two 12,000 volt diodes on the xfmr
board? Anyone ever sucessfully dig into the HV block or try to replace
it with a multiplier from another source, like a discarded computer
monitor?

Any help greatly appreciated.

Mark Palmquist




HV multiplier failures were common in the 485. The only reliable way to
test the HV caps is to run them up on a curve tracer such as a 576 or
577,same for the HV diodes.

The parts list should tell what the multiplication is for the 485 HV
multiplier,probably 4X. You might be able to build one yourself,but careful
attention of HV layout and insulation(potting) is very important. Any
substitute will not have the HV Anode connector,unless you can fit another
TEK HV multiplier in there.

--
Jim Yanik,NRA member
jyanik-at-kua.net