DIYbanter - View Single Post - TEK 485 -2950VDC Supply Short

(TekMan) wrote in
om:

Jim Yanik wrote in message
...
"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.

As Jim already stated: The HV Multiplier is the most propable part.
It's a x4 Multiplier, but has a very small size. So it'S a little bit
tricky to rebuild.
So I agree with Tom: parts scope is a god idea. This should ease the
supply of replacement parts.

Ebay.com is a good place to look at. Or, if your are located in
europe, try ebay.de and the UK site.

Good luck & HTH,

Andreas

If you could figure out how to work the beam current sensing,a TEK 152-
0900-00(or -01) would probably work.It's smaller than the 485 HV mult.,has
the same anode connector.It's used in 1720/1730 video waveform monitors.

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