TEK 485 -2950VDC Supply Short
 

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

TEK 485 -2950VDC Supply Short
 

"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

TEK 485 -2950VDC Supply Short
 

"MPalmquist" wrote in message link.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

-----------------------------

Mark,

You might want to also post your question to the TekScopes group, at
http://www.yahoogroups.com .

There is also a searchable archive, there, that may already have some
answers.

If you could find a cheap 485 "parts unit" scope that has a good HV
section, you could compare in-circuit measurements between them. And
then you'd also have a parts donor.

You should be able to find a 485 parts scope for less than $100, maybe
on ebay.com. Note that you can put an "autosearch" on ebay, so you'd
be automatically emailed whenever anything matching your search-spec
came up for auction. Just go to "My Ebay" and then to "Favorites".
You'd probably be wise to email the seller(s), to try to find out if
the HV section is intact and working.

Also, if you have another scope that you can use, you should be able
to rig up a "quick and dirty curve tracer", along the lines of the one
described at:

http://www.repairfaq.org/REPAIR/F_semitest.html#stqdc

Probably all you'd need would be a transformer and a couple of
resistors.

The curve tracer could also be useful for doing in-circuit
comparisons, if you found a 485 parts scope that had a good HV section
(with power OFF, of course).

Good luck.

Regards,

Tom Gootee

http://www.fullnet.com/u/tomg (Lots of good used Tek and other test
equipment)


"There are 10 types of people in the world: those who understand
binary and those who don't."

"Electricity is nice, but don't get any on you."

-----------------------------

TEK 485 -2950VDC Supply Short
 

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

TEK 485 -2950VDC Supply Short