Tektronix 2445 HV discharges on A9

[Hunter Gratzner]

I am scratching my head over an old Tek 2445.

It might work for hours without problems, then it might do a few HV
discharges in the A9 high-voltage assembly and then it continue to
work. Or it doesn't and just continues with a rapid series of these
discharges. With every discharge the CRT goes dark for a moment. This
of course I understand.

It is A9 making the trouble, because I removed the cabinet and I can
hear static building up and then I can see a flash somewhere on the HV
side under the A9 board shield when the discharge happens. And I hear
the discharge. I can't see exactly where the flash happens, because of
the board shield. However, when removing the board shield I can still
hear static building up, but no discharge happens any more. So I have
no clue where the problem is.

After removing the A9 board I did a thorough (at least I think so)
visual inspection for burn marks, broken insulation of the anode
cable, burned traces on both PCB sides, cracks in the HV multiplier's
case or any of the other compontens on the HV side of the multiplier.
Noting. Everything looks fine.

I checked the board shield and the chassis for marks, too. Nothing. I
checked the resistors on the HV side of the PCB. All have their
nominal values. I checked the capacitors for shorts. Non. I have no
clue what is going on.

Before I start to gamble and hunt for a replacement multiplier or a
complete A9 board, is there anything else I could check? Any other
ideas?

Hunter

 

[JW]

I am scratching my head over an old Tek 2445.

It might work for hours without problems, then it might do a few HV
discharges in the A9 high-voltage assembly and then it continue to
work. Or it doesn't and just continues with a rapid series of these
discharges. With every discharge the CRT goes dark for a moment. This
of course I understand.

It is A9 making the trouble, because I removed the cabinet and I can
hear static building up and then I can see a flash somewhere on the HV
side under the A9 board shield when the discharge happens. And I hear
the discharge. I can't see exactly where the flash happens, because of
the board shield. However, when removing the board shield I can still
hear static building up, but no discharge happens any more. So I have
no clue where the problem is.

After removing the A9 board I did a thorough (at least I think so)
visual inspection for burn marks, broken insulation of the anode
cable, burned traces on both PCB sides, cracks in the HV multiplier's
case or any of the other compontens on the HV side of the multiplier.
Noting. Everything looks fine.

I checked the board shield and the chassis for marks, too. Nothing. I
checked the resistors on the HV side of the PCB. All have their
nominal values. I checked the capacitors for shorts. Non. I have no
clue what is going on.

Before I start to gamble and hunt for a replacement multiplier or a
complete A9 board, is there anything else I could check? Any other
ideas?

I'd try to verify that the -1900V regulation (U1956 etc.) circuit is
functioning correctly.

 

[Sarason]

I'd try to verify that the -1900V regulation (U1956 etc.) circuit is
functioning correctly.

Sounds like the break is in the epoxy underneath the Multiplier. A not
uncommon problem. As the epoxy dries out and moves with the heat/cool
cycles, it cracks, which then leads to sparks.

regards Andrew