8568A No display, Both INSTR CHECK LED off

[Thirtykay]

Hello everyone,

I have recently acquired an HP 8568A and his 85662 IF/Display. As usual with AS-IS item/Found in a trash item, they are not working.

I have no CRT activity and both INSTR CHECK LED off, keyboard inoperative after start up, so not an easy failure.

What i did so far:

Checked both RF/IF Section supplies - OK
Checked 50-wire interconnect cable - OK
Checked keyboard while startup - OK
Ran HP-IB Test - OK
Ran LONG-POP Test - OK
Ran Digital storage Test - NOK, No test pattern but INSTR CHECK LED left on, as expected.
Hooked an external XY monitor - OK but it seems to lack some data
Checked CRT Filament resistance - 19 Ohms, OK
Checked CRT Voltage on A1A3TP1AB - 0V , NOK
Checked +18V on A1A6TP100-5 - +18.12V, OK
Checked DRIVE on A1A6TP100-4 - No oscillation, NOK
Checked CRT DSBL at 0V on A1A6 - 4.5V, CRT is disabled, NOK
Visual check of A4A9 Board and U6 in charge of CRT DSBL signal - OK

Now i can say my HV Power supply and my CRT are good, they are just disabled by the RF Section but why ?

Like i said display is working on an external XY display, but it lacks quite a lot of data, see attachments.

Have anyone encountered this problem before ?

In the waiting for your kind answers, i'm going back to the service manual !

Best regards,
May the force be with you.

[Thirtykay]

Having real trouble to narrow down the failure, here's what i found recently:

By shunting CRT Disable line, the CRT indeed start, so definitely no power supply issues.

I did a display test pattern, it's ok so far.

I also did a reseat of all the A3 Board. Gave me nothing.

And still the keyboard is inactive after selftest.

Now i dont where to go next, if someone can give me an advice i would thank him a lot.

[m k]

My guess is some sort of a memory problem.

It's like an occasional dot has an incorrect address.
Not very typical that a dot has an address, but that's how it looks to be.

The dead center of wrong crossing may be just a bad coincidence.