C.Itoh CIT-80 no high voltage to CRT

[FrHilderbrand]

Merry (belated) Christmas! I have a C.Itoh CIT-80 terminal, which I understand is a DEC VT100-compatible device (and that's why I'm posting it here). I can't find a maintenance manual for the device. The CRT doesn't turn on and there is no power to the -28V, +12V, or -12V rails on the mainboard. The +5V rail and the keyboard power rail look fine, and the NEC 8085AC processor seems to be working well. Caps throughout the machine look fine. There's a diode on the board with the flyback transformer that looks a little crunchy (pictured), but I don't know how to (i) determine what kind of diode it is or (ii) determine what diode, if any, would constitute a drop-in replacement.

Does anyone have access to anything like a repair manual or at parts list for this device? I'm kinda stuck on what to check for and I don't want to rush in and replace parts willy-nilly. Any resources you could point me to would be most appreciated! And if I need to provide more info, please let me know.

 

[MauriceH]

I haven't found any schematics , but all those monitors have basic same setup circuits.
Isn't there a video out connector on that CPU board?
Then just unplug power to that monitor and see if cpu it self is running and all power is back to normal on that supply.
Or with a scope see if there is video out to that monitor pcb.
There must be signals Horizontal and vertical drive signals also.
With that horizontal drive signal that high voltage transformer is driven.

See VT100 , MP00633_VT100_Mar80.pdf at bitsavers.
As C.itho is kind of a copy indeed

Then indeed those high frequent switch diode are know to corrode on those leads.
Think if you touch it , you can push that round easy out of its leads.
Just replace with any high frequent switching diode and enough current.

Try to find any C.itoh manuals of same age, it should be simular in setup.I think.

 

[Hugo Holden]

Some of the early Sinterglass diodes had a problem. Corrosion of the leads results in volume expansion of the lead surface and it can fracture the glass and the diode falls apart.

You can replace that diode (if it is defective) with a BY228, this diode also has the large diameter lead wires.

www.vishay.com/docs/86003/by228.pdf

For smaller versions of this diode with the thinner lead wires, use a BY448.

(I just happen to know this from decades of VDU repairs, you might not find it documented elsewhere, but you can trust the advice, one reason is very few if any of the earlier sinterglass diodes outspec'd the BY448 and the higher current BY228)

 

[daver2]

And quite a few people on VCFED have found these diodes to just "fall apart" when attempting repairs to monitors and/or power supplies that use them.

Dave