Osborne monitor service information ?

[Tony Duell]

> 
> Hi Guys,
> 
> I have an Osborne-1 with a bad monitor - I have the technical
> manual, which gives schematics for the main logic board and
> the disk board, but NOT the monitor.
> 
> The monitor is a: Zenith model DS5NF3

I don't recognise the number, but maybe the IBM Portable PC monitor is 
similar in design (it is a Zenith chassis)

> 
> Symptoms are:
> 
>   Its Powered through a single +12v line on the connector,
>   filiment lights, which suggests power is present.

I would guess this CRT has a 12V (or close) heater (most 7 pin ones do) that 
runs directly off the power input line (maybe through a resistor, but 
nothing more).

> 
>   Horizontal drive, vertical drive and video signals are all
>   present on the monitor connector.
> 
>   No light at all, no "buzz" typical of flyback transformer.
 
>   Looks like horizontal/high-voltage circuit is dead.
> 
> Unfortunately I have no schematic - At least the only IC on
> the board is a 555 - hopefully no unobtainium used in this
> one (unless it's the flyback :-(

I would start by tracing the horizontal drive signal. My guess (without 
seeing it) is that the 555 is part of the vertical circuit and can be 
ignored for the moment. You may find the horizontal drive signal goes to 
the base of the drive transsitor, transformer coupled to the base of the 
horizontal output transistor, thence to the flyback.

I ahve had the horizontal drive transformer go open-circuit in a Zenith 
monitor. It's not hard to rewind, though.


> 
> anyone got diagrams or other service information on a
> "Zenith DS5NF3" monitor.
> 
> Suggestions/debug hints welcome - I'm guessing horizontal
> output transistor or flyback ...

You should be able to trst the former fairly easily. Of course if it's 
failed, there may well be other problems that killed it.

-tony