Great, my VT52 is shot.

[Mattis Lind]

måndag 20 april 2020 skrev Chris Zach via cctalk <cctalk at classiccmp.org>:

> As I wrote earlier in the thread I think it is a good idea to check the V
>> sync and H sync signals to check that they are right in pulse length and
>> shape.
>>
>
> Good starting point. From the schematics would that be scope probe to J1
> pin 5 for horizontal and j1 pin 9 for vertical? Also which is J1, and how
> are they numbered :-) Also is there a good ground reference point?


I have a picture of the connector on my page. Have a look there. The
connector have the standard color codes to map to numbers. Black for zero,
Brown for one. Etc.


>
> Also is there a document that links the part numbers on the boards with
> what's on the schematics?


No DEC document. When I did my repair I mapped all ICs to the board but not
the connectors, sorry. You can find all here. Including links to schematis
and maintenance manual: http://www.datormuseum.se/home/dec-vt5x


> There are indeed electrolytic capacitors in the vertical deflection circuit
>> but I am not sure if that would make the beam move much faster since for a
>> faster move of the beam also requires a higher voltage over the deflection
>> coil to create a faster ramp up of the current trough it. But it is
>> definitely worth checking. Especially since the scan lines are sloping a
>> bit and not straight as one would expect.
>>
>
> I would believe a capacitor failure due to heat, any ones in particular I
> should check?


C45 and C42 could be useful to check.


>
> So do I. And now we need Chris to do some measurements on it to get
>> further.
>>
>
> More than happy, just haven't worked on TV sets before and would rather
> not blow off my hand. I'm guessing the really dangerous voltage is the one
> on the bottom left that comes off the step up transformer over to the odd
> plug going into the VT52 rear bulkhead (HV to display)


What you could do is what I did. To remove all control board on the bottom
and lay it out flat on the table. Make a small circuit that create a
composite video signal and feed it into a standard composite video monitor
or TV set. If you remove a jumper to th CRT logic there will be no HV at
all generated.

/Mattis


>
> C
>