20 Apr
2020
20 Apr
'20
4:22 a.m.
m?ndag 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
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.
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?
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
C45 and C42 could be useful to check.
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?
So do I. And now we need Chris to do some measurements on it to get
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
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)
C