The raster seems folded at the left and at the top, which indicates a
problem with both the vertical and horizontal sweep.
As Mattis Lind already mentioned, the VT52 uses direct drive of the
vertical and horizontal circuitry (no oscillators to synchronize) yet
the character rows in the video are running diagonally totally out of
sync with the sweep, so I'd check the timing generator (pages 4-17 and
4-18 of the Maintenance Manual, pages 19-21 of the Schematics PDF). But
that's after checking that the power voltages are solid and ripple-free.
I'd also check a few components around the horizontal output stage, e.g.
if capacitor C44 goes bad (page 10) then the pulses from the horizontal
output will get into other parts of the circuit via the power supply
lines and/or the driver stages, affecting the logic's work.
A final note: don't keep the terminal running like that for a long time,
since the horizontal frequency is out of specs the fly-back can get
damaged (the core can saturate, increasing the current drawn; that may
also explain the noise you're hearing).
Hector.
On 4/20/2020 7:40 PM, Mattis Lind via cctalk wrote:
I looked at the video once more and paused around 0:21. It appears to be
some text there on the uppermost row.
Maybe the vertical sweep is highly non-linear. Check all capacitors in the
vertical stage. C43, C41,C40, C42, C35 and C45. Would probably be good to
measure the voltage over the current sensing resistor R73 and see if it is
a proper ramp as it should.
What about the R92 linearity potentiometer. Is it ok? Diode D31? Actually
check all semiconductors.
/Mattis
måndag 20 april 2020 skrev Mattis Lind :
Something else that might help with diagnosis is taking pictures of the
screen with known, simple elements on the screen.
e.g.:
- clear the screen
- take pic if it doesn't clear
- type a simple character like a "-"or "1"
- pic
- type a half line or full line of characters
- pic
- type different characters on two different lines
- pic
The idea being to find out, from simple known patterns, where the pixels
end up being displayed,
so it may be possible to figure out what's happening with the scan,
whether it's getting stretched, folding over, etc.
That is a good idea. Loop the terminal and create a bunch of "known
references".
Might add that it would be interesting to see one single character on a
lets say 4 different positions. Column 0, Column 20, Column 40 and end of
line. Approximately since it is hard to be exact.
/Mattis
--
Hector Peraza
BrightSpec NV/SA
Waterfront Researchpark
Galileilaan 15 (Darwin)
2845 Niel
BTW/VAT: BE 0846.102.393
http://www.brightspec.be
