Great, my VT52 is shot.
cz at alembic.crystel.com
Tue Apr 21 09:57:52 CDT 2020
Well I'm starting to walk through this. First I took an IR picture of
the boards in operation, then started troubleshooting.
First I started checking voltages. J2 is easily accessible so I put a
ground probe on pin 10, and checked voltages as follows:
J2-3 Should be -5v, reading -3.8v
J2-2 Should be -12v reading -15v
J2-4 Should be 5v, reading 5.04v
J1-10 Should be 15v reading 14.84
Hm. That's odd. Looks like the negative voltages are a bit off.
According to the schematic the key transistor on the 5v rail is Q6,
which is a 2n3055. The key transistor on +15 is Q10 and that circuit
looks ok. However whatever is running the -5 and -12 volts is not
working right.... Looks like Q12 is the key power transistor there with
E2 serving as the control.
On 4/20/2020 1:40 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.
> måndag 20 april 2020 skrev Mattis Lind <mattislind at gmail.com>:
>>> Something else that might help with diagnosis is taking pictures of the
>>> screen with known, simple elements on the screen.
>>> - 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
>> 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.
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