Great, my VT52 is shot.

[Chris Zach]

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.

Hm....

C




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.
> 
> /Mattis
> 
> 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.
>>> 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
>>