21 Apr
2020
21 Apr
'20
7:57 a.m.
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
m?ndag 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
>