29 Apr
2017
29 Apr
'17
7:42 a.m.
On Sat, Apr 29, 2017 at 2:34 PM, Aaron Jackson <aaron at aaronsplace.co.uk> wrote:
The heater and control pins appear to be giving
sensible voltages. The
logic of the board is fine, I can type on the terminals keyboard and I
get the correct characters on the other end of the serial cable. The PSU
is putting out 31v which seems fine?
Err, the PSU has 3 voltage outputs, +5V, +12V, -12V. I am not sure where you
are measuring 31V between, but that doesn't sound 'fine' to me.
On the other hand, if the +12V rail was 31V the CRT heater would be burnt
out. If the +5V was 31V then the logic ICs would be totally fried so it wouldn't
respond to the keyboard.
So I suspect a measurement error....
I'm used to discharging the tube before fixing things inside CRTs
(usually I only attempt to fix simple things like a broken toggle
switch), but I have not managed to get a spark off this monitor. The
tube doesn't seem to get charged up at all. I've measured the
capacitance of nearly all caps and they seem fine, diodes seem to be
working fine. Does this mean it is most likely the flyback transformer?
According to the schematic of the monitor section (p16 of the 17 page
.pdf file I have) there is a bleeder (discharging) resistor between the EHT
output and ground inside the flyback. This would discharge the CRT in
a few seconds I think. So you probably wouldn't be able to get a spark.
I assume you don't have an EHT meter.
Are there any other bits I should be wary of and test
properly?
Thanks again for your help. Your voltage listing and advice in general
has been very useful.
Do you have a TV-rate video monitor with a composite video input? If
so, connect it to the BNC socket on the logic board. That's a video
output. If you get no video there, then you need to troubleshoot the
video logic first.
-tony