11 May
2015
11 May
'15
10:02 a.m.
On May 11, 2015, at 11:55 AM, Jules Richardson <jules.richardson99 at gmail.com>
wrote:
On 05/11/2015 11:22 AM, tony duell wrote:
Jules,
Not an expert at all, Tony?s advice is worth more than mine. That said, here are a couple
of thoughts.
Surface contamination in a geometry like that is a pretty good suspect. Any hydrocarbons
(coughnicotinecough) that deposited on the plastic between the traces could lower the
effective resistance a long way; worse, once the first arc takes place, it?s not unlikely
it burns a carbon filament across the contaminant which is even lower resistance. That
might be a reasonably hard thing to clean off and could be narrow enough it?s hard to
spot.
Geometry is a big deal in this case; wherever the sharpest point or bend radius is is a
good place to look. However, I would definitely clean the whole thing pretty carefully
(from the CRT terminals all the way to whatever the connections are at the other end of
the strip) as part of the debug process.
One other possibility is relative humidity; dry air insulates a lot better, so if there
is a way to turn down the thermostat for a while and let the air conditioner dry out the
room (and hence unit) under test, that could help.
- Mark
[...]
The CRT end is soldered to three terminals which protrude from the front of the CRT - one
is presumably the normal anode connection, one related to the storage mesh which I gather
that such tubes have, and I'm not sure about the third (but possibly to do with erase
functionality? I've not found an explanation of how that typically works)
However, there's a flexible plastic strip
running between the CRT face and
the chassis which carries three lines (it's a storage tube rather than a
conventional CRT) and I'm still getting periodic arcing across these lines
which of course upsets the display's operation. As far as I can tell,
there's no 'sandwich' (and hence glue) involved - it's just a single
plastic strip with conductive traces drawn onto it.
What does this strip connect to? Is it part of the CRT (or connected to the CRT), does
it connect to a coil around the CRT, or what? I am wondering if there is something else leaking
or flashing over which is causing
excessive voltage to appear here.
Hmm, yes it could be that the HV is just too high and so it arcs over periodically (and
by "periodically", it actually spends more time arcing than not) - although
there's no obvious sign (change in brightness etc.) in terms of the displayed image
immediately prior to it flashing over.
cheers
Jules