HV tracking across flexible PCB strips

[Paul Koning]

> On May 11, 2015, at 9:40 AM, Jules Richardson <jules.richardson99 at gmail.com> wrote:
> 
> 
> I really couldn't think how best to word the subject.  I pulled the CRT out of the HP 1331 X-Y display that I got the other day, then took all the old tape off, cleaned it up, and reassembled with new tape. It's no longer arcing around the CRT face, which is good.
> 
> 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.
> 
> Does anyone have experience of these kinds of strips in an HV environment, and what (if anything) can be done for them when they start to fail? Possibilities seem to be:
> 
> 1) Try some more cleaning,
> 2) Reworking with conductive paint (I wondered if one or more of the traces have gone high resistance in certain spots and this is encouraging the HV to arc between them 'upstream' of such areas),
> 3) Plastic itself has broken down in some way, requiring replacement; has anyone managed to make a replacement strip from scratch?

It sounds like a very strange way to construct high voltage wiring.  The major rule for high voltage work is to avoid sharp edges.  A strip like you describe seems to violate that rule, big time.  It presumably worked when new, but it sounds marginal.

The most obvious fix is to replace it by a set of regular (round) wires.  Use fairly large wires if you can, not because of the current carrying capacity but for the roundness.  Along the same lines, when soldering, make sure the joint is smooth, no pointy protrusions on the solder.

	paul