From: email@example.com (N. Thornton)
Subject: Re: CRT double peaking?
Date: 22 Dec 2002 17:37:20 -0800
NNTP-Posting-Date: 23 Dec 2002 01:37:20 GMT
A E wrote in message news:<3E04A1C6.54249E32@videotron.ca>...
> Is the mechanism of CRT end-of-life double peaking the same as 'cathode
> interface' in a signal tube, ie barium orthosilicate seperating the
> cathode coating from the nickel sleeve?
> If so, is there a reason why I shouldn't drive the cathode of a scope
> CRT with a DC-biased AC signal (notwithstanding aliasing for the moment)
> to squeeze out more electron juice from the cathode?
> I'm guessing the A/cm^2 wouldn't be exceeded since those parts of the
> cathode that don't pass DC will pass AC, therefore increasing active
> I did put a Wavetek generator on my scope Z input and definitely got an
> increase in brightness, but when I disconnected the Z drive, the trace
> took 3 seconds to come back, I'm trying to figure out if that's the
> input circuit on the Z or the tube doesn't like my idea...
> I grew up on ICs, so tubes are not my thing, but does this make sense?
> Can I add a permanent mod to the scope to get brightness this way? I'm
> trying to NOT replace the tube, it's a storage tube, so it's even MORE
> complex than changing the tube in a regular scope.
> Please, no 'get a digital storage scope' replies, I don't like those
> things and I feel they are fragile, unserviceable, over-engineered,
> Windows appliances (shudder) and expensive overkill for checking switch
> bounce and I/O pins on 10MHz PIC circuits. :)
It'd be nice if you could explain all this to us/me.
Re tube boosting, that I do know about.
Best method is generally some sizeable heater voltage boost. The old
recommendations of +20% are very tame. The highest boost I ever ued
was +70% heater voltage boost - strictly experimental of course, but
it worked great for years. I would generally start with +30%, and
raise the stakes as needed. Once you hit +50%, you are definitely into
risk territory. Other than tube death, overheating of scan assembly /
plastic mountings are the things to watch for.
Having said all that I never had a tube die from overvolting, they
routinely went down from zapper type boosting, which I would avoid
until it is absolutely the last resort.
Next trick is to boost the EHT to he final anode a bit. This is
frowned on since it does increase x-ray emission to some xtent, but it
is an option when all else fails. Also increases V on EHT parts, which
is a definite risk.
Nother trick is to understand that its average current thats limited
as much as peak, so with TV, just reducing the background level will
give more peak. With a scope, maybe you could ensure the beam is cut
off wherever it isnt essential.
I'm trying to think what other tricks there are. What colour is the
screen when no beam is showing? If its not real dark, a beam coloured
filter would help. It'll cut down beam bright a bit, but it cuts down
background bri a lot more. More contrast, easier to see, despite being
a tad dimmer.
I'm sure there were other ways... cant think of them tho. One last
very basic one, a cardboard or sheet plastic cowling, to reduce
ambient light on the tube face.
Let us know how it goes.