Intermittent Fault :(

[Cursitor Doom]

Gentlemen,

I'd welcome any constructive suggestions on how to proceed with
tracking down this elusive and annoying fault.
The problem presents as a blank CRT screen on the network analyser I'm
working on currently. Inititally I discovered the screen was being
blanked not just during re-traces, but *permanently* - there appeared
to be a rogue logic signal somewhere responsible. However, it turns
out the problem goes away and the screen instantly comes back when I
press and tap on various areas of the motherboard or the cards that
slot into it. The trouble is, there doesn't seem to be one specific
area that's susceptible to this tapping and bending. First it appears
to be a bad joint/ connection on the motherboard, then it's one of the
slot-in boards and then next time again, a different board - but all
capable of misleadingly appearing to be the site of the poor
connection.
Any ideas as to how to narrow the search area right down to something
manageable?
Thanks.

[abrsvc]

My first guess is that there is a line on the bus that carries that signal and it is being interrupted. Check for any cards that are in pressure type connectors where the board provides the "jumper" between the fingers. Could be as simple as some corrosion on the fingers or the pads on the boards.

Look for the simple stuff first. Something as simple as removing and re-inserting the cards a few times might be enough. It doesn't take much to mess up a signal.

Dan

[John Robertson]

Monitor the picture tube pins.

You may have a problem with the HV transformer that is losing one of
various supplies that the tube needs to work.

Or a simple connection failure - such as is often found with circuit
boards that have only a single sided PCB and Molex style pin connectors
(solder cracks at the terminals)...

A dual trace scope helps with this sort of troubleshooting. Leave one
lead tied to the picture tube cathode so you can see the signal at the
tube, and use the other lead to check the signal path while tapping. The
idea is to divide in half the circuits relating to the tube, while
watching the cathode to make sure it is something unrelated to the video
signal that is causing the image to vanish.

John :-#)#

[Cursitor Doom]

On Mon, 2 Mar 2020 15:19:31 -0800, John Robertson <sp...@flippers.com>
wrote:

>Monitor the picture tube pins.

I'd prefer to avoid the EHT areas until every other possibility has
been eliminated! I don't think that will be necessary anyway.

In the absence of any better ideas, I've decided I'm going to have at
it with Arctic Spray in the hope of exposing a dry joint somewhere.
The appeal here is that there's no mechanical stress being applied
except in the immediate area of application and I've already found
that this fault is unusually sensitive to any form of physical stress.

[Mike Coon]

In article <648s5f9ejs8j7qfvd...@4ax.com>,
cu...@notformail.com says...

I was going to suggest spray for exactly that reason. Maybe in
conbination with a heat gun to increase the thermal shock.

For poking EHT I suggest large knitting needles, even if not EHT
certified. That's why my long-late Dad had one in his toolkit, anyway...

Mike.

[Cursitor Doom]

On Tue, 3 Mar 2020 10:59:59 -0000, Mike Coon <gra...@mjcoon.plus.com>
wrote:

>I was going to suggest spray for exactly that reason. Maybe in
>conbination with a heat gun to increase the thermal shock.
>
>For poking EHT I suggest large knitting needles, even if not EHT
>certified. That's why my long-late Dad had one in his toolkit, anyway...

I have a couple of 30kV high voltage AVO probes with long spikes for
that sort of thing if necessary.
Anyway, I didn't need the spray. It looks like the problem is/was an
insufficiently snug edge connector to the motherboard. These are
pretty much identical to the ones you see in desktop computers; same
pitch etc. I took the suspect board out and cleaned it's edge
connections with IPA but noticed on re-insertion there is no
positivity about the fit whatsoever unlike the other boards I've
removed and re-fitted before. You cannot really tell whether it's
seated fully by feel. Not good. Anyway, problem's gone away -- but for
how long, who knows?

[John Robertson]

CRT tube cathode is typically under 200VDC, not exactly EHV, and the
screen may run in at 400V. Focus is likely 2K or so, but that is why one
stocks a HV meter. The 'scope is for monitoring the cathode, not the EHT.

John :-#)#

[Cursitor Doom]

On Tue, 3 Mar 2020 08:36:34 -0800, John Robertson <sp...@flippers.com>
wrote:

>CRT tube cathode is typically under 200VDC, not exactly EHV, and the
>screen may run in at 400V. Focus is likely 2K or so, but that is why one
>stocks a HV meter. The 'scope is for monitoring the cathode, not the EHT.

I think you need to be more careful with the advice you're dishing out
here. The cathode on this tube runs at -2850VDC with the anode at only
+84VDC. Please don't generalise about the tube potentials on these
pieces of test equipment. Someone else may read your remarks on Google
a year or more in the future and end up killing themselves.

[John Robertson]

I was not aware that those tubes used such a high negative voltage.
Thank you for pointing it out. I am certainly not afraid to admit when I
make mistakes!

John :-#)#

[Cursitor Doom]

On Tue, 3 Mar 2020 11:16:13 -0800, John Robertson <sp...@flippers.com>
wrote:

>I was not aware that those tubes used such a high negative voltage.
>Thank you for pointing it out. I am certainly not afraid to admit when I
>make mistakes!

That marks you out as totally unique in the Usenet community; well
done! :-D

[mako...@yahoo.com]

If the logic is CMOS, you could also have an OPEN input someplace.

I have seen these change state seemingly at random due to static electricity.

m

[Cursitor Doom]

On Wed, 4 Mar 2020 10:02:41 -0800 (PST), mako...@yahoo.com wrote:

>
>If the logic is CMOS, you could also have an OPEN input someplace.
>
>I have seen these change state seemingly at random due to static electricity.

Well, this is certainly the time of the year for plenty of static - if
you're in the northern hemisphere anyway. ;-)

[Jeff Urban]

>I think you need to be more careful with the advice you're dishing >out here. The cathode on this tube runs at -2850VDC with the anode >at only +84VDC. Please don't generalise about the tube potentials on >these pieces of test equipment. Someone else may read your remarks >on Google a year or more in the future and end up killing >themselves.

So he is supposed to read your fucking mind ? MAKE, MODEL, CRT NUMBER, are you sending that telepathically or what ?

My TV is broke. It has no make or model but it has c1 to 5154, which one is bad ?

ANSWER ME ! What is wrong with this TV ! COME ON, WHAT IS TAKING YOU SO LONG ? I transmitted my phone number to you five seconds ago. CALL ME AND HELP ME THIS IS A TV.

Go ahead, ANYONE tell me how much of an asshole I am, that will certainly prove something. And you go ahead and read minds, I won't because there is nothing in them.

[bruce2...@gmail.com]

No, seriously. Its just tracing circuitry. Or are you scared of the fire marshall bill image from the show "In Living Color" (that showed from 1990-2006) where them and electricians have spiked hair and are accidentally shocked or burned every day. Gimmie a break.