Two VERY general questions!

Thread Starter

Nicholas

Joined Mar 24, 2005
139
Hey all!

I have a monitor board that isn't working, so I have two questions I would
like to ask you!

The first is about a resistor on the board. It's color coding says it should
be 1Mohm, but it reads as 247ohm every time! What could be wrong here?

The other is a picture of the solder side of the board, at what seem to be
one of the HV legs. What is going on here, what is that splotch?:



Thanks,

Nicholas
 

beenthere

Joined Apr 20, 2004
15,819
You may need to desolder one leg of the resistor and lift it to get a good measurement. There could be anything in parallel with it, which would give an incorrect reading in circuit.

That blotch could be bad, but there is not quite enough resolution to make it out. What is connected to the other side of the hole?
 

Thread Starter

Nicholas

Joined Mar 24, 2005
139
Hey! Thanks!

I'll try to remove it then.

The high voltage thingy that powers the CRT is connected to the other side of the
hole. Do you think I should re-solder it? And what IS that brown stuff anyway?

Thanks,

Nicholas
 

beenthere

Joined Apr 20, 2004
15,819
Not enough detail visible to say. The size of the copper trace might suggest it's circuit ground, but that's a guess. The split peg visible could be a locating pin. The brown stuff around the hole might be the PCB substrate, some kind of phenolic.

A high voltage thingy is more properly called a flyback transformer, by the way. Problems with them are usually more expensive to fix than getting a new monitor.
 

mrmeval

Joined Jun 30, 2006
833
It looks like a ground pin. Power off the unit, unplug it. Soak a qtip in alcohol and see if the brown gunk will remove. If it does it's flux. It's probably a wire that was hand soldered from the CRT board to the main board.
 

Management

Joined Sep 18, 2007
306
Thanks!

Is flux from a solder joint conductive??

Nicholas
I don't think so. I thought it was only for causing a better flow of the solder when melted.

EDIT: Looked this up. I guess it depends on the sensitivity of your circuit. Best to clean it off a little after soldering.

www.qprox.com said:
Are Your Chips Sensitive To Soldering Flux Residue?

This FAQ applies to all QTouch and QMatrix devices.

All capacitive sensors should be treated as highly sensitive circuits which can be easily influenced by stray conductive leakage paths. QT devices have a basic resolution in the femtofarad range; with these devices, there is no such thing as 'no clean flux', regardless of what it says on the can.

ALL fluxes leave a residue, the only questions are 'of what' and 'how much'. An unwashed PCB always has a sticky feel, a slightly dull look, and leaves fingerprints easily. The reason is obvious.

Flux residue films absorb moisture (they are in fact highly hydroscopic) and become conductive between solder joints, causing signal drift and resultant false detections or temporary loss of sensitivity.

Conformal coatings will trap in existing amounts of moisture which will then become highly temperature sensitive. The problems appear when there is a rapid change in temperature or humidity, and especially if the board temperature falls below the dew point.

QMatrix devices (QT60486 etc) are naturally more resistant to these effects. 'Simple electrode' devices like QT110, QT160 etc are more prone to these problems, especially if there are leakage paths from the electrode wiring to adjacent conductors (ie ground, power, other signal lines or keys, etc).

The vast majority of consumer applications will not suffer from these effects in normal use even with a dirty board. However the designer should be aware of these issues and test the product in a chamber with these possible problems in mind.

The designer should strongly consider ultrasonic cleaning as part of the manufacturing process, and in more extreme cases, the use of conformal coatings after cleaning and baking.
 

JoeJester

Joined Apr 26, 2005
4,259
The brown stuff looks like flux to me. I certainly would remove that solder, make sure it's supposed to be connected to that pcb run, and renew the connection. It does look like the tab of a flyback transformer.
 
Top