Television repair

[Sinewave]

Hello folks.

I'm repairing my 10 year old CRT tv (yes I've already been told to throw it out - I refuse ).

I've found a resistor that's gone open circuit, it looks as though a small chip has been blown out of it.

What's the likely hood that it's just component failure rather than another event causing it to blow?

I know this may be a bit of an ambiguous question. I've tested around the circuit where some diodes and poly capacitors are in circuit with it and they appear fine.

Thanks.

 

[(*steve*)]

If the failure looks energetic (a chip blown out of it?) then the chances are that this is a symptom of the real fault.

I'd be looking for a transistor (connected to this resistor) that has shorted across emitter and collector, but I'd also be looking at the schematic to see if there were other possibilities.

 

[Sinewave]

Hi Steve,

Looking at the schematic, the circuit from the resistor can be traced to a mosfet, but first by going through an inductor, though one transformer winding, then out of that into another transformer winding and then to the drain of the mosfet.

 

[(*steve*)]

Can you post the circuit (or at least the relevant part) with the affected resistor highlighted?

 

[Sinewave]

Sure, here's some photos I've just taken of them.

The resistor is number 3110, where the 'OC' is I've written to remind me that's the one that's gone open.

If you need more detailed image of a specific area, let me know and I'll sort that for you.

Thanks.

 

[(*steve*)]

Looks like there should be 166V on one side of that resistor. With a resistance of 2.2 ohms, I suggest it's there as a fuse...

I need to know what it's connected to. The connection goes off the page. You need to determine where it reappears on another diagram and show that to me

 

[Sinewave]

Looks like there should be 166V on one side of that resistor. With a resistance of 2.2 ohms, I suggest it's there as a fuse...

I need to know what it's connected to. The connection goes off the page. You need to determine where it reappears on another diagram and show that to me

What I can do, if you like, if you PM me your email addy I can email you the entire PDF, that may make it easier to read too.

Thanks.

 

[Sinewave]

Here's a link to where I've uploaded the PDF.

http://www.pdfhost.net/index.php?Action=Download&File=374811b3bd66ccd8f65336af5a2e48e6

The page we've been looking at so far is page 69 of the PDF file, standby supply.

Thanks again.

 

[(*steve*)]

I'm having trouble understanding the schematic!

It looks like the point labelled on A2 as A1-8 connects to the point on A1 that is labelled A2-8. This point is 315V direct from the bridge rectified mains (via a fuse).

On A2, there is a link labelled 9109 that I can only presume is not connected since it routes the 315V to a point labelled at 166V. Since a 2R2 resistor is all that separates them... I think not!

Another connection from here goes to the tranformers marked as 5100 and 5102.

It appears that the mosfet 7102 switches the current through them to ground.

And I'm struggling to find what the low impedance path is from the source to ground. I feel it is probably via the pair of 10 ohm resistors 3108 and 3118.

I would check that mosfet to see if it is shorted across drain and source, and I would also check the other components: 5113, 6147, 3108 and 3118.

It seems unusual that the 2R2 resistor is damaged without blowing the fuse 1503 or damaging the 2 resistors 3108 and 3118.

Sorry I can't be more precise. Can you take a look at those components and see if they look OK or not?

 

[Sinewave]

I'm having trouble understanding the schematic!

It looks like the point labelled on A2 as A1-8 connects to the point on A1 that is labelled A2-8. This point is 315V direct from the bridge rectified mains (via a fuse).

On A2, there is a link labelled 9109 that I can only presume is not connected since it routes the 315V to a point labelled at 166V. Since a 2R2 resistor is all that separates them... I think not!

Another connection from here goes to the tranformers marked as 5100 and 5102.

It appears that the mosfet 7102 switches the current through them to ground.

And I'm struggling to find what the low impedance path is from the source to ground. I feel it is probably via the pair of 10 ohm resistors 3108 and 3118.

I would check that mosfet to see if it is shorted across drain and source, and I would also check the other components: 5113, 6147, 3108 and 3118.

It seems unusual that the 2R2 resistor is damaged without blowing the fuse 1503 or damaging the 2 resistors 3108 and 3118.

Sorry I can't be more precise. Can you take a look at those components and see if they look OK or not?

Thanks Steve. I'll take a look tonight and test the components you've suggested. I'll post back my findings.

Thanks again.

 

[Sinewave]

Okay, it does appear as mosfet 7102 is down.

I'm getting 137 ohms between source and gate, 340 ohms between drain and gate and 466 ohms between gate and drain.

On diode test I'm getting 0.127v between source and gate (neg lead to source) and to drain 0.231 with no change when gate and source are shorted.

It's an STP3NB60 Mosfet, closest I can get at the moment is 5NK80Z

http://uk.farnell.com/stmicroelectr...nb60&matchedProduct=3nb60&whydiditmatch=rel_3

Thanks.

 

[Sinewave]

Or there's this.

http://uk.rs-online.com/web/p/mosfet-transistors/7084844/

500v max drain source V instead of 600 though. I'll keep looking though.


The specs on these mosfets, are they abosolute maximums? Or do my specifications really need to be bang on precise?

 

[(*steve*)]

The specs are maximums. But a replacement part should ideally have the same or greater maximum voltages (but other specs should be same or lower, or as close as possible to the same, depending on what you're talking about)

I'd try to get a direct replacement of the same part.

Did you check the resistance between these pins both ways? There is a diode (internal) across drain and source, so measure it both ways and tell me what the largest value is.

There is also an external diode across source and gate, so check that both ways.

I can't see any other path between gate and drain unless the zener 6105 is damaged, so that reading is curious -- but measure it both ways just to be sure.

Also, since this part is under suspicion, if you don't get very different readings when reading the resistance both ways (especially D-S and S-G), remove it and check again out of circuit. There may be other components in the circuit that affect the reading that I haven't spotted.

Remember that out of circuit the mosfet is sensitive to static, so work on an anti-static mat if you have one.

 

[Sinewave]

Hi Steve, yes all testing was done out of circuit and following ESD rules.

I'll test the resistance again later making note of any differences in polarity and report back.

Thanks.

 

[Sinewave]

Okay

Negative to gate and positive to source 149 ohms, reversed 137. Certainly shorted.

 

[Sinewave]

What I find confusing, perhaps you might be able to help me understand this...

...is that on the spare parts list for the large signal panel where this standby supply is, that for example, two resistors I've looked at 3110 and 3108, the values on the schematic say 3110 to be 2R2 and 3108 to be 10ohms, yet the parts list say completely otherwise, 3110 10ohms and 3108 15ohms.

Add to that confusion that I've just tested out of circuit, 3108 and am getting a reading of 523ohms. It's so tiny I'm struggling to see the colour code, so I've taken a macro photo and will look at that and see if it's any better. I'm not sure if resistors can go high in value prior to a failure, or if that would be a failed resistor symptom.

 

[(*steve*)]

I'd probably go with what was on the board (since -- presumably -- it has been running OK for some time). If you can't figure out what was there originally, try with the highest value first, then try the lower value if you have problems.

Yes, a resistor can be expected to go high in value after it has been stressed.

If it's reading 532 ohms, then it's gone *way* high.

 

[Sinewave]

Hi Steve,

Here's the resistor

I'm not sure I'm reading this one correctly. I'm not afraid to admit, with all these 3 band, 4 bad, 5 band resistors about, I do get confused on reading them and which end to start.

I looks as though to me to have some gold bands, I made this 1.5ohms, so I know I'm not reading this correctly.

 

[(*steve*)]

Brown, green, gold? -- That would be 1R5.

It doesn't look damaged in that photo.

 

[Sinewave]

So you read the value as I read too, but I've measured it differently, the schematic says different and the parts list which is in the same document says different. This really is a brain tickler