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Pet video board 320064-02 bad

Dwight Elvey said:
A 16% increase in voltage is about a 36% increase in dissipated power.
Dwight
Click to expand...

That is about right if the load on the power supply was an ohmic resistance due to the V(squared)/R power relation.

But a lot of electronic circuitry doesn't operate like that. Take the vertical deflection amplifiers for example. These class A-B amplifiers are very much like their audio amplifier counterparts. They have a small quiescent current in their output stages to avoid cross over distortion (which would otherwise cramp the few scanning lines near the center of the screen in the raster together).The power delivered to the load though is dependent on the drive level, or in this case the height control. So the power consumption of this is merely dependent on the height of the raster, not the supply voltage to the amplifier.

On the other hand, the H output stage is a different matter and one could argue that if the peak to peak currents there increase linearly with increasing power supply voltage, then the I(squared)R losses would increase with the square law relationship.

As far as the video signal amp stages go, these draw so little power it hardly matters.

So if the monitor's voltage increases by 14/12, the power dissipation will not increase by 36%, it will be a lot less.
 
ilpaninaro said:
Good morning guys!
I have now removed Vr1 and i measure 25V on the board but i can't found 12v or 14v now....
Where should i read 12v?
Thanks at all for precious help!
Click to expand...

Hi
We don't expect to measure 12V. We are looking to see what voltage you measure where 14V used to be. I assume you are saying the locations that used to be 12V are now 0V? ( please respond with a clear message, like " I am now measuring xx volts where I used to measure 14V ). I'm hoping you are now measuring 0V. Where it used to be 14V. If so, I believe I know what the issues was.
In the layout picture, they show the regulator with the 3 leads, having the center lead cut. It is using the thermal mount for the DC path of the ground of the regulator. Because of dissimilar metals, I've seen this connection fail in the past with high resistance on pinball machines ( sound boards for one of the machines ). The regulation floats instead of being held at the desired level. In your picture, I see that there is some corrosion from moisture on many of the parts. We may be looking at a corrosion issue. The ground lead doesn't carry much current from the regulator. It is typically in the milliamp range but a high resistance path is bad. If we connect a wire from the cut center lead of the regulator to ground, we may find things are back to normal. Do you think you can reinstall the regulator but add a wire from the center lead of the regulator to a ground point of a near by component?
Dwight
 
It looks like the lead of R3, closest to the edge of the board is the ground lead. A small wire from that lead to the regulator should be fine. Take a picture of the work and show your work.
As for Q14 I suspect it has been damaged with the floating regulator voltage. I think it would be wise to remove it and use the diode function to check the junctions. Are you familiar with this test? We will replace it if needed.
It needs to be removed to properly test with a diode measurement.
Dwight
 
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I thought I might make a note about the layout picture. The components are suppose to be over a greyed out picture of the traces. I suspect the intent was to make it look like a xray picture. Looking at it for some time, I've been trying to make sense of the component leads relative to the traces.
I see now that the traces are totally flipped left to right, as they would be seen from the bottom view of the board, while the components are seen from the top of the board. I don't think that was the original intention. I've seen it done so it looked like one was looking through the board at the traces. I guess when they made the copy, they had the trace picture flipped. So, don't try to make sense of the traces relative to the components.
Dwight
 
I suspect, looking at the traces, that I got the wrong end of R3. Please check with an ohm meter as to which end is ground.
Dwight
 
Yes, I got R3 backwards. The end of R3, closest to the regulator is ground!!
Sorry about the confusion. I don't have a real board to look at.
Dwight
 
Dwight Elvey said:
Yes, I got R3 backwards. The end of R3, closest to the regulator is ground!!
Sorry about the confusion. I don't have a real board to look at.
Dwight
Click to expand...

Hello Dwight, yes I am now measuring 0V where I used to measure 14V....
Now, must i resold Vr1 and central pin with wire to R3 ok?
Thanks!
 
You are missing the screw to screw the regulator down. That is important to get good thermal contact of the regulator to the heat sink. The connection to R3 looks right. After you replace the screw we can power things up and run some experiments with Q11, Q12 and Q13.
Check that the wire at the regulator isn't shorting to the other lead and tidy up your R3 lead by removing the tail of wire.
Dwight
 
Dwight Elvey said:
You are missing the screw to screw the regulator down. That is important to get good thermal contact of the regulator to the heat sink. The connection to R3 looks right. After you replace the screw we can power things up and run some experiments with Q11, Q12 and Q13.
Check that the wire at the regulator isn't shorting to the other lead and tidy up your R3 lead by removing the tail of wire.
Dwight
Click to expand...

Ok Sir,
screw are now ok...
Can i turn on and can i measure voltage at Q11,12,13 collectors?
 
Nice catch on that flipped layout picture! Years of looking at those schematics etc. and I never noticed that.

Re VR1 12/14: yeah, never assume anything, however obvious.

I'm curious: what is the board number? Mine says 320084-02 F CPX-1
I've found at least one difference from the published schematic, but not relevant to this AFAIK.
 
We have clearly all fallen into a parallel universe where the Laws of Physics are different!

As Dwight has said, something is clearly wrong somewhere and we need to track it down. The 7812 is rated up to +30V at the input, so what you are reading is high, but within the limits for the 7812 - albeit with increased power dissipation.

I liked the idea of a high resistance joint on the GND side of the 7812; but, assuming the wire has been connected correctly, that should now rule that out.

Just thinking if measuring the input to GND voltage, the output to GND voltage and the differential across the regulator from input to output will tell us anything? The voltages measured should (at least) be consistent...

Dave
 
daver2 said:
We have clearly all fallen into a parallel universe where the Laws of Physics are different!

As Dwight has said, something is clearly wrong somewhere and we need to track it down. The 7812 is rated up to +30V at the input, so what you are reading is high, but within the limits for the 7812 - albeit with increased power dissipation.

I liked the idea of a high resistance joint on the GND side of the 7812; but, assuming the wire has been connected correctly, that should now rule that out.

Just thinking if measuring the input to GND voltage, the output to GND voltage and the differential across the regulator from input to output will tell us anything? The voltages measured should (at least) be consistent...

Dave
Click to expand...

Now i ve connected again central pin of Vr on board...
i ve measured Q11,12 AND 13 collectors and i ve these measures:

Q11=14v
Q12=0v
Q13=14v
 
I suspect some type of oscillation. We have coils and such around. If thing were pulsing, it is possible to pump the voltage up. On the youtube, one of the others was noting that the filter capacitors were dry. I wonder if this is related to what we are seeing. I'd sure like to see things on a scope.
Linear regulators don't regulate backwards, only forwards.
Dwight
 
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