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Pet 8296D no signs of life

Hi Dave, for the first step i wanna check the video connector with scope... I ll check for the three video signals...
 
Good morning, I'm trying to measure the voltages on the J8 connector but I don't understand well the differences compared to the diagram ...
On the diagram J8 has 5 pins while the Pet connector j8 has only 4 and 1 is not connected .... how should I do to check the voltages?IMG-20221023-WA0002.jpeg
 

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The schematic you have attached is J7 and you're looking at J8 on the motherboard. I've attached the schematic for the power supply, which has J8 on there.
 

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eight088 said:
The schematic you have attached is J7 and you're looking at J8 on the motherboard. I've attached the schematic for the power supply, which has J8 on there.
Click to expand...
Sorry I was wrong to attach the image ... the diagram of J8 is still different from my connector :(
 
Oh yes I see. I think if you measured the voltage across the yellow wire and ground, that should be 5v. If this is true then I think the AC line would be good, because the ac line feeds the 7805 regulator via a diode and then the output goes to J8 pin 5.
 
eight088 said:
Oh yes I see. I think if you measured the voltage across the yellow wire and ground, that should be 5v. If this is true then I think the AC line would be good, because the ac line feeds the 7805 regulator via a diode and then the output goes to J8 pin 5.
Click to expand...
unfortunately I have not yet measured anything because I do not understand how to do it ....
 
I'm not familiar with the 8296, but it looks like there is no 7805 regulator at VR1 or VR2? That seems strange when I look at the schematic. It might be best if you wait for someone else to help you.
 
oh wait - you must have a switching power supply, hence no VR1 or VR2. That makes sense now. I think if you still measure the yellow wire to a ground point with your multimeter (red probe touching the metal clip on the yellow wire on the connector at J8) and then the black probe on a ground point on the motherboard it should read +5v (the blue wire will be ground)
 
eight088 said:
oh wait - you must have a switching power supply, hence no VR1 or VR2. That makes sense now. I think if you still measure the yellow wire to a ground point with your multimeter (red probe touching the metal clip on the yellow wire on the connector at J8) and then the black probe on a ground point on the motherboard it should read +5v
Click to expand...
yes i have switching power supply!
 
Ok i measured J8 connector (from power supply):

YELLOW= 4,98 V
ORANGE= 12,5 V
BLUE= 0V
 
Unfortunately i checked J7 pins 1,3,5 with scope and i haven't waveforms...only high signals :(
 

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I concur, the DC voltages are fine. Good guidance @eight088.

So @Desperado, we have power, but no video signals at all. It is, therefore, not surprising that the monitor screen is black and that the initial fault is on the PET logic board.

So, what is your next step?

Dave
 
daver2 said:
I concur, the DC voltages are fine. Good guidance @eight088.

So @Desperado, we have power, but no video signals at all. It is, therefore, not surprising that the monitor screen is black and that the initial fault is on the PET logic board.

So, what is your next step?

Dave
Click to expand...
Cpu signals???
 
ok so cpu 6502 with scope:

PINS:
40: LOW and HIGH after 1 second
39: correct waveform
37: correct waveform
7: no waveform
6: no waveform ( high signal )
4: no waveform ( high signal )
3: correct waveform
 
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