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C64 IN PET S' CASE

Desperado said:
can I try putting the ICs back on to see if the signal reappears?
Click to expand...

The IC's at the moment have nothing to do with the price of Fish. Because the input signal (video) has gone awry somehow.

No, put the board aside for a while.

Try to get the video output from the C64 working properly first. Put the pull up resistor to +5V back on the video signal line and see if you can see the video signal on the scope looking normal or not.

Only when the video signal is working normally does the board stand any chance of working.
 
Hugo Holden said:
Try to get the video output from the C64 working properly first. Put the pull up resistor to +5V back on the video signal line and see if you can see the video signal on the scope looking normal or not.
Click to expand...
1K resistor Hugo?
 
I saw in a previous post that the RF modulator had been removed, and it seems from this point on the video signal is no longer on pin 15 on the VIC? The RF modulator is required to get a signal from the VIC. Maybe resolder the modulator back in and check pin 15 again?
 
Did you read my post #1,735? I asked you to do something that you haven't done yet...

Inserting any ICs should NOT have any effect on the C64 video signal, unless there is a further PCB fault (unlikely), you haven't performed the track cutting and wiring correctly, or your soldering is inadvertently shorting out something that it shouldn't be doing.

The other possibility is that we modified the original value of R4 from 10k and that has not been noted down anywhere - and you have thrown the old PCB in the bin, so we haven't got it to refer to now even if we wanted to.

This is why you don't destroy a prototype or throw it in the bin until the thing you are designing works correctly.

Dave
 
eight088 said:
I saw in a previous post that the RF modulator had been removed, and it seems from this point on the video signal is no longer on pin 15 on the VIC? The RF modulator is required to get a signal from the VIC. Maybe resolder the modulator back in and check pin 15 again?
Click to expand...

Not quite true. The modulator was removed a long while ago (hundreds of posts back) and the video signal taken directly from the VIC chip pin 15.

Either something is different on this build of the PET monitor interface board, or the new board itself has killed the VIC chip when it was connected up due to the incorrect PCB tracking of the video signal.

Dave
 
What was the part number of the modulator that you removed? I will look up the schematic for it.

What would make sense is that we changed the value of R4 for something like a 470R or 680R resistor (or soldered another resistor in parallel with R4 to reduce its resistance significantly). But, if the old board has gone in the bin...

Dave
 
daver2 said:
What would make sense is that we changed the value of R4 for something like a 470R or 680R resistor (or soldered another resistor in parallel with R4 to reduce its resistance significantly). But, if the old board has gone in the bin...
Click to expand...
I remember well that R4 was 10 Kohm !
 
Did we ever install a separate pull-up resistor though on the C64 video signal that is now missing?

I am not going back and re-reading 1,700 odd posts to find out though...

Dave
 
daver2 said:
Right,

The errors are not the fault of the PCB, and the schematic (well, the front end) looks right to me.

We have no ICs installed, so they shouldn't be affecting us.

Disconnect your video interface card from the C64 and put it on the bench.

Use your multimeter (set to read resistance) and measure the following points on JP1.

Negative lead to pin 1 and positive lead to pin 2.

Reverse the leads and take another resistance measurement.

Negative lead to pin 4 and positive lead to pin 2.

Reverse the leads again and take another resistance measurement.

Report the results.

Dave
Click to expand...
Negative lead to pin 1 and positive lead to pin 2 = 1.105 Kohm

Reverse the leads and take another resistance measurement = 1.105 Kohm

Negative lead to pin 4 and positive lead to pin 2 = 0.67 Mohm

Reverse the leads again and take another resistance measurement = 0.33 Mohm
 
Pins 1 and 2 make sense.

Pins 2 and 4 not quite sense yet.

Can you just check from JP1 pin 4 to both sides of resistor R4. One end of R4 should indicate 0 Ohms and the other end should indicate 10k Ohms.

Dave
 
daver2 said:
Can you just check from JP1 pin 4 to both sides of resistor R4. One end of R4 should indicate 0 Ohms and the other end should indicate 10k Ohms.
Click to expand...
Yes, 9,7 Kohm on one side of R4 and 0 Ohm in the other side :(
 
Ok,

Why the unhappy face? Your readings are OK.

Now measure the resistance from JP1 pin 4 to the non-banded end of both D1 and D2.

The resistance should be approximately 10k Ohms in both cases.

Dave
 
Ok,

Can you now put the multimeter probes across D1 and measure the resistance.

Then, reverse the multimeter probes across D1 and measure the resistance again.

Repeat the above with D2.

Report all four (4) resistance readings.

Dave
 
daver2 said:
Ok,

Can you now put the multimeter probes across D1 and measure the resistance.

Then, reverse the multimeter probes across D1 and measure the resistance again.

Repeat the above with D2.

Report all four (4) resistance readings.

Dave
Click to expand...
D1: 53,5K and 105.3 K
D2: 0.300 Mohm and 0.680 Mohm
 
We have seen this before with your multimeter. We seem to get high resistance readings when measuring diodes.

Right, with your video board disconnected from the C64, can you attach a 470 Ohm resistor from the C64 video signal to +5V, and measure the C64 video signal with your oscilloscope set to 1 [V/div].

Dave
 
So, that looks a good, healthy 4.2 Volt signal.

Leave the 470 Ohm pull-up resistor to +5V connected, connect up the video interface board, and remeasure the C64 video signal again with your oscilloscope.

Dave
 
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