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C64 IN PET S' CASE

daver2

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Didn't you replace C1 (capacitor between JP1 pin 3 and 0V) with a potentiometer also?

I don't see that on the new schematic...

What happened to it?

Dave
 

daver2

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I would normally wire JP1/3 to one end of the potentiometer. Ground to the other end of the potentiometer. And the wiper of the potentiometer to (in this case) D1 and D3. I would further wire the potentiometer so that turning the potentiometer clockwise increases the signal (moves the wiper closer to the JP1/3 signal source).

This prevents the source signal being accidentally shorted out, and would (in general) provide less of a load change to the signal source as the potentiometer was adjusted).

Dave
 

Hugo Holden

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I would normally wire JP1/3 to one end of the potentiometer. Ground to the other end of the potentiometer. And the wiper of the potentiometer to (in this case) D1 and D3. I would further wire the potentiometer so that turning the potentiometer clockwise increases the signal (moves the wiper closer to the JP1/3 signal source).

This prevents the source signal being accidentally shorted out, and would (in general) provide less of a load change to the signal source as the potentiometer was adjusted).

Dave
In this oddball case that might not be a good idea, because regardless of the amplitude of the signal at the diode's D1 and D3, the circuit relies to an extent on the input voltage falling close to ground as the reference point where the syncs are.

If it was set up like a "volume" or amplitude control at the input, it might lift the signal's zero voltage point somewhat as well as adjusting the amplitude. I think the signal needs to start at zero volts on the sync tips and have two thresholds where in one case the syncs get separated where the voltage approaches zero, and at the higher level the video data gets picked off.

Still, altering the preset wiring could be worth a try. I don't think we ever found out what the capacitor value there was supposed to be, I think likely it was a 100pF not a 100nF.

Maybe just put a 100R resistor in series with the preset to prevent a total short ?
 

daver2

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I think that was what we recommended waaaaay back wasn't it (install a low value series resistor)?

Good call regarding the zero Volt component.

Dave
 

antony

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@daver2 @Hugo Holden the trimmers can be replaced with resistors very easily once the image is well centered, just read the value of the trimmer with a multimeter and look for the closest commercial ohm value
 

Desperado

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thanks for the title, but I'm not a doctor, I'm a simple electronics technician who can carry out the instructions of an engineer but not competent in design
You are too modest! You worked at Commodore in the 90s!!
 
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