C64 IN PET S' CASE

[Desperado]

Ok, I managed to fix the videoboard and i installed the two multi-turn potentiometers, I managed to center the writing but you can always hear the buzz and every now and then the writing gets deformed or shakes.... how can I fix it?

 

[daver2]

2?

I thought there were going to be 4?

Dave

 

[Desperado]

2?

I thought there were going to be 4?

Dave

there were two at the moment....the one in place of C1 and another in place of R14...

 

[daver2]

We were going to add 2 more replacing the other two fixed resistors associated with the monostables.

In order to correct your problem, you need to vary these also.

Dave

 

[Desperado]

We were going to add 2 more replacing the other two fixed resistors associated with the monostables.

In order to correct your problem, you need to vary these also.

Dave

ah ok, R11 and R15?

 

[daver2]

I don't remember.

We covered this in an earlier post.

Dave

 

[Desperado]

Ok now i am really desperate because i inserted instead of:

R11= 10K potentiometer
C1= 1K potentiometer
R15= 20K potentiometer
R14= 5K potentiometer

i have always noise from crt....i tried to set these potentiometer but i can't erase this noise

 

[daver2]

What we're the original fixed resistor values?

Dave

 

[Desperado]

What we're the original fixed resistor values?

Dave

Original or the ones we had modified then?

 

[Desperado]

However, with potentiometers it is easier to adjust the screen but the noise still remains... I think there is a problem with some capacitor perhaps?

 

[daver2]

What fixed resistors were in there (i.e. what value did you replace with what value of potentiometer)?

What is your procedure you are adopting for adjusting the potentiometers?

Do you know what part of the signal each potentiometer adjusts?

Dave

 

[Desperado]

Please Dave help me to solve this noise!!

 

[Hugo Holden]

Something is going wrong with the Horizontal drive pulse most likely and the monostable producing it must be erratically triggering for some reason. Monostables can sometimes be finicky especially if the power supply bypassing is not good. Perhaps look at the H drive pulse with the scope on one channel and the scope locked to the input video signal on the other channel, to see any instability in the H drive pulse.

 

[daver2]

You have the board and you have the oscilloscope...

Work out, from looking at the input and output of each monostable with the two channels of your oscilloscope (using the monostable input as the trigger) which monostable is going wrong.

Set the potentiometer so that the text is on the left, but the HDRIVE is correct (no noise), and look at what the outputs are like.

Then, tweak the potentiometer so the text is correct - but you have the noise - and see what has changed.

YOU need to take the measurements and then WE can help.

Dave

 

[daver2]

What is the problem in doing what I have asked you to do?

Dave

 

[Desperado]

Hi Dave, if you remember I had already tried to carry out these measurements but I didn't see any differences....I think this noise is here to stay...this 9" monitor is not designed to work with this videoboard
I am desperate for this!!

 

[daver2]

Rubbish,

You did not try...

You observed the HDRIVE frequency halve. That MUST have come from one of the monostables.

Stop feeling desparate all the time and start learning how to use the tools you have at your disposal properly, and learning how things work - or (more correctly) how to learn how to find out how things work...

I am on holiday learning to program in a 'new' (but old) computer language...

Dave

 

[antony]

Dear @daver2 my friend @Desperado is deperado because he doesn't have any electronics background, unfortunately we live more than 900km away. It would have been nice to be able to teach him but he needs simple explanations like connecting wire from point A to point B, he will never learn on his own. Please help a poor desperate and just friend, thank you!

 

[antony]

@Desperado Dave vuole sapere in pratica cosa entra nell'oscillatore monostabile e cosa esce escludendo tutto tranne il rumore cosi da poterlo isolare, in pratica collega una sonda dell'oscilloscopio su i piedini indicati da Dave e un'altra sonda dell'altro canale in modo da fare una differenza tra i due per esempio A-B=C , il valore di C sarà il nostro rumore che verrà visualizzato sullo schermo come un impulso. devi concentrarti è importante eseguire questa misura per completare questo progetto, ti rendi conto che nessuno ha realizzato questo?

 

[daver2]

This is the problem - it is not as simple as 'wire from point A to point B'.

It involves measuring signals (pulses) with an oscilloscope and adjusting potentiometers correctly based upon the measurements.

I have asked for measurements. Firstly, with the monitor working - but with the text too far too the left; and secondly the same set of measurements, but with the text correctly aligned - but with the noise.

The things to measure are the Q outputs from each monostable (4 off) and the VIDEO signal to the monitor itself under both conditions.

If the monitor is 'getting upset' then there MUST be an observed effect in in one (or more) of the signals from the interface board to the monitor.

It is as simple as that...

What I need to know is 'what has changed'...

It may be as simple as we need to adjust a potentiometer - but I don't know what @Desperado has done to replace the fixed resistors with potentiometers.

Alternatively, one of the monostables may be 'on the edge' of stability and we may have to adjust the capacitor to compensate. We have to make sure that the capacitor and resistor values stay within the acceptable limits (based upon the datasheet) for the IC types we are using.

The only person in the universe (!) with this design of interface board is @Desperado, so the only person in the universe that can make the measurements is @Desperado...

Dave