Bad Cbm 3008 PET, time to desperation again?

[Desperado]

Good morning guys and happy summer !
In these sunny days i bought a used CBM 3008 Pet,
when I turned it on i saw bad screen, i cleaned the board but i have same problem...
I'll start by checking the sync on the CPU, right?

Thanks!

 

[daver2]

Start by checking all of the power supplies.

Then the SYNC pin of the CPU.

The screen image is not random though is it?

Every other character has the most significant bit set (well, most of the characters at least) - so that smacks of a fault in the video circuitry somewhere.

In fact, the ones that do not are not very random either...

The monitor is 'squashed' in the Y direction, so that could indicate a monitor issue as well.

Dave

 

[Desperado]

Hi Dave how are you? Are you on holiday?

I have correct voltages but i haven't Sync signal on CPU pin 40...

The screen image is not random though is it?

Correct, i have always same screen...

The monitor is 'squashed' in the Y direction, so that could indicate a monitor issue as well.

Exactly, the screen is also very flickering....there will also be something to fix on the monitor board I'm afraid...

 

[daver2]

So, what does no SYNC on the CPU tell you where to go next?

Dave

 

[Desperado]

So, what does no SYNC on the CPU tell you where to go next?

Dave

I suspect that I might have problems with the reset signal...
The signal should be low when turned on for a few moments and then go high, correct?

 

[daver2]

Correct...

Also, check the clock pin of the CPU for a 1 MHz clock.

Dave

 

[Desperado]

I have always LOW signal on CPU pin 40, and i have LOW also on A10 pin 12,14 and on A3 pin 2. I have HIGH signal on A3 pin1 and A2 pin 3.

 

[daver2]

OK,

So what would you check next?

By the way, it is A10 pins 12 and 13, not pins 12 and 14.

Dave

 

[Desperado]

OK,

So what would you check next?

By the way, it is A10 pins 12 and 13, not pins 12 and 14.

Dave

Yes sorry A10 pin 12 and 13...

So now what can i check?

 

[daver2]

Obviously, something on the NE555 reset circuit isn't working correctly is it?

Check the logic levels on A2 pins 1, 4 and 8 using a logic probe.

Then check the voltage levels on A2 pins 2, 6 and 7 with your oscilloscope.

We then need to think about a faulty C67, C68 or A2 (NE555)...

Before replacing anything, however, check the solder joints and PCB tracks for damage around A2 and the associated components. Someone could have been playing before you...

Dave

 

[Desperado]

ok, thanks!

A2:

1: L
4: H
8: H

 

[daver2]

Correct, these are the NE555 power pins, so they look OK.

Dave

 

[Desperado]

A2:

2: 4,5 V
6: 4,6 V
7: 4,5 V

 

[daver2]

The pull-up resistors (R16 and R15) are OK then.

What happens on A2 pin 3 (the output of the NE555) if you short A2 pin 2 to 0V via a 100 Ohm (or so) resistor?

Dave

 

[Desperado]

What happens on A2 pin 3 (the output of the NE555) if you short A2 pin 2 to 0V via a 100 Ohm (or so) resistor?

Always HIGH signal!

 

[daver2]

I suspect A2 (the NE555) is toast.

Dave

 

[Desperado]

I suspect A2 (the NE555) is toast.

Dave

OK thanks! I ll try to remove to test with logic tester!

 

[Desperado]

Hi Dave, yes you were right as usual, the 555 was broken. I changed to a new one and now it works!
There is still some problem with the screen shaking...I need to check the monitor board now..

 

[daver2]

The 'none random' nature of the video RAM must be 'normal' for the devices fitted then.

Yes, the monitor either requires adjustment or some testing/repairing...

Dave

 

[Desperado]

Good morning,
I started to replace some capacitors on the monitor board, but there are these two capacitors that are different from the others. Can't I replace them with the classic electrolytic ones?