Cbm 2001 Pet strange boot

[daver2]

Yep, that's where we are heading I think...

But I am trying to take the other Dave on the fault-finding journey.

Dave

 

[Desperado]

Just been out for the evening.

Those signals look OK.

Can you just move forwards now and check G1 pins 1, 2 and 3.

Dave

G1
PIN1 : 5V square wave
PIN2 : 4,5V pulse
PIN3 : 1,7V pulse

 

[daver2]

Interesting.

So, somewhere between G1/3 and G8/12 we loose our signal?

Can you double-check those two (2) pins again please.

Dave

 

[Desperado]

Interesting.

So, somewhere between G1/3 and G8/12 we loose our signal?

Can you double-check those two (2) pins again please.

Dave

I double checked and i have same signal on the two pins.... 1,7V and some pulse.
On G8 pin 12 i have found a little rust....maybe i i had a bad read before

 

[daver2]

Well, that doesn't help me - because now your readings are potentially inconsistent. Can you recheck your readings again around G8 (see post #14 for the pins to measure)?

Dave

 

[Desperado]

I try with logic probe now Dave:
G8

pin8: high
pin9: low
pin11: pulse
pin12: low
pin13: pulse

 

[Nivag Swerdna]

OK so I probably should have concentrated more when I blamed the monitor.

The readings above look slightly spurious...

If you consider H8 pin 13 (which should be wired directly to pin 10 also) then this is just nINIT which is simply pulled up to 5V. That should be sitting happily high.
H8 p13 needs to be nicely HIGH to get any flip flop action?
It would also be interesting to check the voltage across 7 and 14 of H8.

 

[Desperado]

OK so I probably should have concentrated more when I blamed the monitor.

The readings above look slightly spurious...

If you consider H8 pin 13 (which should be wired directly to pin 10 also) then this is just nINIT which is simply pulled up to 5V. That should be sitting happily high.
H8 p13 needs to be nicely HIGH to get any flip flop action?
It would also be interesting to check the voltage across 7 and 14 of H8.

Hi Nivag,
H8 pin 13: High
voltage across H8 pin 7 & 14= 5,12 V

Thanks

 

[daver2]

G8/12 appears inconsistent.

In post #26 you state it is LOW.
In post #14 you state it is also LOW.
In post #24 you state (with me reading between the lines) that you did (eventually) find a signal on G8/12.

We need to resolve this inconsistency...

G8 pins 8 and 9 should be 'in antiphase' - so one output should be HIGH whilst the other output should be LOW. This is the case, but the flip-flop is not clocking...

Pin 11 (the clock) appears to be there.

This just leaves pin 12 (assuming pin 10 is HIGH and pin 13 is not permanently LOW).

If you have some 'rust' on the pin - you will need to clean the pin. Last time I had 'rust' on a pin of a 7474 IC the pin had been completely 'eaten through' and wasn't making contact with the actual IC!

Dave

 

[Desperado]

In post #24 you state (with me reading between the lines) that you did (eventually) find a signal on G8/12.

Sorry,I explained myself badly...
In post #24 i measured with scope and it was probably not a pulsating signal but just noise ....

In post #26 i measured with logic probe and i saw low signal! From now on, if it is not necessary, I will measure only with the logic probe...ok?

 

[Hugo Holden]

Daver,
Ah, the dreaded state sequencer hang up. If anyone can track it down, you will. My bet is a bad 74177 counter on sheet 7, but it could be any one of a dozen ICs that went bad.

Before I got my PET I did some research on the 4 state machine. After I got it I confirmed the functions on the scope. (see attached)

Apart from anything else one thing that makes just this part of the circuit super tricky is the relatively widely spaced but narrow clock pulses which are in fact only about 100nS wide. Very difficult to visualize, only just did it with a Tek 2465B scope, though it was obviously being triggered. I don't think a digital scope would help greatly, though a logic probe with the typical pulse stretcher would determine if they were there, or not.

I have not assessed the circuitry that ultimately generates these unusual clock pulses, yet, but at least it is a start. It might be worth checking that the /INIT line is staying high after booting.

 

[Nivag Swerdna]

@Hugo Holden I think I wasn't very helpful due to not concentrating and not processing what I did read... so sorry for the Red Herrings.

My money is now on @dave_m .... check that 2,5,9 and 12 of F2 and F4 are pulsing?

 

[Desperado]

ok so:
F2 pins 2-5-9-12 no signal (no low, no high and no pulse)
F4 pins 2-5-9-12 pulsing!

 

[Desperado]

It might be worth checking that the /INIT line is staying high after booting.

Hi Hugo, how are you?
Please, where i can check INIT line? Thanks!

 

[Nivag Swerdna]

ok so:
F2 pins 2-5-9-12 no signal (no low, no high and no pulse)

So for the record please describe the signals at all pins of F2.

 

[Desperado]

So for the record please describe the signals at all pins of F2.

ok,
F2:

pin1: pulse
pin2: no signal
pin3: high
pin4: high
pin5: no signal
pin6: no signal
pin7: low
pin8: pulse
pin9: no signal
pin10: high
pin11: high
pin12: no signal
pin13: high
pin14: high

 

[daver2]

F2 has probably gone to that great TTL chip store in the sky. RIP...

Does everyone agree?

Dave

 

[Desperado]

F2 has probably gone to that great TTL chip store in the sky. RIP...

Does everyone agree?

Dave

Dave you suggest to replace this sn177 ic?

 

[daver2]

Yep - device F2, but the correct part number is SN74177.

Dave

 

[Desperado]

Yep - device F2, but the correct part number is SN74177.

Dave

Yes!
I changed F2 ic and now i have full screen! Unfortunately i have random chars screen but i think that's a good step!
Thank so much!