My Falcon went to black screen

[exxos]

I don't understand why your previous RESET tests all show "backwards" readings compared to Videl...

- Pin 17 (Reset) :
- Just after power on, I see 5V and then rapidly 0V (instead of 5V)
- when I press the reset button, I see -5v that then goes rapidly to 0V

- Pin 18: (HALT)
- just after power on, I see +5V that goes rapidly to 0V (instead of 5V)
-

After turning on the machine reset will go to 0v for a short time, then 5V. I don't see how your getting -5 to 0v ? Do you have your meter probes the right way around ?!

[christophe.griard]

I saw these voltages using my small oscilloscope...

But when I just measure the voltage using my multimeter I have different figures...
And with my meter, I have a stable +5V on pin 17 (reset), a stable +5V on pin 18 (HALT) and a stable +5V on pin 19 (Bus error)... A totaly different picture... To be sure, I double check these values with my meter.
And when I push the reset button the pin 17 goes to 0 and then raise to +5V: normal behaviour...

I checked also that pin 24 and 25 show a stable +5V: it is the case.

Clearly the values I got from my small oscilloscope seem to be wrong... It is with this oscilloscope I got the 8 Mhz on pin 23 of expansion port. I just checked again the signal on pin 23 and I found again 8 Mhz. But I don't know why voltage are different between my small oscilloscope and my meter for pin 17, 18 and 19...


At least, the behviour seen with my meter is more coherent with what is waited from the Falcon...

All other measurements (VIDEL, DALLAS, U1, U11, etc.) have been done with my multimeter.

Now I have to completely restart my search for the failure...

Sorry for that... I still not understand the reason of figures I got from my oscilloscope...

[czietz]

Sorry for that... I still not understand the reason of figures I got from my oscilloscope...

My guess: On the oscilloscope, AC coupling is turned on, instead of DC coupling.

[exxos]

And when I push the reset button the pin 17 goes to 0 and then raise to +5V: normal behaviour...

So reset appears to be working correctly then....

[christophe.griard]

Yes you are right AC coupling was on... Now I have obviously the same results between the scope and the meter...

Sorry for that forgetfulness in handling my small scope...

And yes the reset circuit works... The CPU is not halted, there is no bus error...

So, only the frequency mesured on pin 23 of expansion port is not at the right level (8 Mhz instead of 16mhz). Everything else seems to be ok but the screen is still with these strange green bars.

I checked on CPU pin 64 (Reset), pin 129 (HALT) and pin 130 (Bus error) and all three are at +5V, therefore ok. And when I push the reset button, for pin 64 goes to 0V and when I release the button goes to a stable +5V.

I checked also the pin 59 (reset) on COMBEL and it works properly also (+5V and 0V when pushing the reset button).

Maybe need to have a look to 25 Mhz crystal that is used byu VIDEL (32 Mhz crystal seems to work as I measure 8 Mhz on the expansion port)...?

If you have some ideas on the next diagnostic step to be done...

[christophe.griard]

Hello everybody,

As lots of signals are finally ok, I followed the Service manual that says:

"If no activity is seen on the RS232 port or display (it is my case with with the diagnosis cartrigde and the VT52 terminal emulator running on a 520 STE) continue with :

- disassemble the computer so that the printed circuit board is exposed. Power up the computer. Using an oscilloscope, verify the 32 Mhz clock to the COMBO IC. If not present, replace the oscillator."

=> so I put my oscilloscope to the pin 20 on the COMBO IC (it is quite easy as there is a track ended by a very small hole connected to pin 20). And I can measure that a 32,09 Mhz clock signal arriving at the COMBO IC (see first picture) => checked.

The service manual then says : Check the 16 Mhz clock to the 68030 CPU. If it is not present replace the COMBO IC.

=> so I put my oscilloscope to pin 6 of 68030 CPU and I do measure a 8 MHz signal (the same frequency and signal shape than the one I measured on port 23 of expansion port; see second picture) and obviously not the waited 16 Mhz...

So the service manual recommends changing the COMBO IC...

Then I checked the pin 44 of the VIDEL IC to see if the 32 MHz clock signal arrives at the inlet of the VIDEL IC : I do measure the 32 MHz clock signal arriving at the VIDEL.
Then I checked the pin 73 of the VIDEL IC to see if the 25 Mhz clock signal arrives at the inlet of the VIDEL IC : I also do measure a 25 Mhz at the inlet of the VIDEL IC on pin 73 (see third picture).

What do you think about of this latest informations ? Could you confirm that pin 6 is the one on which to check the clock signal frequency for the 68030 CPU, and do you think that the diagnosis leads to having to replace the COMBO IC?

Thanks a lot for your help and advice.

Have a nice evening.

VIDEL IC pin 73 25,17Mhz.jpg

CPU Pin 6 clock signal.jpg

COMBO IC pin 20 32MHz clock signal.jpg

[exxos]

I'm not 100% sure, maybe someone can check.. But I think the Falcon boots firstly into 8MHz mode and switches into 16MHz later on.

You need to see what the CPU /AS and /DTACK pins are doing to see if the CPU is even half alive.

[christophe.griard]

Thanks Exxos.

I will check that tomorrow morning. I see AS pin on CPU (pin n°125) and I assume the DTACK you mentioned is DS pin (pin n°124).

Have a nice end of the day.

Nice thanks.

[exxos]

Capture.JPG

PIns 3,132,125 for starters.. but bare in mind you need to set your scope to trigger on RESET.. because those pins will likely only trigger after reset for a few cycles.

[christophe.griard]

Thanks a lot Exxos : I will do so and try to measure properly the signals on these 3 pins.

See you.