My Falcon went to black screen

[christophe.griard]

Hello,

I started to have a look at the reset circuit.

As a start, the reset switch.
Below are the resistances values I measured across the 3 pins of the reset switch:
- when the reset switch is not pressed:
- between pin 1 (the most à the rear of the Falcon) and pin 2: 0 Ohms
- between pin 1 and pin 3 : about 10 kOhms
- between pin 2 and pin 3 : about 10 kOhms
- when the reset switch is pressed:
- between pin 1 (the most à the rear of the Falcon) and pin 2: 10 kOhms
- between pin 1 and pin 3 : about 10kOhms
- between pin 2 and pin 3 : 0 Ohms

At least the switch seems to works but I don't know if the 10kOhms is enough to make the motherboard understand there is a reset...

For the next steps in testing the reset circuit, I would like to have a look to :
- first, the U1 Chip (LH 555)
- second, U2 and U11 chips (7406)

But I have a question to test these components : as they are under the PSU, and as the ground is brought to the motherboard by the metal socket of the PSU that is screwed on the motherboard, I wanted to know how to test these two components avoiding a new failure due to a ground not properly set for the motherboard.

Thanks a lot in advance for your help.

Christophe.

[sety]

But I have a question to test these components : as they are under the PSU, and as the ground is brought to the motherboard by the metal socket of the PSU that is screwed on the motherboard, I wanted to know how to test these two components avoiding a new failure due to a ground not properly set for the motherboard.

I've run it with the PSU outside of the case before and it's worked fine. :)

You just need to be careful as the lead isn't very long. I normally put something under it to insulate like a mouse pad just incase the PSU falls back onto the board.

[christophe.griard]

Hello Sety,

Thanks for the advice. I will proceed carefully as proposed.

Do you confirm that in normal operation pin 17 (reset) of expansion port should be at +5V (and not at 0V; in my case, just after the reset I see a +5V than goes rapidly to 0V)? Just to prevent me from looking for something not relevant...

Thanks for your help Sety.

Have a nice day.

[PhilC]

Hi @christophe.griard , you'll find that all /resets and /halts are connected together as the reset is applied to all chips at the same time.

[christophe.griard]

Thanks PhilC. Understood for the reset to all chips.

But when the reset button is not pressed, should I find +5V on the pin 17 of the expansion port if my Falcon was operating correctly? Just for me to know if I should find why the reset signal is not the correct one, that is to say have a look at the whole reset circuit.

Thanks for your help PhilC

[sety]

Yup. All of those are active low so they should be TTL high when things are normal. Honestly, I would just change out U11 and U26 anyway because they're really easy to find and pretty easy to change.

According to a very old post on AF by Simbo, it's recommended you change U24 at the same time as U11. He said that one can damage the other and vice versa. I don't know how accurate that is, he hasn't been active for a very long time and I haven't encountered the problem myself. But It's not hard to do.. You can socket U24 and ST ACIA will work in the Falcon so they're not hard to find. :)

Cheers

[christophe.griard]

Thanks a lot Sety.

Now that I am sure the reset circuit is not operating correctly, I will go further and I will check the whole reset circuit and especially the chips you mentioned (U11, U26 and U24).

I keep you uptodate once done (I will do it this afternoon).

Nice thanks.

[sety]

You are very polite, and you love your Atari. I like you. :)

[christophe.griard]

Hello Sety,

Oh... It is very kind of you... Yes I love this beauty even if it make anxious to see it being sick...

And I very much appreciate your help (and the help of the other members) : it is very kind... And I like you too... :)

With your help, I will do my best/our best before asking Mr. Czuba if he can help me: this will be the last chance if we do not manage to revive my bottle ... I do not want to bother him without having done my best/our best to solve the problem before asking for his help ... And maybe he will not be available or able to help me...

As promised in my previous message, I started to have a look to the reset circuit. Below are my first observations, observations I submit to your insight : I started to have a look U1 (LM555 timer):

- below is current measured on the 8 pins of the chip when the reset button is not pressed:

---- 5v-------0v--------0v------3,36v
-----|______|______|______|
+----------------------------------------+
|---|------------------------------------|
|---|-----------LM555-----------------|
|---|------------------------------------|
+----------------------------------------+
-----|______|______|______|
-----0v-------5v ------10mv----5v

- below is current measured on the 8 pins of the chip when the reset button is pressed and released:

--------------3,36v----3,36v
---- 5v------then 0---then 0---3,36v
-----|______|______|______|
+----------------------------------------+
|---|------------------------------------|
|---|-----------LM555-----------------|
|---|------------------------------------|
+----------------------------------------+
-----|______|______|______|
----0v--------0v------4,4v------5v
--------------then----then
---------------5v-----10mv

Then I had a look to U11 (7406) chip:

- below is the curent on the 14 pins of U11 chip when the reset button is not pressed :

-----5v------- 5v------90mv----90mv----80mv--80mv---80mv
-----|______|______|______|______|______|______|
+-------------------------------------------------------------------+
|---|---------------------------------------------------------------|
|---|---------------------------7406------------------------------|
|---|---------------------------------------------------------------|
+-------------------------------------------------------------------+
-----|______|______|______|______|______|______|
----10mv----5v-----10mv------5v-------5v-----80mv-----0v

- below is the curent on the 14 pins of U11 chip when the reset button is pressed and released:

-----5v------- 5v------90mv----90mv----80mv--80mv---80mv (same as when the reset is not pressed)
-----|______|______|______|______|______|______|
+-------------------------------------------------------------------+
|---|---------------------------------------------------------------|
|---|---------------------------7406------------------------------|
|---|---------------------------------------------------------------|
+-------------------------------------------------------------------+
-----|______|______|______|______|______|______|
--- 4,4v----110mv---4,4v-----110mv---5v------80mv-----0v
----then----then-----then-----then
----10mv----5v-------10mv-----5v

At this stage, I am reaching my skills limit to understand if these two chips work properly. It seems for me that U1 works correctly (not sure; could you confirm?) but for U11 no idea at all at this stage...

Another question: could you confirm that if Dallas is out of order, the Falcon would boot (with english keyboard, time been reset, etc.)?

Thanks a lot again for your help...

Have a nice evening.

PS: sorry for the small chips sktech (I had top replace spaces signs by '-' to be more understandable...

[frank.lukas]

You can try to Reset the Dallas chip when the Falcon is powered up. There are two Reset pins. Pin18 /Reset and Pin21 /RCLR. Put these pins to GND for a little moment of time and put the gerneral reset button. Switch not the power off ...