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 ...