Atari STE only boot in High Res

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Re: Atari STE only boot in High Res

Post by neanderthal »

And since we are on the learn how the stuff works side of things,here is a typical data sheet for a 257,combine that with the schematic takeout above and you begin to get what it does,pretty much just first page that is of interest here.
Pinout and function table.
In the STE we only use 2 (1 and 2) of the selectors and they are always on.And yea /OE like in the schema TI calls /G.
http://www.ti.com/lit/ds/symlink/sn74ls257b.pdf
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Re: Atari STE only boot in High Res

Post by tzok »

1,3V on any output of TTL chip indicates a bad chip... as /OE is hard tied to GND on this chip all outputs should be "solid" 0 V or 5 V (well, it may be little different, but within TTL accepted range). So it looks like U405 is faulty. Of course 1 Ω resistance between any input and GND also confirms that.
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Re: Atari STE only boot in High Res

Post by joska »

Mr Nours wrote:Here's the backside of the u405 area.
U405.jpg
See the area highlighted in yellow. What's this? It looks like the trace from the videoport to pin 2 and 6 on U405 is shorted to the groundplane. If that really is the case then this is a production error and this STE has never worked with colour monitors.
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Re: Atari STE only boot in High Res

Post by Mr Nours »

Good news here,

I've tested to connect pin 5 (5v) to pin 7 and successfully boot in low resolution ( Gool ol' green desktop ).

So two options :
- Cut the trace to see where it is connected to GND
- remove the U405 an change it ( tzok says 1.3v on ttl is a bad chip... )

What's the best?

Joska, the dot you have seen is like a little impact on the motherboard, do you think that this where the pin4 of monitor connector is grounded?

Neanderthal, thanks for the documentation.Now I know the refs for ordering a brand new chip if i need it :)

Thanks folks, where a near the victory :)
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Re: Atari STE only boot in High Res

Post by joska »

Mr Nours wrote:So two options :
- Cut the trace to see where it is connected to GND
- remove the U405 an change it ( tzok says 1.3v on ttl is a bad chip... )
Cut pin 6 and measure resistance between the pin and GND, and the pad and GND. But yes, it's a good chance that the chip is broken.
Mr Nours wrote:Joska, the dot you have seen is like a little impact on the motherboard, do you think that this where the pin4 of monitor connector is grounded?
Use a sharp knife and make a cut between the groundplane and trace at this point just to make sure there's not a short here. I would start with this.
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Re: Atari STE only boot in High Res

Post by frank.lukas »

Pin4 is the 32Mhz Buffer Mainclock Output. So the 1,3V are OK but the chip is faulty I think. The Pin1 is when a SM124 connect low (0V) and high (5V) if no Monitor or a RGB Display is connected and Pin7 must change.
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Re: Atari STE only boot in High Res

Post by Mr Nours »

Ok,

With a cutter i cut the trace between video pin 4 and pin 2 and 6 of the u405.
pin 2 and 6 has continuity with GND. Pin Video 4 has not.
I cutted the link between pin 6 and 2.
Pin 2 has continuity with GND, Not 6.

Well, i had cutted deeper and always pin2 connected to GND. I i checked the schematics, it should not be grounded for me...
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Re: Atari STE only boot in High Res

Post by joska »

Sounds like the chip is dead.
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Re: Atari STE only boot in High Res

Post by Mr Nours »

I agree!

I will order a new chip if i found a seller.

Meanwhile, i had soldered u405 pin 5 to u405 pin 7 to force the ST to boot in low res, works well.

Thanks for your help, i had learned a bit about how our atari detect monitors and auto set their resolution properly.

I will update this thread if i successfully replace the U405 chip.
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Re: Atari STE only boot in High Res

Post by frank.lukas »

There are run 32Mhz over the chip. It must be fast like a 74S257 or a 74F257 ...


Use the Google Translator: Der Computer erkennt keinen Farbbildschirm

Im 1040STE sitzt normalerweise ein 74S257 am Monochrom-Detect Ein-
gang und zwar ohne Pullup-Widerstand am Eingang. Geht beim S-Typ
noch gut, der erkennt bei offen (Farbbetrieb) H-Pegel). Beim F-Typ
(von Atari eingebaut in einige 1040STE) wird bei offenem Eingang L
erkannt, diese 1040STEs machten immer nur monochrom, bis zum Ein-
löten eines Pullups (10 Kiloohm Widerstand zwischen die Pins 2 und
16 einlöten)).

Ich habe das jetzt schon oft genug gesehen, daß der 74S257 nen Teil-
ausfall hat, einmal war z.B. nur noch externer Takt (über Genlock)
möglich, einmal "klemmte" der Rechner in Farbe, ein weiterer Rechner
"klemmte" in Monochrom, d.h. das Mono-Detect-Signal am Ausgang hatte
nix mehr mit dem Pin an der Monitorbuchse zu tun. Die für den Takt
zuständige Hälfte des Chips war dabei jedoch in Ordnung, in der
"geklemmten" Betriebsart lief der Rechner tadellos...

Was macht der Rechner beim Reset?
Wenn ein weißer 60Hz-Bildschirm zu sehen ist, WÄHREND Reset gedrückt
wird (das TOS startet ja immer in Mono und fragt erst nach 'ner Weile
den MonoDetect ab), dann solltest Du Dir den 257 mal ganz genau
ankucken. Kannste ja auch mal mit 'nem Multimeter testen, wenn der
Pegel an Pin 7 des 257ers mit und ohne Monomoni gleich ist, kannste
dir schon mal nen neuen besorgen.
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Re: Atari STE only boot in High Res

Post by neanderthal »

frank.lukas wrote:Pin4 is the 32Mhz Buffer Mainclock Output. So the 1,3V are OK but the chip is faulty I think. The Pin1 is when a SM124 connect low (0V) and high (5V) if no Monitor or a RGB Display is connected and Pin7 must change.
Yeap,fault tracing with a multimeter is a bit dodgy like that.One has to think that on a live machine there are clocks and bus-stuff going all the time so the only thing one can see with a multimeter is the avarage DC voltage on the place where you measure.Why I normally pull out my scope when doing fault tracing.Then one can 'see' whats going on instead of semi mystical dc-levels :)
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Re: Atari STE only boot in High Res

Post by tzok »

If this 1,3 V has been measured using multi-meter, then it is not meaningful at all...
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Re: Atari STE only boot in High Res

Post by Mr Nours »

Hello all, :D

Some good news and some bad news...

Good news is that i had received the new u405 chip... i have removed the old one, put a socket, insert the new one and fixed( see photos ) the traces that i had once cutten for testing. Liike a butcher, but seems ok with my multimeter....

Yay! The automatic screen detection now works just fine.
So thanks you for all your help, this was the solution.... changing the bad u405 chip.

Now come the bad new : Now images flickers as hell, seems that by changing the old chip i had triggered a new problem.

Do you have an idea where to look to fix this one?


Here"s somes photo :

Image
Initial modification, with cutted traces and old chip. Works fine, there were no sync problems with this one. But no screen detection.

Image
New chip fitted in his socket in all his glory.

Image
Modification of the traces to re-enable auto detection of the monitors kinds. Yes, this is butcher work, but this must do the same than the old traces.

Image
Low Res sync problem.

Image
High res Sync problem.
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Re: Atari STE only boot in High Res

Post by joska »

Maybe frank lukas has the answer here:

http://www.atari-forum.com/viewtopic.ph ... 24#p394318
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Re: Atari STE only boot in High Res

Post by Mr Nours »

frank.lukas wrote:
joska wrote:
Mr Nours wrote:This is a 74LS257
It is a 74S257 or a 74F257 ...
Thanks joska, you mean that i ordered a bad reference?

The new chip is labelled DM74LS257BN, this seems good to me to replace a 74LS257... no? :shrug:
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Re: Atari STE only boot in High Res

Post by czietz »

Didn't you write yourself earlier: "the U405 is labelled as a a 74S257N"? "LS" is not equal to "S". "LS" is much slower. Since the 32 MHz clock passes through this IC you need to use reasonably fast logic family.
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Re: Atari STE only boot in High Res

Post by Mr Nours »

Thanks, i understand my error! Quite dumb... bad ref.
So the poor chip sn't fast enough so it causes flickering!

Hopefully i had socketed this chip, i will then just had to order a good chip!

This one seems ok :
https://www.littlediode.com/components/74S257N.html <-it is not, this were i ordered...
Last edited by Mr Nours on Sun Mar 15, 2020 2:13 pm, edited 1 time in total.
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Re: Atari STE only boot in High Res

Post by Mr Nours »

Well,

I had a doubt, so i had checked my last order, and i do ordered a "74S257" @ littlediode.com
They send me a 74LS257 chip instead... wich seems no compatible at all...

Well... do you guys know where i could order a "real" 74S257 ?

Edit :
Found here by pack of 10 ... https://www.ebay.fr/itm/SN74S257N-TI-Qu ... 1673367460

Does this seems ok to you? This labelled SN74S257 instead of 74S257 but they says this is a "full" equivalent... :roll:
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Re: Atari STE only boot in High Res

Post by czietz »

Maybe you should complain to the original seller then, since you bought a 74S257. "SN" is just a manufacturer prefix (for Texas Instrument), so yes SN74S257 and 74S257 are interchangeable.
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Re: Atari STE only boot in High Res

Post by Mr Nours »

Yes, I made a complaint and ordered a bunch of ten 74S257 chips on ebay.

As always i will keep you informed when i received and replaced this chip.

I hope this will be the end of this topic and a new beginning for this STe :)
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Re: Atari STE only boot in High Res

Post by Mr Nours »

Hello,

I made a complaint to the seller who promise me to send me the right chip. But i received the ten ones before...

So as I had received the new chip , i removed the old one with bad ref and put the new 74S257 ( ref : SN74S257N ) in his socket.

and...

Booted with a a mono screen :

Image

Booted with a a color screen :

Image

It is just working now!

I must thanks you all for your help, i learned a lot. ( I just apologize for the quality of my soldering work... i must practice. :oops: )

I hope this topic will help some people with the same problem in the future.

Cheers :cheers:
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