Mega STE, white screen in colour mode or TV, black screen in high res

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Chandler
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Mega STE, white screen in colour mode or TV, black screen in high res

Postby Chandler » Fri Jul 05, 2019 7:03 pm

Another motherboard i've been trying to repair lately is this Mega STE PCB

All it get is, a white screen (with black lines which look like they are moving / scrolling) when connected to either a TV or colour monitor

or black screen when connected to high res

No floppy drive activity

ALL socketed IC's are tested ok and the sockets are ok, ram, TOS,CPU, floppy IC ok

PSU OK, I have even changed the U501 glue SMD IC

Any help appreciated :)

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby czietz » Fri Jul 05, 2019 7:24 pm

Always the first thing to check: Diagnostic cartridge output over the serial port. Then go to the Service Manual, section "Troubleshooting a dead unit".

Since you mention changing the GSTMCU/Combo IC U501: Be aware that in some cases (like in my MegaSTE), the IC needs to be installed differently than you might think at first: the marking on the IC that denotes pin 1 must point to the "pin 73"(!) marking on the mainboard.

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby czietz » Fri Jul 05, 2019 7:33 pm

PS: What I wrote above applies when installing the Combo IC C302183:
combo.PNG
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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby Chandler » Fri Jul 05, 2019 7:41 pm

Hi

Thanks for your reply

Yes i know this about U501 and i have installed the IC correctly

The early type IC C300589-001 the IC marking relates to pin 1

The later IC C302183-001 fits the opposite way around

I have tried using a test cartridge but seeing as there is no desktop it's not of much use

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby czietz » Fri Jul 05, 2019 8:04 pm

The test cartridge will output to the serial port (9600, 8N1) even if there is no screen display at all.

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby Chandler » Sat Jul 06, 2019 8:55 am

So how do you set this up then? how do i get to read any data from the serial port

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby czietz » Sat Jul 06, 2019 9:16 am

Use a null-modem cable and connect the defective Mega STE to another machine with a serial port, e.g., another Atari. Start a terminal program on the other machine, set the serial port on that machine to 9600 bit/s, 8 bits per character, no parity, one stop bit ("8N1"). Switch on the Mega STE with the diagnostic cartridge inserted. See if there's any output in the terminal program.

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby Chandler » Tue Jul 09, 2019 6:31 pm

Can you tell me what is this terminal program? is that the one that comes with the Language disk? VT52

Also what's the exact pinout for this cable? it says in the Falcon service manual pin 2 of (U.U.T) to pin 3 (of terminal) and vice versa and 5 to pin 7

So is this pin 5 of U.U.T to pin 7 of the terminal?

Nothing is clear

Also which oscillator is for the CPU on the Mega STE?

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby czietz » Tue Jul 09, 2019 8:19 pm

Yes, you can use the VT52 terminal accessory that comes with the language disk. Use the Control accessory to set the serial port to 9600 8N1, as I said. (Or you can use any of the ton of terminal programs that are available for the ST.) As for the cable: It's a standard null-modem cable. The internet will have the pinout.

As with every ST, the 32.x MHz crystal is the one the CPU clock is derived from. Goes through the Shifter and the GSTMCU to get 16 MHz and 8 MHz. PAL U12 chooses which of the two frequencies is applied to the CPU. It defaults to 8 MHz at boot.

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby Chandler » Tue Jul 09, 2019 9:29 pm

OK so been playing with a couple of other terminal programs (TAZ and FLASH) and a null modem cable and i have the set up working using a working ST to test the set up

So i connected the Mega STE mobo and switch it on and...nothing

No text or data on the screen even after pressing reset a few times

When i used a working ST with test cart i had the full page test cart text on screen so i know my set up works

So guess it's back to as the service manual says in "trouble shooting a dead unit"

To be honest after changing the OSC / Crystal i can't do anything else without a scope

Can anyone recommend a scope that will work for this kind of diagnosis?

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby Chandler » Wed Jul 10, 2019 7:32 am

Looking at the crystals / Osc on the board there are 4 in total

U408 - C398767- 001 / 16.021226 MHZ
UA12 - C398107- 001 / 3.672MHZ
Y202 - C070176 - 001 / 32.084988
Y301 - C025993- 001 / 2.4576

Also another question i have is will a regular STE 8MHZ CPU work in the Mega STE?

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby frank.lukas » Wed Jul 10, 2019 8:19 am

I have the same problem with my Mega STE.

The CPU are not working. So I have to check CLK, RESET and HALT on the CPU. I think one of the three 22V10 PAL/GALs are faulty, I not know?
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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby czietz » Wed Jul 10, 2019 9:31 am

A 8 MHz CPU will work as only as you don't switch to 16 MHz. So, for testing purposes it's OK to use it.

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby Chandler » Wed Jul 10, 2019 8:45 pm

frank.lukas wrote:I have the same problem with my Mega STE.

The CPU are not working. So I have to check CLK, RESET and HALT on the CPU. I think one of the three 22V10 PAL/GALs are faulty, I not know?


Hey Frank

Do you get a white screen with black limes moving using a colour monitor or TV port?

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frank.lukas
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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby frank.lukas » Fri Jul 12, 2019 1:07 pm

I have no TV. With ST-LOW I have a white screen and on ST-HIGH a black screen.
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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby Chandler » Fri Jul 12, 2019 6:09 pm

OK Frank, same as me then!

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frank.lukas
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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby frank.lukas » Fri Jul 12, 2019 6:45 pm

Hello Chandler, my Atari Mega STE works again with help from czietz. A PLCC CPU socket pin (D12) was bent and have no longer contact to the CPU.

-> https://forum.atari-home.de/index.php/t ... #msg242691
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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby Chandler » Sat Jul 13, 2019 9:29 am

I have already checked all PLCC sockets are they are clean with no bent pins, chips also are clean with no bent pins

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby czietz » Sun Jul 14, 2019 10:12 am

In Frank's case, the socket also passed visual inspection. However, he was able to capture some traces with his logic analyzer that led me to conclude that the CPU was executing wrong instructions. With that information (and a tiny bit of luck) I was able to make the guess that some pin didn't make contact. And I was right.

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby siriushardware » Thu Jul 18, 2019 10:27 pm

I had problems with an STE which had been put away working, was dusted off after about 15 years, worked for about 20 minutes from switch-on and then died. Long story cut short, it was due to pins on the square CPU socket failing to make contact with some of the corresponding contacts on the CPU, although this was not visually obvious and the fault even survived removal, inspection, cleaning and reinsertion of the CPU.

The sockets chosen by Atari have a non-standard 'weird' pinout which is different to that of the majority of easily available PLCC sockets, so this is an added complication - I bought a genuine Atari socket from Best over in the USA, at great expense. Shortly after that, Exxos produced a useful little adaptor PCB which allows a 'normal' easily available PLCC socket to be used in Atari PCBs.

But anyway... assuming the availability of a scope and a needlepoint probe, a steady hand and a bucket load of patience, one way to diagnose or rule out socket problems is to start at pin one on the chip, look at the signal, then move to the pin 1 contact on the socket - both are accessable next to each other from the top side, but the probe point must be fine enough for you to be able to choose whether to monitor the pin or the socket contact, not both at the same time. Don't put any pressure on either the pin or the socket contact, use only the lightest touch needed to get a picture of the signal.

Essentially, if you see the same 'picture' on each pin and its corresponding socket contact then they are OK, but if you find any where the 'picture' on the pin and the 'picture' on the corresponding socket contact are different, that pin / socket contact is faulty.

The inherent problem with the PLCC sockets is that the four sides have a lot of pressure applied to the insides by the springy pins of the IC, but there is no equal and opposite pressure on the outsides of the four sides of the sockets so over time, the sides of the sockets are prone to bow slightly outwards and the connections in the middle of each side in particular may lose their connection.

Chandler, time you had a scope. Even an old 20Mhz or 40Mhz analogue CRT scope (Hitachi V212 / V412 or Hameg HM-203, etc), would be better than not having one at all.

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby Chandler » Fri Jul 19, 2019 9:22 am

Thanks for your detailed response

regarding the scopes

Would the Hitachi V212 be of any use for the Falcon?

I am looking for a scope, would either of these on ebay be any good?

https://www.ebay.co.uk/itm/Dual-Channel ... 3527130792

https://www.ebay.co.uk/itm/OWON-Vds1022 ... 2976441933

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby frank.lukas » Fri Jul 19, 2019 11:13 am

Has your Mainboard two patchwire?

These going to U905 a 74LS244 chip.
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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby Chandler » Fri Jul 19, 2019 6:07 pm

no patch wire

only a factory green wire underneath going from 1 large chip to another

What's the patch wire you mention for anyway?

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Re: Mega STE, white screen in colour mode or TV, black screen in high res

Postby frank.lukas » Fri Jul 19, 2019 7:12 pm

There are in some Boards is a buffer ...

U905.png


When the 74LS244 is faulty there is no longer a clock
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