Atari 520ST black screen with white border

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mattsoft
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Atari 520ST black screen with white border

Post by mattsoft »

Hi all -

Picked up a 520ST (North American version) recently and it only displays a black screen with a white border on an SC1224 RGB monitor. No sound of keys clicking when typed on keyboard. Power LED is on.

I've checked the PSU voltage and all rails are good. No hot chips on the mainboard. DRAM chip voltages are all roughly +5VDC. I've removed, cleaned, and reseated all socketed chips. Same problem. Gonna bring out the logic probe this weekend and check the DRAMs.

Any other ideas? Thanks!
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Arne
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Re: Atari 520ST black screen with white border

Post by Arne »

mattsoft wrote:Gonna bring out the logic probe this weekend and check the DRAMs.
Good idea. As it seems that all data lines are affected I suggest to look for unusual behaviour on /RAS0 (if it is a "real" 520 with 512KB) otherwise on /RAS1 if it got a memory upgrade using the 2nd bank.
Additionally you may check if GLUE asserts /ROM2 accessing the lower TOS ROMs.
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mattsoft
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Re: Atari 520ST black screen with white border

Post by mattsoft »

Arne wrote:
mattsoft wrote:Gonna bring out the logic probe this weekend and check the DRAMs.
Good idea. As it seems that all data lines are affected I suggest to look for unusual behaviour on /RAS0 (if it is a "real" 520 with 512KB) otherwise on /RAS1 if it got a memory upgrade using the 2nd bank.
Additionally you may check if GLUE asserts /ROM2 accessing the lower TOS ROMs.
Thanks! Yeah, it's a stock 512kb model. RAS0 on all DRAMs are pulsing OK. ROM2 on the GLUE is high. Any other ideas?

The big 470uf 16v cap in the bottom right looks like it has some white stuff coming out of the anode. I've got a replacement on order. All I have is a multi for checking capacitance and this big one in the bottom right seems stuck at 0.0 -- doesn't seem to take a charge from the multi. Hrmmm...
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Arne
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Re: Atari 520ST black screen with white border

Post by Arne »

mattsoft wrote:ROM2 on the GLUE is high.
Is it constantly high - all the time? There should be at least four low pulses (CPU fetching 32bit SP and 32bit PC = four 16bit accesses).
Check the PCB trace of /ROM2 and look at Glue's PLCC socket if that pin has contact.
I have no SC1224 at hand. Try pulling all TOS ROMs and see if the screen (black with white border) is the same without ROMs. This could be an indicator that the ROMs aren't accessed at all.
This is how /ROM2 looks on my 1040STFM without HI-0/LO-0 ROMs to keep it from booting:

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mattsoft
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Re: Atari 520ST black screen with white border

Post by mattsoft »

Arne wrote:
mattsoft wrote:ROM2 on the GLUE is high.
Is it constantly high - all the time? There should be at least four low pulses (CPU fetching 32bit SP and 32bit PC = four 16bit accesses).
Check the PCB trace of /ROM2 and look at Glue's PLCC socket if that pin has contact.
I have no SC1224 at hand. Try pulling all TOS ROMs and see if the screen (black with white border) is the same without ROMs. This could be an indicator that the ROMs aren't accessed at all.
This is how /ROM2 looks on my 1040STFM without HI-0/LO-0 ROMs to keep it from booting:

Image
Yes, ROM2 (pin 19) of the GLUE is always high. No pulsing.

This is a 6 ROM 520ST (CO26160-CO26165). If I removed all 6 ROMs, the screen is now white. If I put CO26165 (hi-0) and CO26162 (lo-o) back in, I get the black screen with white border. I'll remove the GLUE again and double-check all the pins. I recall everything looking OK, but I'll doublecheck.
mattsoft
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Re: Atari 520ST black screen with white border

Post by mattsoft »

I've double-checked the GLUE PLCC, all pins are making contact. Pin 19 of the GLUE /ROM2 has continuity to pin 20 on both U7 and U4 (hi-0 and lo-o) ROMs.

I'm wondering if the GLUE is bad, or maybe the ROMs. I'm trying to get a working 520ST mainboard so I can swap the GLUE and see what I get.
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Arne
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Re: Atari 520ST black screen with white border

Post by Arne »

I forgot to mention:
what about /RESET and /HALT on the CPU? These should go low when pressing the Reset button and then back to high.
Is there activity on CPU's /AS, /UDS and /LDS lines when releasing the Reset button? CPU will try to read SP from address $0 first so it is a typical read access. You may check some 68000 online 'library' how that looks like. Glue should acknowledge that SP fetch access with /DTACK.
Does Glue get 8MHz clock on pin 34? And does it generate 2MHz (pin 54) and 500kHz (pin 43)? Is there activity on HSync/VSync/DE/BLANK?
Check the whole clock tree with 32MHz running into Shifter and so on.
What about pin 22 of all ROMs? These should have continuity to either /UDS (HI-x) or /LDS (LO-x).
And of course Glue needs to monitor all necessary control lines (/AS, /UDS, /LDS, R/W, CLK8) and address bus to generate /ROMx signals. Check for continuity to CPU.
So at least you can verify if Glue is (partially) working. The Service Manuals all contain a section "Troubleshooting a dead unit". You can get the PDF from my homepage.
Hope that's enough input for now. :coffe:
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mattsoft
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Re: Atari 520ST black screen with white border

Post by mattsoft »

Awesome. Thanks Arne, I will check all those out! MUCH appreciated!!!
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Re: Atari 520ST black screen with white border

Post by mattsoft »

Welp, it looks like the CPU is halted. /RESET is high and blips low on reset while /HALT is low then blips high on reset. /BERR is always high regardless which looking at the manual implies that the reset circuit is working OK. The board looks super clean, but maybe a bad trace/joint somewhere. I'll check all continuity between CPU, ROM and GLUE to be sure. Weee!
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Arne
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Re: Atari 520ST black screen with white border

Post by Arne »

/RESET and /HALT should go high in conjunction after reset button was released. I will take a screenshot of my scope with these signals tomorrow.
Check the clock tree. Maybe MMU does not output 8MHz or does not receive 16Mhz from Shifter or....
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Arne
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Re: Atari 520ST black screen with white border

Post by Arne »

So here it is: /RESET and /HALT after reset.

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As you can see /RESET goes low after approx. 7µs. The next low-phase of /RESET is triggered by TOS. Second instruction is a Reset instruction. http://info.sonicretro.org/SCHG:68000_Instruction_Set That is the reason why http://www.atari-forum.com/viewtopic.ph ... 14#p391669 is a pretty bad idea. :roll:
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