MegaSTE repair

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atari030
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Joined: Mon Feb 12, 2018 12:43 pm

Re: MegaSTE repair

Post by atari030 »

Cheers Steve. It is the mongrel square DMA. I have reflowed the legs, I was also thinking of changing out the mains cap again. The one I got was all I could get in lockdown and I'm not convinced it's much chop. The ground cap is a good idea too. Thanks.
atari030
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Re: MegaSTE repair

Post by atari030 »

I'm going over the DMA chip in the MegaSTE. The schematics are up the wazoo. Thank god the REV 6 schematic is right, I was wondering why the hell the DMA chip had 40 pins. :?

I've dropped a cap and extra ground and it seems a bit happier, demo's get a bit further. Mostly getting 2 bombs booting off an HDD though and xcontrol goes transparent. Compared to a white screen I guess I'm making progress.

Xcontrol Crash.jpg
Xcontrol Crash.jpg (180.53 KiB) Viewed 1644 times
Steve
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Re: MegaSTE repair

Post by Steve »

Have you buzzed out every leg on the DMA to where it is supposed to go? Perhaps change your testing methodology. Just have a clear disk, run the CHECKDISK program for Ultrasatan to see if it corrupts an sd card. Don't have any software installed at all, just clear partition.

viewtopic.php?f=25&t=43
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atari030
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Re: MegaSTE repair

Post by atari030 »

I buzzed a few legs out last night, got to midnight then needed to pack it in. I'll finish it off tonight and revisit the diag cart. Off a clean disk it boots, and I can format a HD floppy without error. I'm using a couple of ICD Links with SCSI drives, no ultrasatan.
Steve
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Re: MegaSTE repair

Post by Steve »

I assume you can confirm there is no issue with these ICD links and/or drives? Just making sure you're chasing the right donkey ;D
atari030
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Re: MegaSTE repair

Post by atari030 »

Luckily I have about 30-40 ST's/Mega's/Falcons/TT's so that wasn't something I had to worry about. :-) I have a few SCSI drives to choose from as well. Perks of working in IT for decades.

Buzzed a bit more out last night now I'm back to white screen. So there has to be a flaky data line in there somewhere. I wish the DMA was socketed.
atari030
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Re: MegaSTE repair

Post by atari030 »

Been a while again. I've invested in some component drawers and tidied up my work area. I never thought I'd get excited about labeling component drawers but here we are. I've got a lot more spares than I thought I did.

OK, so I'm stuck at home and dived into the Mega STE pretty hard in the last couple of days. I've been buzzing it out and made a few more changes. Some quite interesting.

I replaced the RAM and the 17-72 with an Ajax. Interestingly I used non parity RAM and while its not prefect it seems to be a bit happier. That is weird as I had used genuine Atari SIMMs before and it wasn't much of an improvement using those.

I've been buzzing the hell out of the machine and I've got a bit further. The machine locks up after a bit of time, and will after that produce a white screen till it cools off or 4 bombs. I went to town buzzing out U1 because I got a bit suspicious and I'm starting to think its failing.

So my next plan is socket, buzz and replace U1. No one has any IC's locally though so that's a PITA. And I still need to replace the bus resistors. Also none locally, more PITA.

All in all it no longer craps itself on the desktop so getting there I suppose. I want to get this sorted as I have the ATX board to finish, a A1000 that needs some attention and a Mega ST to repair.
atari030
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Re: MegaSTE repair

Post by atari030 »

It's been an interesting day. I found a couple more crappy joints on the blitter. So while I was at it I pulled the blitter and CPU again, used a pin to splay out the socket and cleaned the CPU and blitter again. Stability has gone up 10 fold. While I was poking around with Flippo I noticed that U407 is getting quite warm. I have buzzed out all the PAL's and IC's around the clock's and U1, it looks pretty good so I think U1 is OK. Problem is the crashes aren't consistent when they happen.

Shoving a 12v fan over U407 doesn't seem to make a huge difference but sitting a heatsink on it seems to. That's just bloody weird. If the PAL is cool it will play ball, if it gets warm its too late and a cold boot is required. I've swapped out the PAL and the replacement does exactly the same thing so its not the IC. 5v looks good, sits around 5.1v. I'll go over the schematics tomorrow and see whats what.

But, in the space of a day I can now run a lot more. Fastbobs seems to be OK if the PAL is cool, Gembench works, Yaart seems to be fine even frontbench is doing its job.

One other interesting thing I did find, I was testing some CPU's and blitters in the spares drawer, the original FN12 is rooted and crashes pretty quick. I tried two ST FN8's and they work fine unless I switch it up to 16Mhz then I get the same result as the control panel mess above. All in all a good day at the office.
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Re: MegaSTE repair

Post by exxos »

Those FN8's etc tend to get red hot in the MSTE. It's possible not all CPU's can even cope at 16MHz. Its amazing they run at all TBH. A lot of people swapped to the HC CPU because of that. It may or may not help with your problems..
https://www.exxosforum.co.uk/atari/ All my hardware guides - mods - games - STOS
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viewtopic.php?f=17&t=1585 Have you done the Mandatory Fixes ?
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atari030
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Joined: Mon Feb 12, 2018 12:43 pm

Re: MegaSTE repair

Post by atari030 »

These FN8's were STE pulls. It might be my imagination but I don't think they were as hot @8mhz in the Mega. All of the machines have HC's in them now.

I'm thinking about the blitter and cpu sockets with funky replacements.
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