Repairing an ST with dead floppy & ACSI

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v01d
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Repairing an ST with dead floppy & ACSI

Postby v01d » Mon Oct 08, 2018 9:46 am

Hey everyone.

I have two STs. One is my original that is no longer working, and another I bought more recently which is. I'm using the working one to try and troubleshoot and figure out what needs replacing in the busted ST.

The busted ST has a dead PSU, and I'm fairly sure it took out some other parts on its way to the afterlife. Floppy drive is one of those, as it doesn't work in the working ST either.

With a working PSU and FDD in the busted ST, I go instantly to the GEM desktop (TOS 1.02), completely skipping the typical floppy checks on boot. The A/B disk icons appear, but when trying to open it just says it failed and the drive doesn't spin or flash. The UltraSatan also is not accessible, although not unexpected with it failing to load the driver. But that does suggest to me the ACSI and FDD are both dead. I don't know the hardware well, do these both go through the same controller or something?

The main board looks fine, I can't see anything visually wrong with it. Everything seems to be in order, otherwise, such as the mouse, keyboard and video output in GEM.

Does anyone have suggestions as to what I should try next? Thanks in advance!

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Re: Repairing an ST with dead floppy & ACSI

Postby Arne » Mon Oct 08, 2018 12:56 pm

v01d wrote:I don't know the hardware well (...)
Does anyone have suggestions as to what I should try next?


Yes... definitely: do not try anything without proper knowledge! Or do you dare to repair your TV set?
Give it to someone capable of repairing it.
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Re: Repairing an ST with dead floppy & ACSI

Postby mpattonm » Mon Oct 08, 2018 1:14 pm

DMA controller is common component to both ACSI and FDD and actually might be the one to blaim. But certainly, remote debugging can be a little tricky ;) WD1772 floppy controller is another suspect, they are easy to kill and actually it could permanently pull down data lines (shared with ACSI port), if dead.
Judging from a fact you did not even consult a schematic you do not have much of the experience, so you might want to start with simpler one and do a blind swap of WD1772. They go fairly cheap on Aliexpress, or eBay sometimes so I would suggest you get a new one, rather then desoldering one from your other ST. Even nice big DIP package of WD1772 can be difficul to desolder.

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Re: Repairing an ST with dead floppy & ACSI

Postby v01d » Tue Oct 09, 2018 11:39 am

Arne wrote:Yes... definitely: do not try anything without proper knowledge! Or do you dare to repair your TV set?
Give it to someone capable of repairing it.


Totally fair comment, but where's the fun in that? ;) And, yes I do dare. I'll be the first to admit I'm a complete noob at this, but the options are: let it sit collecting dust, sell it for nothing considering it's a dead board, or have fun and learn something. I want to learn. :)

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Re: Repairing an ST with dead floppy & ACSI

Postby v01d » Tue Oct 09, 2018 11:49 am

mpattonm wrote:DMA controller is common component to both ACSI and FDD and actually might be the one to blaim. But certainly, remote debugging can be a little tricky ;) WD1772 floppy controller is another suspect, they are easy to kill and actually it could permanently pull down data lines (shared with ACSI port), if dead.
Judging from a fact you did not even consult a schematic you do not have much of the experience, so you might want to start with simpler one and do a blind swap of WD1772. They go fairly cheap on Aliexpress, or eBay sometimes so I would suggest you get a new one, rather then desoldering one from your other ST. Even nice big DIP package of WD1772 can be difficul to desolder.


Ah, cool. Thanks heaps for the info.

With my level of electronics I'm more likely to infer the wrong thing from a schematic, so I prefer to hear from someone with experience, and I think they're more likely to point me in the right direction, despite remote debugging ;)

Can I get useful info using a multimeter to check the data lines of the WD1772? I really can't expend the cash on an oscilloscope for a fun project.

I do understand I'm asking a lot from people who don't even know me, so I don't expect anything at all from people here; I just want to learn and if I manage to fix my original ST board, that'd freaking awesome. :D

Blind swap seems like the go, though, especially if they're not too expensive.

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Re: Repairing an ST with dead floppy & ACSI

Postby mpattonm » Tue Oct 09, 2018 2:04 pm

Well you can measure the impedance on each data line against ground, they should not be 0 ohms. Also you can measure them between each other, again there should be at least couple of kOhms, certainly not short. Thats as far as you can get with just multimeter.

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Re: Repairing an ST with dead floppy & ACSI

Postby Greenious » Tue Oct 09, 2018 3:07 pm

I'd just like to add that you don't need a state of the art oscilloscope to troubleshoot these old computers.

There are cheap PC based ones to be found at aliexpress, banggood and other chinese sites, aswell as cheap older used ones.

Depending on where you live, you might also have an active computerclub or ham radio club nearby, they often have meetings where they tinker electronics and sometimes equipment you can borrow. It's also an excellent way of meeting people that knows electronics and it's a lot easier learning that way than from forums.
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v01d
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Re: Repairing an ST with dead floppy & ACSI

Postby v01d » Sat Oct 13, 2018 2:23 am

mpattonm wrote:Well you can measure the impedance on each data line against ground, they should not be 0 ohms. Also you can measure them between each other, again there should be at least couple of kOhms, certainly not short. Thats as far as you can get with just multimeter.


I checked the data lines on the 1772, all were ~30M ohms. Nothing weird between each of them, either.

The only odd thing was pin 3 (3rd data line on the memory side) on the DMA chip at 20M ohms, and pin 1 at 4.7K, yet all other lines were 10K (these are the lines on the HDD and 1772 side). This was weird but perfectly identical to my working ST. Interesting side note, they're identical revision main boards, and appear to be quite hard to find info on specifically (C070789-001 Rev D). Schematic for Rev C seems to be close enough.

My DMA chip is C025913-38 PH23-030, but I haven't been able to find it online (it is in the working ST of course, heh).

Can I swap it out for an IMP C100110-001? I can't seem to find confirmation that these two chips are compatible, or with my board revision.

Other findings so far are that no chips appear to be getting hot, and trying to access the floppy _does_ actually spin it up for a short while, but instantly shows the error "The data on the disk in drive A: may be damaged", etc.

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Re: Repairing an ST with dead floppy & ACSI

Postby v01d » Sat Oct 13, 2018 2:24 am

Greenious wrote:I'd just like to add that you don't need a state of the art oscilloscope to troubleshoot these old computers.

There are cheap PC based ones to be found at aliexpress, banggood and other chinese sites, aswell as cheap older used ones.

Depending on where you live, you might also have an active computerclub or ham radio club nearby, they often have meetings where they tinker electronics and sometimes equipment you can borrow. It's also an excellent way of meeting people that knows electronics and it's a lot easier learning that way than from forums.



Hey, that's true. Thanks for the advice.

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Re: Repairing an ST with dead floppy & ACSI

Postby rcamp48 » Wed Oct 17, 2018 2:20 am

Dead floppy is my problem , Greenious: I tried t adjust the speed screw with a magnetic screwdriver and it sorted out the case and the metal screw: Is this fixable not?

Russ


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