I borked my 520ST Help?!?

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Blur2040
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I borked my 520ST Help?!?

Postby Blur2040 » Tue Dec 19, 2017 3:54 pm

Posted on Atariage as well, so I can soak up as much Atari wisdom as possible: http://atariage.com/forums/topic/273305 ... arn-fault/ I was more upset when I posted there, so forgive my rambling style.

Executive summary: I borked my DMA chip in my 520 ST, but maybe did worse and borked something else. All it does now is give me a blank screen, gray, sometimes the display area comes up as black (with gray outline) and some wavy lines made up of rectangles moving across the screen.

Any advice/tips/etc? Will it boot at all with a borked DMA chip (which I've ordered)? Or is no activity a sign that I killed the 68000?

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Re: I borked my 520ST Help?!?

Postby Dal » Tue Dec 19, 2017 4:01 pm

The fact you're getting any display at all is good news. A lot has to work to even get any display up - so I'd say the 68000 itself is likely to be fine.
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Re: I borked my 520ST Help?!?

Postby czietz » Tue Dec 19, 2017 5:45 pm

Service manual is here: http://www.dev-docs.org/docs/files/Atar ... Manual.pdf

The troubleshooting procedures described there do however assume that you have access to some equipment: oscilloscope (or logic analyzer) and optimally a diagnostic cartridge.

You have a color monitor, I presume? I ask because switching to a mono monitor only happens sometime during boot, so if you hooked up a mono monitor and still got some display, you could be sure that the CPU is still working. On the other hand, on a color monitor the Shifter will output a signal even if not initialized by the CPU.

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Re: I borked my 520ST Help?!?

Postby siriushardware » Tue Dec 19, 2017 7:51 pm

The DMA IC is quite involved in all sorts of things so I wouldn't expect an otherwise working machine to work in any meaningful way without one in. You're going to have to wait until the replacement arrives before you can make progress, but at least you will have had time to go on a voyage around the circuit diagrams by the time the part arrives.

At work we make some equipment which runs on 12VDC but, due to a mixture of supply voltages in our systems (which was a bad design choice by us in my opinion) end users often accidentally connect it to 24VDC. I'm always moaning that they do this.

Today, I did it myself for the very first time in many years of working on the things, damaging a PCB on which I had just repaired a much more minor fault.

So don't worry, we all have bad days.

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Re: I borked my 520ST Help?!?

Postby Blur2040 » Tue Dec 19, 2017 8:24 pm

Thanks for the replies, gang.

Looking back at my post history here, I posted for the first time in 2006 and have been trying to get this thing working on and off since then. This ST deserves better, darnit. It was just so upsetting to finally (I think) figure it out and them have it all go up in smoke.

Edit/Update: As I mentioned elsewhere, I had a line on a 1040STF for $100 USD...and I purchased it. Got it, a monitor, two extra mice and a TON of random DS/DD floppies--which I need-- so I'm a happy camper. Still my 520 ST is giving me dirty looks, so I'll do right by it as soon as the new DMA chip comes in.

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Re: I borked my 520ST Help?!?

Postby Blur2040 » Sun Jan 07, 2018 5:01 pm

Well, new DMA chip here, no go. Still boots to a white screen with a border, about 20 or 21 dark vertical lines evenly spaced across the screen.

Searching around suggests RAM is the most likely candidate. Seems painful to desolder/ replace em all. Any suggestions on how to narrow it down?

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Re: I borked my 520ST Help?!?

Postby troed » Sun Jan 07, 2018 5:58 pm

Blur2040 wrote:Well, new DMA chip here, no go. Still boots to a white screen with a border, about 20 or 21 dark vertical lines evenly spaced across the screen.

Searching around suggests RAM is the most likely candidate. Seems painful to desolder/ replace em all. Any suggestions on how to narrow it down?


If you have a friend around with a diagnostic cartridge that could indeed help you figure out which chip(s). Otherwise it's difficult.

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Re: I borked my 520ST Help?!?

Postby Blur2040 » Sun Jan 07, 2018 6:10 pm

Well, I wouldn't even know how to find someone with an ST near me. North America isn't exactly ST country.

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Re: I borked my 520ST Help?!?

Postby Wayne123 » Sun Jan 07, 2018 8:41 pm

If it will help Bruce at B & C ComputerVisions has diagnostic cartridges listed on his website. http://www.myatari.com/ataristh.txt
I imagine Best Electronics has them also.
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Re: I borked my 520ST Help?!?

Postby Blur2040 » Mon Jan 08, 2018 5:16 pm

For whatever reason, and despite that my 1040 ST works just fine, I've gone ahead and reached out to Best electronics to get a diagnostic cart. When this is done, I'll be an ST tech guru.

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Re: I borked my 520ST Help?!?

Postby Arne » Mon Jan 08, 2018 6:00 pm

Blur2040 wrote:For whatever reason, and despite that my 1040 ST works just fine, I've gone ahead and reached out to Best electronics to get a diagnostic cart.

Still have some available in my shop.
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Re: I borked my 520ST Help?!?

Postby Blur2040 » Fri Jan 12, 2018 4:17 am

**double post, delete if possible***
Last edited by Blur2040 on Fri Jan 12, 2018 4:20 am, edited 1 time in total.

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Re: I borked my 520ST Help?!?

Postby Blur2040 » Fri Jan 12, 2018 4:19 am

Well, I got my cartridge. It actually does something, which is more than I expected.

Upon power up, it says "I6 Bus Error handling failed"
Next, it moves to a screen with the heading "Testing MFP, Glue Timing, Video" ...and then prints...on the next line..
"T0 MFP Timer"

It makes it no further than that. The screen simply stays locked on that image. No key press will move past the screen. I would also note that there are still vertical bars in the image.

Per the service manual (thanks!) the diagnostic cart also writes to RS232 the same information (or is it more information?) it prints on screen and looks to it for input there as well. Seems that might allow me to push space or another input after the error message is received to move past it...if the keyboard is caught up in the error in my ST. It may be as the referenced MFP does seem to handle keyboard interrupt.

That being said, I'll have to either dig up, build or buy an atari to 9 pin serial adapter to hook up to the RS232.

Any other suggestions? Or is protocol here to just remove/replace the 68901 MFP? (on that note, it seems that I can find tons of 68901P, but I can't find anywhere that tells me what that P is for/if it will make a difference).

***Also, Arne, thank you, and sorry. I had already ordered from Best by the time a saw your post.

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Re: I borked my 520ST Help?!?

Postby Arne » Fri Jan 12, 2018 4:50 am

Blur2040 wrote:"T0 MFP Timer"

Service Manual says that's a problem with the MFP timer(s).

Blur2040 wrote:Or is protocol here to just remove/replace the 68901 MFP?

NO. Protocol is to find the error.
Take a scope, the schematics and check the clocks(2.xMHz and 4MHz), the /CS line and see if /DTACK is activated shortly after.
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Re: I borked my 520ST Help?!?

Postby Blur2040 » Sat Feb 17, 2018 10:08 pm

Long delay, but some progress.

Got a scope. Went on a clock hunt. 4mhz is available on the 68901...pin 35 as I recall, but not the 2.x mhz at 17/18. I was able to easily find that on my 1040st, so I thought I had it. Bought a new crystal and replaced.

Still no love, still no 2.x mhz coming off the crystal. Looking at the schematic, there's not much going on there---just the crystal and two capacitors at c10 and c11. Maybe bad capacitors? Maybe just a bunk crystal? Should I socket the pins for the crystal and try another?

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Re: I borked my 520ST Help?!?

Postby ranix » Sun Feb 18, 2018 12:40 am

replacing the capacitors isn't a bad idea but I would not do it now. It's a destructive operation - it modifies the system under test. What you want is to bring the system into a known state so that you can make modifications to it in order to increase its level of functionality.

right now you have a computer that can't "show you the desktop", or whatever known state you expect a computer to be in before you start using it. You're trying to increase the computer's functionality to return it to that state by repairing random components hoping you can nurse it back to that state. Instead, learn enough about the machine so that you're comfortable operating with the system in the state that it is in now

if you can understand what's going on physically on the motherboard with your scope, and if you can know which chips are receiving power and twiddling their outputs as they should, and you can read the output of your diagnostic cartridge, then your computer will be in a known state and you will be comfortable making any modification to it that you want. You should have all the tools you need now

one or more of the components on the motherboard is going to be bad, or there is going to be one or more broken motherboard traces due to some incident that you will be able to figure out from evidence

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Re: I borked my 520ST Help?!?

Postby Blur2040 » Mon Feb 19, 2018 3:53 pm

To be clear, I am trying to learn how the computer works and determine where the problem is. While I've pulled and replaced random socketed components, those were relatively easy and at low cost. A defensible practice, I'd argue, considering how often it works.

Upon the advice of the last poster, Arne (thanks!), zoomed in on the 68901 with a scope, and seemed to have zeroed in on the issue, perhaps. The lack of clock, both before and after replacement made me look into how crystals work.

It would seem that the crystal and the caps next to it form an oscillator circuit. Probably a pierce Oscillator https://en.wikipedia.org/wiki/Pierce_oscillator That probably makes those caps quite important.

However, I'm still trying to grasp what makes the oscillator do it's thing....leading to the ultimate question: will a bad 68901 prevent the clock from going, or is it the other way around?

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Re: I borked my 520ST Help?!?

Postby Foxie » Mon Feb 19, 2018 6:34 pm

Blur2040 wrote:However, I'm still trying to grasp what makes the oscillator do it's thing....leading to the ultimate question: will a bad 68901 prevent the clock from going, or is it the other way around?


Either could be the case. Without the clock crystal, I thnk the 68901 will still work - just the internal timers won't run. Of course that will cause a serious problem for TOS which will probably fail to boot. But AFAIK the other functions of the MFP (interrupt handling) will continue to function without the clock crystal, and you should be able to see some bus activity.

The basic concept of a crystal oscillator is pretty simple. There's an inverter connected between the two pins of the crystal. Normally if you attach the output of an inverter to its input, it will settle down at the mid-point due to negative feedback. The crystal exhibits a 180 degree phase shift at its working frequency, which turns the negative feedback into positive feedback - causing it to oscillate.

The capacitors are pretty important to making it resonate, but their value isn't very critical. It will probably run with anything from 15pF to 100pF, but the speed may vary slightly with the change in capacitance. Exact speed isn't too critical for the ST because STs have different clock speeds depending on the model. You'll often see a trim capacitor used in a real time clock circuit to get it as close as possible.


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