Atari STFM dead

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CRG
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Atari STFM dead

Postby CRG » Sun Jul 21, 2019 4:35 pm

Hello all, this is my first post here and I'm looking some help.

I recently picked up a spare repairs STFM from eBay for £20 (plus postage). I tried to power it on and the power LED flashed for a split second then off. There was also a small crackling type sound out of the power supply to I quickly turned it off and went looking.

The PSU is the Mitsumi SR98 and I've been reading the PSU thread and had a look at the following page - http://www.exxoshost.co.uk/atari/last/p ... x.htm#SR98 but before I go spending money on replacement caps etc is it worth trying to repair the PSU or would it be better to replace it with something new? Considering the ST seems to use +5v and +12v could I in theory wire up to an ATX PSU just to test the machine posts OK.

I think I've traced the snap crackle and pop to C101 (a large ceramic by the looks of it) that has a crack and a burnt patch on it. I suppose it might just be this cap is bad but after reading the above site its probably best to replace them all anyway. Is there a data sheet for this PSU anywhere so I can get the value of this cap or could someone help with this?

Hopefully the PSU hasn't fried anything hence why I'd prefer to test the machine first before spending money. I suppose regardless I'll probably try to get it up and running but I'd prefer to know where I stand before hand.

One other quick query if I may. This is my first ST and looking around the board there are multiple empty spots for chips. I've seen that I can add a blitter for some performance boost which I may do but the part that caught my eye is over beside the 512k ram there is space for what looks like another 512k. Is this something that can be added?

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Re: Atari STFM dead

Postby Atarieterno » Sun Jul 21, 2019 11:55 pm

ST/fm/e, STacy, Mega ST/e, TT, Falcon, C-Lab MKX... and more music tools.

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Re: Atari STFM dead

Postby tzok » Mon Jul 22, 2019 5:43 am

12V is only for floppy, STf itself will run fine just on 5V.

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Re: Atari STFM dead

Postby mpattonm » Mon Jul 22, 2019 6:06 am

Something new to consider: https://centuriontech.eu/product/stpsu-st/

From your description there is a short somewhere. It could be dead dry PSU cap as well as something on the motherboard (i.e. elcap as well). How does PSU perform when detached from mainboard, what are voltage readings?

Either way, I highly suggest to recap both - PSU and mainboard.

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Re: Atari STFM dead

Postby aszasza » Mon Jan 06, 2020 10:13 pm

Hi all, newbie here so much so that I've never actually used any ataris, never had one but I just recently found an 520 STfm it was left behind on the new place we have moved in. I'm an electronic enthusiast so I was quite happy to see one of these pieces of computer history and of course wanted to do something with it. The place I moved into used to be a music studio so I guess they used it for music production. There is also a red dongle without any labels on it but I suspect its a cubase hw key.

Anyway before I turned it on took it apart, cleaned it and recapped the psu with good quality caps, all is good with the psu now, tested it with a dummy load even checked the ripple with an oscilloscope its quite high actually to todays standards a few hundred mV-s on full load(tested it up to 3A)

I got myself a gotek drive flashed it with FlashFloppy and started playing around. The ST turned on, first thing sysinfo, showed 2.5Mb of ram (when I took it apart I noticed that there are 2 simm modules on a separate board with with ribbon cables over the mmu and video shifter.

After about half an hour of messing around I loaded cubase3 it worked but I wanted to try If I can do something with it so I hooked it up to a yamaha keyboard with a midi cable, keyboard midi out to ST midi in and the whole computer went dark, its not turning on ever since. The power LED is on I've checked the psu, all ok but the computer is not trying to access the floppy or do anything really. Can it just be a coincidence that it happened while I was messing around with the midi cables or can something like this actually kill the ST? I had a quick look on the schematic but the midi in port starts with an opto coupler so maybe I had a groundloop problem? Can this kill an ST? Its weird, any ideas? What should I start checking? Should I try to reinsert all the chips? I do have skills and tools just lack the experience with this computer.

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Re: Atari STFM dead

Postby trashwolf » Sun Jan 12, 2020 7:10 pm

Very cool find!

Is there any video output at all, or just a black screen? Any beeps from the keys (assuming you have it hooked up to some kind of monitor with speakers)?

You might as well try re-seating any socketed chips, yes. It sometimes fixes a bad contact and it's not like it costs anything. Be gentle when disassembling and reassembling the machine, the plastic case can go brittle with age.

You say you have a scope, so a good next step is to try checking the following pins on the 68000 CPU:
Pins 14 and 49: Vcc (should be 5V)
Pin 15: 8MHz clock signal
Pin 17: /HALT (should be high)
Pin 18: /RESET (should be high)
Pins 1-5 and 54-64: data lines (should show activity)
Pins 29-48 and 50-52: address lines (should show activity)

There is a service manual out there. It does assume that you'll have a diagnostic cartridge and a ready supply of custom Atari ICs, but it still has some useful troubleshooting guidelines and an overview of system operation: http://www.atarimania.com/documents/ata ... manual.pdf

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dma
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Re: Atari STFM dead

Postby dma » Sun Jan 12, 2020 8:11 pm

The only time i saw a instantly killed ST, was when unplugging a cartridge (ST replay) while it was running (Sound Lab software).

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Re: Atari STFM dead

Postby aszasza » Wed Jan 15, 2020 10:45 pm

Thank you for the answers!

Black screen on a b&w high res atari monitor, no beeps on keypress.

5V ok, pin15: well, its 8mhz spot on but more like a triangle wave instead of square, and not a full 5V swing. Halt/reset both low-ish, i can see some very little change when pressing the reset button but they are both under 1V at all times. Same thing if I take the roms/shifter/mmu/glu out, no matter what combination I leave them in place. Dead cpu I guess?

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Re: Atari STFM dead

Postby aszasza » Fri Jan 17, 2020 3:48 pm

Okay I found it, it was the 47hctls05 inverter ic in the reset circuit. Don't know how it died but replaced it and everything is ok now.

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dma
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Re: Atari STFM dead

Postby dma » Sat Jan 18, 2020 10:48 am

Ah great, congratulations on putting this ST back to life. :)

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Re: Atari STFM dead

Postby trashwolf » Sun Jan 19, 2020 5:05 am

Nice work! And glad to hear it was a component that's reasonably cheap and obtainable :)

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Re: Atari STFM dead

Postby aszasza » Sun Jan 19, 2020 5:34 pm

I just had a 7414 in my random parts bin so used that instead. I don't really get why they used an open collector inverter, the one I used is actively driving the reset line high or low and it works just fine of course. Maybe the 7405 was cheaper back than?

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Re: Atari STFM dead

Postby Arne » Sun Jan 19, 2020 5:44 pm

aszasza wrote:the one I used is actively driving the reset line high or low and it works just fine of course.

/RESET line is bi-directional :!: Think about it.
Image

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Re: Atari STFM dead

Postby aszasza » Sun Jan 19, 2020 7:04 pm

Is it? I've never came across any processor(I've only worked with microcontrollers) that can drive the reset line, its always a high impedance input only. What else is driving the reset line in the ST apart from this chip? And why? :D

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Re: Atari STFM dead

Postby czietz » Sun Jan 19, 2020 7:08 pm

aszasza wrote:Is it? I've never came across any processor(I've only worked with microcontrollers) that can drive the reset line, its always a high impedance input only. What else is driving the reset line in the ST apart from this chip? And why? :D


The CPU is also driving the reset line. Now you're overloading its output driver by actively driving the line high. PS: HALT is bidirectional, too.

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Re: Atari STFM dead

Postby aszasza » Sun Jan 19, 2020 7:13 pm

Very useful info, thank you! I'll get the proper part than. I've soldered a turned pin socket for it so its gonna be easy to swap it.

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Re: Atari STFM dead

Postby aszasza » Mon Jan 20, 2020 9:48 pm

Turns out that this machine once belonged to the band Apollo440 or @440 if anyone wants to know. Probably the first few of their albums was made using this very machine. Cool huh? :)

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dma
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Re: Atari STFM dead

Postby dma » Tue Jan 21, 2020 8:33 am

Pretty cool indeed!


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