[solved]Help identifying faulty RAM chip in 1040stfm

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allenbailey
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[solved]Help identifying faulty RAM chip in 1040stfm

Postby allenbailey » Sat Jul 13, 2019 10:47 am

Updated to fix screenshots.

Hello there,

been lurking for a little while and now hoping some wiser heads can help me. In the 80s and 90s I was a BBC then an Amstrad user, I missed out on the 16bit Micros and became a PC user.

I'm currently the (almost) proud owner of my first Atari. It's an STFM1040 purchased from ebay.
It boots to the green desktop and seems to attempt to load floppies. I've actually replaced the Floppy drive now with a modded gotek and oled display.

Loading software always causes a crash (various bombs).

I've tried using some diag software which has indicated a RAM fault. As I'm hoping to get a little more into the care and repair of this Atari I recently also purchased the diagnostic ROM cartridge. This seems to confirm there is a RAM issue. See the picture below, or at https://ibb.co/8bjSkdp
It's suggesting that bit 14 is causing the error?

Image

The layout of RAM chips on my 1030 is 4 banks of 8 chips.

This is not my board although the RAM layout is the same
Image

I'm hoping for some help in relating the address to a particular chip please?
Which of the 32 RAM chips would hold bit 14 in the memory range listed in the diag screen?

Cheers
Last edited by allenbailey on Mon Jul 15, 2019 5:23 am, edited 2 times in total.

czietz
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Re: Help identifying faulty RAM chip in 1040stfm

Postby czietz » Sat Jul 13, 2019 11:09 am

EDIT: Seems that the original post I replied to (below) has vanished. Huh?

Unfortunately, the screenshot of the diagnostic cartridge did not make it to the post and the link to ibb.co just shows a very (blurry and grainy) photo of the mainboard. Generally, because there are so many different board layouts, if you have a continuity tester (or a multimeter), the fastest way to find out which RAM chips are connected to a specific data bit is to measure from the Shifter (the big chip inside the metal shielding). See the schematic (https://docs.dev-docs.org/htm/search.ph ... M+Schemati, p. 2) for the Shifter pin numbers.
Last edited by czietz on Sat Jul 13, 2019 11:34 am, edited 1 time in total.

allenbailey
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Re: Help identifying faulty RAM chip in 1040stfm

Postby allenbailey » Sat Jul 13, 2019 11:33 am

Thanks very much for taking time to reply.
I'll take a look at the link you've posted.
In the meantime the correct link for the output of the diag. cart is here
https://ibb.co/8bjSkdp

Does this make it clear which ram chip is likely faulty? I'm afraid I'm an Atari noob and don't know what maps to what bank and chip.

Cheers

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Re: Help identifying faulty RAM chip in 1040stfm

Postby Greenious » Sat Jul 13, 2019 11:53 am

czietz wrote:EDIT: Seems that the original post I replied to (below) has vanished. Huh?


Yes, new user, edited his first post so it had to be approved again. A funny quirk with the approval system. lol
Updated my guides as of june 28th, 2016. Check'em out and feedback!
viewtopic.php?t=5040

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Re: Help identifying faulty RAM chip in 1040stfm

Postby Greenious » Sat Jul 13, 2019 12:04 pm

allenbailey wrote:Thanks very much for taking time to reply.
I'll take a look at the link you've posted.
In the meantime the correct link for the output of the diag. cart is here
https://ibb.co/8bjSkdp

Does this make it clear which ram chip is likely faulty? I'm afraid I'm an Atari noob and don't know what maps to what bank and chip.

Cheers


Well, yes, it's bit 14 that's bad. However, to tell which chip it is with certainty is more difficult.

At https://docs.dev-docs.org/ you can find schematics, and the only one I find is a 1040STFM rev A, and on that board, bit 14 is U44 or U52 depending on which memory bank is affected.
Updated my guides as of june 28th, 2016. Check'em out and feedback!
viewtopic.php?t=5040

czietz
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Re: Re: Help identifying faulty RAM chip in 1040stfm

Postby czietz » Sat Jul 13, 2019 12:08 pm

Again, the image is extremely blurry! I cannot make out the address with certainty. Next time please make photos that are actually in focus.

Let's assume the address starts with "02". That would place the fault in the lower bank (000000 - 07FFFF). Addresses 080000 - 0FFFFF are in the upper bank. Considering that there are over a dozen of different board layouts, I cannot tell you the designator of the affected RAM chip. You have to figure it out yourself.

Shifter pin 18 is connected to data bit 14. Find the (two) RAM chips that have their pin 2 connected to pin 18 of the Shifter. Those are the chips for bit 14 in the lower and upper bank. To find the lower bank, trace pin 4 of the RAM chip back to the MMU chip, possibly across a resistor. The one going to pin 8 of the MMU is the chip in the lower bank.

The schematic I posted, while probably not 100% applicable to your specific board layout, will help you understand what I wrote above. Another option is to go through the different schematics on https://docs.dev-docs.org/ and see if there's one that fully matches your board.

allenbailey
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Re: Re: Help identifying faulty RAM chip in 1040stfm

Postby allenbailey » Sat Jul 13, 2019 12:16 pm

Edit for exact board match

Thanks very much for your help, I appreciate you taking the time to assist me. I'm sorry about the photo quality, sadly I don't think that the image is actually blurry but reflects the poor quality of the video output I'm getting, oddly it looks better in high res?

I'll have a look at the schematics / links you've kindly posted.

My board is this one. So I'll search for schematics for the C070859

http://www.atari-wiki.com/images/5/55/S ... sh2fit.jpg

Image

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Re: Re: Help identifying faulty RAM chip in 1040stfm

Postby czietz » Sat Jul 13, 2019 2:30 pm

Oh, that's a rather rare board with SMD Glue and MMU. While this schematic does not match the board, the numbers of the RAM chips (page 5) might be the same as on your board: U3...U6 up to U44..U47.
https://docs.dev-docs.org/htm/search.ph ... 28Rev+C%29

In case not only the numbers but also the connections are similar, lower bank, bit 14 would be U10.

allenbailey
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Re: Re: Help identifying faulty RAM chip in 1040stfm

Postby allenbailey » Sat Jul 13, 2019 4:24 pm

@czietz - thank you so much for holding my hand with this. Thanks to your help I'm now the proud owner of a working Atari STFM 1040.

I desoldered the chip from U10 using my awful solder sucker and a lot of swearing. I'm reminded why I'm going to ask my wife for a desoldering gun for Christmas! Anyway I soldered a socket in its place, added a chip and rebooted the diag cart. Lots of screen artifacts and complete crash during RAM test. Horrible sinking feeling. Whilst tasting failure and sipping a cup of tea I realised I'd stupidly put a 4164 instead of a 41256 chip in. Quick change, reboot and hooray!

Screen shots below of Shinobi booted up and my imperfect gotek mod.

Thanks to everyone who took time to reply. As a result of your knowledge and kindness another Atari lives! Another quick question, although it probably needs a new thread? As you've seen the graphics are a bit fuzzy on a modern LCD. I'm using a cable from monitor to Scart. This seemed marginally better that using the RF modulator but neither really seems very good. Is this indicative of a problem of just the reality of using the STFM?

Anyway - thanks again.

https://ibb.co/3dBMQHf
https://ibb.co/Nsc7KyS
https://ibb.co/dj8NBgN


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