Rescuing an Ebay "for parts or spares" 520STFM board

[Elethiomel]

When I was preparing to build my Phoenix H5C1, I mistakenly thought that the donor board I had was unusable. I browsed Ebay and found a board in its shield for £40 marked as not working. I figured that for £40 I could give it a shot and at least get some good chips off it. After ordering it I figured out I COULD use my original donor board for my Phoenix, so I went ahead and did that.

That left me with a board that outputted this on the screen

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I had a gut feeling it was RAM related as the shifter was obviously running and had been at least partly programmed. That meant ROM, CPU, MMU, GLU etc were probably OK.

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Apart from a little bit of dust and the odd removal of part of the shifter shield (maybe a marpet was previous fitted? - The MMU still had a clip though...) , the board looked to be in pretty good pretty good shape with nothing obviously wrong. I reseated all of the socketed chips to no avail. No chips getting hot to the touch.

I then grabbed my trusty Exxos diagnostic cart and booted up.

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A red screen is looking good. I'd expect that with no keyboard attached. Time to look at the serial console.

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Looks like my intuition was correct! We have a bogey RAM chip. Looks like it's the one for bit 5.

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Now which chip is that!? It could be the 6th chip in the first column, counting from the top. It could be the 6th in the 2nd column counting from the right. It could go left to right, then down. It could go any number of ways to be honest. I could try and sound things with the schematic, but I had a simpler idea.

I grabbed the data sheet from https://pdf1.alldatasheet.com/datasheet ... AJ-15.html and I made myself a little probe tied to ground.

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By shorting the data out pin (pin 14) to ground, I can see if anything changes on the running memory test.

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Sure enough, on the 6th chip in the first column...

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OK! we have our culprit! I reflowed the solder on it just in case. No joy.

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socket installed.

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Luckily I had a spare compatible chip from a system I had pulled the RAM from to install one of Exxos' under the PSU 4MB RAM expansions. I popped it in.


Fingers crossed. Time to try a boot.

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Looking very promising!

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I'm going to call that success!

[exxos]

:bravo: :goodpost:


I did a video on locating RAM chips on my TY channel. Nobody likes me though, it's fine :lol:


But in the case of that series board : https://www.exxosforum.co.uk/forum/viewt ... f=31&t=100 1040ST_Schematic_12-26-1987 C070787

It tells you bit per chip ;)

Actually I sectioned that off in a new image *somewhere* on the forum :shrug:

[Badwolf]

Another one resurrected!

Something something Easter something…

Great work! :)

BW

[Elethiomel]

Thanks guys!

:bravo: :goodpost:

I did a video on locating RAM chips on my TY channel. Nobody likes me though, its fine :lol:

But in the case of that series board : https://www.exxosforum.co.uk/forum/viewt ... f=31&t=100 1040ST_Schematic_12-26-1987 C070787

Aww, I like you :D Got a link to the channel? I felt like experimenting a bit by using my probe so no cheating and using schematics :D

This board will appear soon in Ebay Board 2 : The mystery of the failed 6 to 2 chip TOS upgrade. STAY TUNED!

[exxos]

Aww, I like you :D Got a link to the channel? I felt like experimenting a bit by using my probe so no cheating and using schematics :D

https://www.youtube.com/user/exxosuk/videos

Must be this video I was waffling on about it in

This board will appear soon in Ebay Board 2 : The mystery of the failed 6 to 2 chip TOS upgrade. STAY TUNED!

Cool!

[derkom]

I did a video on locating RAM chips on my TY channel. Nobody likes me though, it's fine :lol:

Being liked and being right are orthogonal.

[russellnash]

Useful information, thanks. I started looking again at several boards that have RAM problems primarily just to get experience for when my currently working STs start playing up. I have a question to RAM diagnostics using the cartridge. Maybe I am missing something here or am just plain stupid but in this example if you read from left right the fault displayed is bit 5. That seems very straight forward. However in the following example

https://www.exxosforum.co.uk/forum/viewt ... &start=220

the output is read from right to left, showing bits 8 and 10 as the error. Should the output be read from left to right or right to left?

[zChris]

Very nice and educational! Thanks!

[exxos]

the output is read from right to left, showing bits 8 and 10 as the error. Should the output be read from left to right or right to left?

Explained here https://www.exxosforum.co.uk/forum/viewt ... 143#p47751