Amstrad 1512 repair, white screen of sad

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Amstrad 1512 repair, white screen of sad

Postby Siggypony on Tue Jul 16, 2019 11:31 pm

Hello,
I am trying to fix my Amstrad 1512.

When I turn it on it just displays a white screen. The last time it ran (about 2 years ago) it booted but stopped, complaining of a ram parity error.
I've seen online that one other person that had an Amstrad that got a white screen was told it was probably ram also.

What I've done so far, is I've removed the old ram chips and replaced them (my amstrad differs from the normal schematic in that it has 2 sets of 2 ram chips and one parity chip), using socketed chips that are known to work. Before I did this I tested continuity on all the traces connecting to the ram, and despite my Amstrad having a slightly different schematic to the standard one, all the traces checked out ok and went where they are meant too.

After replacing the ram chips (and rechecking all the traces after socketing), there is no change.

I've tested all the voltages I know to find and their good, all 3 crystals on the motherboard are at least doing something but I don't have the gear to get accurate readings from them unfortunately (yet).
When I pull the reset pin on the 8086 to ground the screen goes blank, and then goes back to being white once the pin is no longer pulled down.

I've ordered a logic probe that hasn't arrived yet at hopefully that will give me more insight into whats going on.

If anyone has any ideas of what else I could check or what it might be it would be very helpful. Unfortunately Amstrads like the PC1512 don't have the following or knowledge base of other lines like the 8-bit CPC amstrads :(
I can find very little information of trouble shooting them.
Siggypony
 
Posts: 10
Joined: Tue Jan 30, 2018 12:52 am
Location: Christchurch

Re: Amstrad 1512 repair, white screen of sad

Postby Siggypony on Tue Jul 16, 2019 11:45 pm

Actually I got smarter lol. All the voltage rails, +5v -5v +12v and -12v are fine, -5 is most out at -5.14 but I don't thinks that's enough to cause any issues.

The original parity chips were 120ns, I replaced them with 100ns chips. I have 150ns chips too but I read slower chips are likely to give issues where faster chips should be ok.

Original ram chips where SIEMENS HYB514256A-80 I replaced them with KM44C256AP-8 (also 80ns)
The parity chips where SIEMENS HYB41256-12 I replaced them with M5M4256P-10
Attachments
IMG_20190716_233811_1.jpg
Ram chips
IMG_20190716_233811_1.jpg (308.73 KiB) Viewed 334 times
Screenshot_20190716_233853_com.simplemobiletools.gallery_1_1.jpg
Motherboard
Screenshot_20190716_233853_com.simplemobiletools.gallery_1_1.jpg (306.8 KiB) Viewed 334 times
Siggypony
 
Posts: 10
Joined: Tue Jan 30, 2018 12:52 am
Location: Christchurch


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