Stuck with repair of Spectrum+

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
HappyLittleDiodes
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Re: Stuck with repair of Spectrum+

Post by HappyLittleDiodes »

Your clip of the machine running with no RAM looks a lot like what I had here


That came down to a broken trace under the RAM sockets.

Have you checked continuity on all the lower RAM pins (Except Din and Dout) between the farthest left (IC13) and the farthest right (IC6)?


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1024MAK
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Re: Stuck with repair of Spectrum+

Post by 1024MAK »

speccyplus wrote:
Fri May 15, 2020 9:27 pm
In order to simplify things, I removed all the upper ram and what I thought were the Logic ICs needed for it, do I still need IC24 in place?
Yes, IC24 is definitely needed for issue 4A, 4B and 4S boards, as two logic gates are used to improve the timing for multiplexer chips IC3 and IC4.

Issue 1 to issue 3B boards don’t use IC24 for this purpose, so IC24 can be removed. Issue 5 and issue 6A boards use a custom chip to replace most of the 74LSxxx series chips, so there is no IC24.

Mark
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speccyplus
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Re: Stuck with repair of Spectrum+

Post by speccyplus »

1024MAK wrote:
Sat May 16, 2020 6:06 pm
speccyplus wrote:
Fri May 15, 2020 9:27 pm
In order to simplify things, I removed all the upper ram and what I thought were the Logic ICs needed for it, do I still need IC24 in place?
Yes, IC24 is definitely needed for issue 4A, 4B and 4S boards, as two logic gates are used to improve the timing for multiplexer chips IC3 and IC4.

Issue 1 to issue 3B boards don’t use IC24 for this purpose, so IC24 can be removed. Issue 5 and issue 6A boards use a custom chip to replace most of the 74LSxxx series chips, so there is no IC24.

Mark
Doh! I was looking at the wiring diagram and saw the NAND gates and was like wait, wah? Thats what I get for thinking a issues 3 and 4 are almost the same.
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MaxS
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Re: Stuck with repair of Spectrum+

Post by MaxS »

Thank you Mark!
I was dealing with an issue 4A, the first 4 I ever had, and it was the worst Speccy motherboard I've ever seen.
Build quality is very poor, traces in the brown upper side without protecting coating and very easy to lift.
And it was very abused by previous owner.. too much love may be..
Anyway after fixing every single trace and checked every IC it won't boot up!!
I was completely puzzled..

when I've read your post, late in the night... very late actually :lol: , I jumped in the basement to check schematics:
I've removed the upper RAM as it was an issue 2/3.. :roll:
put IC24 back in his socket...
and TA-DA!!!
another Speccy of my growing collection is now fully working again!
Many Many thanks
max
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MaxS
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Re: Stuck with repair of Spectrum+

Post by MaxS »

MaxS wrote:
Tue May 19, 2020 12:01 pm
Thank you Mark!
I was dealing with an issue 4A, the first 4 I ever had, and it was the worst Speccy motherboard I've ever seen.
Build quality is very poor, traces in the brown upper side without protecting coating and very easy to lift.
And it was very abused by previous owner.. too much love may be..
Anyway after fixing every single trace and checked every IC it won't boot up!!
I was completely puzzled..

when I've read your post, late in the night... very late actually :lol: , I jumped in the basement to check schematics:
I had removed the upper RAM as it was an issue 2/3.. :roll:
put IC24 back in his socket...
and TA-DA!!!
another Speccy of my growing collection is now fully working again!
Many Many thanks
max
0 x

speccyplus
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Re: Stuck with repair of Spectrum+

Post by speccyplus »

Ok, have some other info.

I have another test rom, the one from retroleum. When I pull IC3 but leave all other memory logic chips for upper and lower ram, it has a corrupt screen, but I can read the message saying diagnostics.

It is able to check upper and lower ram and says all green.

When I put in the ic3, it says that the first 3 bits fail, second 3 bits are good and last bit fails from what I recall. The upper memory it says is good still.

I have put the ULA in another board, and it tested OK.
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1024MAK
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Re: Stuck with repair of Spectrum+

Post by 1024MAK »

Keep in mind that with IC3 missing, the testing of the RAM that IC3 feeds address lines to will lead to misleading results, as some of the RAM address line inputs will no longer be driven, hence the Z80 and the test program may ‘think’ it is testing all of this RAM, but in fact, it is only testing half of it.

Because of the way the hardware works, the test program doesn’t know that when it tries to access the area of RAM that IC3 would normally be used for, instead the test program is actually accessing the half that it does have access to. So it tests one half twice. The half that needs IC3 is never tested.

Mark
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