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Re: 520STFM: After replacing memory: White screen of death.
Posted: 26 Sep 2020 20:52
by stephen_usher
I've now checked all lines to the ROMs as well and then decoded the only two memory accesses the CPU makes.
It reads the first two words of the ROM and then stops reading from the ROMs. About a second later the machine crashes and starts again.
It's almost as if the GLUE is stopping the assertion of the ROM lines too early, after only two reads, before the CPU has the address of the start of TOS.
Re: 520STFM: After replacing memory: White screen of death.
Posted: 26 Sep 2020 21:17
by stephen_usher
Success! I removed the Blitter socket and the machine sprung to life!
Re: 520STFM: After replacing memory: White screen of death.
Posted: 26 Sep 2020 21:20
by PhilC
stephen_usher wrote: 26 Sep 2020 21:17
Success! I removed the Blitter socket and the machine sprung to life!
Well done, now to find out why :D
Re: 520STFM: After replacing memory: White screen of death.
Posted: 26 Sep 2020 21:22
by stephen_usher
The data buses are now clean and don't have any noise. I think the empty socket was acting as an antenna picking up RF interference.
Re: 520STFM: After replacing memory: White screen of death.
Posted: 26 Sep 2020 21:33
by JezC
Glad to hear that you sorted it out...though all these problems are one of the major driving forces behind the H4/H5 etc.
I've already seen how diagnosing one or two faulty machines can take more time than building the H4... :shock: :roll:
Re: 520STFM: After replacing memory: White screen of death.
Posted: 26 Sep 2020 22:10
by stephen_usher
Actually, RF issues like this would happen on the new board as well. It merely needs the socket leads or internal interconnects to be the right length and you suddenly pick up Radio 2 on the data bus. :lol:
Re: 520STFM: After replacing memory: White screen of death.
Posted: 26 Sep 2020 22:27
by tzok
I rather think, there was a short under the socket... was it original "funky" socket, a modified modern one, or a modern one on the adapter board?
Re: 520STFM: After replacing memory: White screen of death.
Posted: 26 Sep 2020 22:31
by JezC
True...but there are lots of inherent noise issues with the older boards as well, so mixing older/aged ICs with the extra noise can easily tip the old boards from (just) working to giving lots of problems/unreliability.
It's pretty impressive how many of these 30+ year old computers work at all - I dread to think how short the lifetime of many of the newer lead-free electronic devices may be!
Re: 520STFM: After replacing memory: White screen of death.
Posted: 26 Sep 2020 23:39
by stephen_usher
Yeah.
Re: 520STFM: After replacing memory: White screen of death.
Posted: 26 Sep 2020 23:41
by stephen_usher
tzok wrote: 26 Sep 2020 22:27
I rather think, there was a short under the socket... was it original "funky" socket, a modified modern one, or a modern one on the adapter board?
No, I put a foam pad underneath (approximately 2mm thick) to make sure that there was no possibility that the underside of the conversion board would get anywhere near the surface.