Faulty (video) ram

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fuasn
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Faulty (video) ram

Post by fuasn »

Hi there,

i recently got a 2nd QL in my hands, unfortunately not working. My assumption is that its a ram-chip problem and i also assume the video-ram is affected.
That is what i get after turning it on (the service manual says likely a ram problem with that pattern), i replaced the ROM with Minerva to figure out which chip it is, but unfortunately not readable. I once got an output that told me address is 0x10000 if i remember correctly, unfortunately written/read was not readable on the screen, but 0x10000 would be even before video-ram if im not wrong...
QLRam.jpg
What i already did
- use another 8031, also didnt work (not sure any longer if the picture was different then, could be that i got that partially readable minerva memory-test with the other 8031)
- socketed/replaced the 2 74LS257, no real change
- socketed/replaced the 74LS254, no real change
- bought 16 replacement 4864P-2 rams, not yet soldered

So now my question:
- Would you also agree with the assumption that at least one of the ram-chips has a problem, and do you also think one of the chips with the video ram is affected ?
- If its at least one of the chips containing video-ram, anyone knows which of the chips on the board contain that video-ram area ? the upper "bank" with the 7 chips or the lower with the 9 chips (well i guess one of the "lowers" is the 8th of the upper bank). i'd like to avoid resoldering/socketing all 16..
- if it makes a difference, it seems to be an Issue5 board, even with that empty IC17 socket


thx for any help in advance,
best
manfred


fuasn
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Re: Faulty (video) ram

Post by fuasn »

forgot: that screen is actually with minerva installed


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tofro
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Re: Faulty (video) ram

Post by tofro »

There's no RAM at $10000 at all on the QL (it actually starts at $20000 with the first video page), so what you see must be wrong.


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Pr0f
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Re: Faulty (video) ram

Post by Pr0f »

that could be down to either a ram chip not being writable (Data in on the RAM shorted or open circuit), or to a shorted or open address line on the DRAM chips - the regular pattern also suggests something like this. Such problems are hard to find.

As you have surmised - the upper and lower banks are actually both 8 chips wide - each one of the 8 chips in a bank looks after one data line on the address bus. The chip on the end nearest the microdrives actually belongs to the upper bank.

Check for any obvious shorted pins on each ram chip (something conductive bridging them) on both the top and bottom of the board.


ones' complement
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Re: Faulty (video) ram

Post by ones' complement »

Looking at the display it appears that (other than green/magenta stripes) the random pixels are placed exactly where Minerva normally shows fault report. So Minerva is trying to report a RAM failure. But surely only one 4164 fails at a time? This looks like something more than a single chip failure. As PrOf mentioned, it looks more likely to be a fault on one of the common address/data lines.


fuasn
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Re: Faulty (video) ram

Post by fuasn »

thx a lot so far !

I checked the 257s Za, Zb, Zc, Zd. No shorts found, all have a connection to somewhere on the ram pins (looked correct regarding which A* pin).
I checked the 254s datapins, no shorts found, each seem to have a connection to one pin of each 2 ram-chips. I also checked the other side to the CPU, also all 8 datapins seem connected. that seems correct to me.

DataIn and DataOut on the ram chips are connected (as they should), no shorts to elsewhere found on them. All ram chips have common VCC and common GND.

Maybe one of the WRITE, CAS, RAS lines ?
seems WRITE goes to 8301 pin40 (WEL), RAS goes to 8031 pin8 (RASL) and CAS should go to 8301 CAS0L(9) or to CAS1L(10), right ? given that picture download/file.php?id=1686&mode=view
i think i'll check that next, i would also have an usb oscilloscope (bitscope, limited bandwidth) if i should check some pins with that

btw, thats how the screen looks like with the JM ROM the QL came with, that is also somehow blinking, means like changing a bit every 1s or so.
QLRam-JM1.jpg
QLRam-JM2.jpg


fuasn
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Re: Faulty (video) ram

Post by fuasn »

the 8 datapins of the 245 bustransceiver seems also be all connected to one of the 8031's pins


fuasn
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Re: Faulty (video) ram

Post by fuasn »

so, all CAS0L are connected to the upper bank, all CAS1L are connected to the lower bank
all RAS lines have a connection
the WRITE line is also beeping on all ram chips

hm....


fuasn
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Re: Faulty (video) ram

Post by fuasn »

could one of the rom pins of those 2 sockets have a problem ?


fuasn
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Re: Faulty (video) ram

Post by fuasn »

all rom sockets have a connection to the counterparts of the other rom socket
all rom databus pins have a connection to the 254
all rom address pins have a connection to one of the 257

i think i need to start accepting that i have to change ram-chips, one by one, or has anyone another idea ?

since CAS0L from the 8031 goes to the upper bank, i will start with that


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