Faulty (video) ram

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Popopo
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Re: Faulty (video) ram

Post by Popopo »

please,
May you show a photo of the naked socket?
Without IC.

Before go harder with another focus (that of course, higly probably will be another issue), but (at least in my experiece) sockets are really really tricky too often.
You have check out the connections between sockets pins and the counterpart (the other side), well done. But... have you checked out how IC legs make contact with the sockets? Very often there is the hide ghost.
I know that is many times useless and paranoid, but here are good reasons for it. And the cost to test it out is so little comparing with labor of de-soldering RAM ICs and test a whole new bunch.


fuasn
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Re: Faulty (video) ram

Post by fuasn »

thx for your comment popopo,

im not sure if i understood that correctly. I've tested IC leg to IC leg with the multimeter and if im not wrong got a beep on each. How could the socket inbetween leg and leg be the reason for smth not connected. Or do you mean the sockets could have bad influence on the signal quality ?Visually, i could until now not really find an issue. Nevertheless, adding some images, the are not very sharp i fear, but maybe good enough.
QLMotherboardTop.jpg
QlRomSockets.jpg
QlMotherboardBottom.jpg
QlMotherboardBottom1.jpg


Btw: i already started socketing/exchanging the DRAM chips, after the first chip the screen (with the JM ROMs) changed a bit towards how it looks like with Minerva. Chip 2,3,4 didn't really change the picture. Also, with the desoldering gun i recently got, its much easier.
Here's how it looks like now (with JM)
QLRam-JM-ChangedChip1.jpg


Chris_68
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Re: Faulty (video) ram

Post by Chris_68 »

Hi,

I think what popopop suggested was to measure with IC in place so you have connection from pad to top of IC, this to verify there is good contact in socket.

Glad you see some progress!!

Most likely nothing but have a look at the solder joint in red plus, if you have solder flux resolder the joints.
Attachments
QlMotherboardBottom1.jpeg


fuasn
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Re: Faulty (video) ram

Post by fuasn »

thx chris,
yes, i measured with IC in place, means from IC leg dram to the 254, 257 IC legs etc.

also thx for that hint you have marked, will do

but: i don't know if this has been also before my solderings, it seems that CAS0L (at least now) has contact to GND. will have to find where and fix it. i assume most likely on the ram chips, as CASL is right next to GND..


martyn_hill
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Re: Faulty (video) ram

Post by martyn_hill »

Hi fuasn!

Just a thought about your probing strategy - for those ICs socketed already, aim to test both with and without them in circuit when you test for shorts.

Sometimes what you end up measuring with ICs still in circuit is their own inherent impedance to Gnd or Vcc, which, whilst a valuable data point, depending on the semiconductor technology used for some of these ICs, might be quite normal to register as an apparent short whilst it's unpowered.

Of course, we're it possible, comparing the equivalent signal line with a known good unit provides the best indicator of '"what's normal".

But I'm sure you've thought of all that already :-)


fuasn
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Re: Faulty (video) ram

Post by fuasn »

thx martyn_hill,

that was new for me. Checked that, the drams & the 8031 seems fine regarding that (check with ICs and without).
In the meanwhile i fixed the CASL to GND problem, can can continue.
I socketed 6 of the upper bank now, had some serious issues with IC5 & IC6, but should be ok again. I think the DRAMs are all okay towards their connection to the 8031. Also not shorts other than pin2 & pin14 (din & dout). No shorts on 8031

But while checking that with and without ICs in sockets, i found that the ROM sockets have a short between Pin1 and Pin28 (might be that exactly what you commented is the reason why i didn't see that before). That would be A15 and VCC. Also fixed another bug made by me (one of the 257 had two legs in 1 socket .... )
Trying to find the short on the ROM sockets now. thx again

update: wait no, rom sockets 1 & 28 are A15 & VCC for the AT27C512R i ordered to burn minerva on. Pin1 & Pin28 are both +5V, so that is okay.


fuasn
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Re: Faulty (video) ram

Post by fuasn »

Ha !
Breakthrough. Im getting a readable RamTest from Minerva. That should allow me to figure out which chip is the remaining problem now.
MinervaRamTest.jpg
FaultChip.jpg
Last edited by fuasn on Sat May 25, 2024 2:11 pm, edited 1 time in total.


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Pr0f
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Post by Pr0f »

The QL motherboard has pin 1 and pin 28 wired to 5V - so the 'expected' A15 signal is not on that pin - instead it's used as a chip select for the ROM's - the ROM's in the QL ROM set - one is active high and one active low - so A15 decodes whether lower 32K or upper 16K rom are selected.

Either ROM can be used in either socket as they are both wired the same. The only problem is you can't just use a 64K EPROM in place of the ROM - not without some kind of adapter board or some tricky mods to the motherboard.


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Pr0f
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Re: Faulty (video) ram

Post by Pr0f »

Well done on the progress with the DRAMS - so now probably just a single faulty chip?


fuasn
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Re: Faulty (video) ram

Post by fuasn »

thx prof,
figured that out already. I also have an adapterboard for the 64k eprom. just need to burn & solder the eprom & the 74hct00n thats underway.
Last edited by fuasn on Sat May 25, 2024 3:04 pm, edited 1 time in total.


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