All RAM chips destroyed??

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Cristian
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Re: All RAM chips destroyed??

Postby Cristian » Thu Feb 15, 2018 7:58 pm

Dave wrote:are the pixel noise patterns fixed (thus stored in RAM) or shimmery (thus happening after RAM)?

The pixel noise is not fixed, it quivers continuously and randomly.
I have strong suspicions about 8302, because:
- it had the dirtiest pins (now cleaned as much as I could);
- it's one of the few chips I could't test nor swap with other QLs (they are iss6 and have solderings, resistors and wiring over the 8302);
- when I keep it touched with my finger, the noise effect starts immediatly and increases, and the very slow startup sequence (after Minerva logo) accelerates...


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Dave
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Re: All RAM chips destroyed??

Postby Dave » Fri Feb 16, 2018 12:47 am

Check for dry solder joints too, on the sockets of 8302, 8301, then 8409.

Another less likely possibility is the three 74LS257s, one path through may be pulling up or pulling down or something? (IC19,20, 21).

The random keyboard input would be logical. The keyboard buffer is getting sparkles too and taking them as keypresses. The keypress sparkles should tell you the character. Look up the binary of the ASCII character to work out which bits are toggling.


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Cristian
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Re: All RAM chips destroyed??

Postby Cristian » Fri Feb 16, 2018 6:18 pm

Dave wrote: one path through may be pulling up or pulling down
Not sure to understand... sorry for my english

Dave wrote:Look up the binary of the ASCII character to work out which bits are toggling.
I'm afraid this is too much for my limited knowledge. Anyway, thank you very much for your suggestions and efforts.


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Re: All RAM chips destroyed??

Postby Cristian » Fri Feb 16, 2018 6:22 pm

Here below is a summary of the repair progress till now. A possible next step might be the modification on 8302 (adding R102 and R103).

Image


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Dave
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Re: All RAM chips destroyed??

Postby Dave » Sat Feb 17, 2018 2:22 am

And you say this had AH ROMs?

It looks like maybe the required mods either haven't been done, or have been done incorrectly?

Explanations:

pull-up/pull-down: A pin may be shorted (even intermittently) to GROUND (pulled down) or to 5V (pulled up). It could just become disconnected (open circuit, or OC). In the

Look up the binary of the ASCII character to work out which bits are toggling: I saw the recognizable characters

i, ò, _, É, [

So:

Code: Select all

CHAR ASCII HEX BINARY
  i   105  $69 01101001
  ò   152  $98 10011000
  _   95   $5F 01011111
  É   131  $83 10000011
  [   91   $5B 01011011


From the above, we can't see a particular bit ALWAYS being pulled to 1 or to 0. So, this didn't provide us with a clue.


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Re: All RAM chips destroyed??

Postby Dave » Sat Feb 17, 2018 2:49 am

Your comments about the 8302 make it highly suspect.

The 8302 is on the data bus, plus A0, A1 and A5. It receives keyboard input from the 8049. If active when video is being read out, it would create this type of shimmery stipple pattern, and its data connections are tri-state. I've seen a lot of these with heavily tarnished pins. At Sandy, and even today, I use a gritty pen eraser to gently clean each side of the pins, while they are supported from the other side by the table or a blade. Some would just.... Suicide.

Places I routinely check for weirdness are:

8302:
Pin 8 /DSMC
Pin 10 /PCEN
Pin 26 /IPL1
Pin 27 /RESINT
Pin 28 /RESET

Any traffic on [D0:7] by 8302 when 8301's Pin 38 (VDA) is supposed to be asserted. That is, the VDA signal that isolates that bus may be defective, or either of IC19 or 20 (74LS257) may not be properly isolating the bus. When the screen is being drawn, the 8301 and DRAM cut themselves off from the ROM and CPU, but the 8302 is in the 8301's little private party and can scribble over things while this is happening. Which sounds most likely to you? ;)

Meanwhile, might as well get a spare 8302. https://www.sellmyretro.com/offer/detai ... -chip-1822


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Re: All RAM chips destroyed??

Postby Cristian » Sat Feb 17, 2018 8:46 am

Yes I confirm that the mods (resistors, wiring) on 8302 have never been done at all. Surely I'm going to apply the mods as soon as possible, anyway I suppose they should not affect the startup process.
Thank you again for your teachings and suggestions. Now I feel so near the truth...


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Re: All RAM chips destroyed??

Postby Cristian » Mon Mar 05, 2018 8:08 pm

Just replaced IC23 with a "new" one and.... no evident progress.
Next step: apply the mandatory modifications adding R102 and R103. Let's hope....


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Re: All RAM chips destroyed??

Postby tcat » Sun Apr 08, 2018 11:11 am

Hi,
I have been following this topic with a great interest, while learning a lot about QL as well. It made me read several chapters from the service manual again, now with slightly more understanding. The chapter on firmware upgrade caught my attention.

4. Remove IC17 (74LS00).
5. Remove IC33 and IC34.
6. Fit links JU2, JU3 and JU4.
7. Fit 32k ROM to IC33 position.
8. Fit 16k ROM to IC34 position.


What is the purpose of JU2,3,4, where are they located?

Many thanks.
Tomas


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Re: All RAM chips destroyed??

Postby martyn_hill » Sun Apr 08, 2018 1:12 pm

Hi Tomas

Without wishing to go too far off Cristian's original topic, those JUx represent wire-links/zero-ohm resistors on the Iss-5 boards to accommodate different EPROM/ROM configurations before the masked ROMs were finalised towards end of life of the Iss-5 release. They appear immediately between the ZX8302 and the right-hand ROM socket.

Iss-6 did away with them altogther.

The links worked in conjunction with the 74-series NAND logic IC17, which itself became redundant once the original (up to AH-) EPROMs were replaced with masked ROMs (JM onwards) and served to adjust the required CS signal based on ROMOE & A15 (IIRC) to suit either EPROM/ROM with their different pin-outs.

You'll only see them in the original Service Manual schematic (with hand-annotated dotted-lines) ad not in the updated PDF version for the Iss-5.

I've re-configured the links successfully in conjunction with a bit of (messy) re-wiring of IC17 to allow a 64k EPROM to be fitted directly to the ROM socket before I later got my hands on a Minerva carrier-board. I wouldn't recommend such DIY mods for the faint of heart...



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