All RAM chips destroyed??

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tofro
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Re: All RAM chips destroyed??

Postby tofro » Sun Jan 28, 2018 1:29 pm

1024MAK wrote:Note that IIRC, one make of 74LS257 chips is not recommended in Sinclair machines, but I'm using a mobile device, so don't fancy going through the service manual.

It was NatSemi they didn't like. But probably make and timing might have changed slightly in the last 30 years. Today, National Semiconductors is TI anyhow ;)

Tobias


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Cristian
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Re: All RAM chips destroyed??

Postby Cristian » Tue Jan 30, 2018 8:14 pm

So I removed old IC19, IC20 and IC21 (74LS257 and 74LS245), soldered the sockets and replaced with new chips. Previously I socketed and replaced also IC1 (Ram).
No visible change at all: the same white screen! :-(
I checked the continuity of my solderings several times. I removed at least two times all the socketed ICs, cleaned the contacts with proper cleaning spray and replaced on their sockets (I found light traces of rust on ZX8302 only).
I tried the CPU on a second QL and it works. I found only a curious behavior of ZX8301: it works perfectly on the second QL in RGB mode, but in antenna-cable mode the colors disappear (grayscale picture). It has also a red pont mark (see pictures). In any case, the second machine boots and works, therefore I suppose this chip can't be the cause of the white screen problem.
If I touch the CPU during white screen, some signs appears (see pictures). I also took some shoots of the first instant after the power on, before the white screen.
I ordered two more 74LS257 chips by different manufacturer, but a little voice in my head is telling me that it won't work again...

the new ICs
Image

first instants
Image
Image

Touching CPU during white screen
Image


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Re: All RAM chips destroyed??

Postby RWAP » Wed Jan 31, 2018 5:38 am

Hmm - I can't remember a QL where the HAL chip is not inserted in the socket (just above the varistor). Does anyone know if the QL can work in this configuration?


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Re: All RAM chips destroyed??

Postby Pr0f » Wed Jan 31, 2018 7:32 am

I think this is the issue 5 board - so no HAL chip, That does mean that the ZX8302 is on the same bus as the memory - so if that's faulty it may be causing problems...


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Re: All RAM chips destroyed??

Postby tofro » Wed Jan 31, 2018 8:12 am

RWAP wrote:Hmm - I can't remember a QL where the HAL chip is not inserted in the socket (just above the varistor). Does anyone know if the QL can work in this configuration?


IC17 (a 74LS00, not the HAL) was removed during the transition from issue 5 to issue 6 - It was apparently used to support various ROM/EPROM configurations through solder bridges and is not needed if you have the "final" 32k+16k ROM configuration. The issue 5 schematic has a note to that.

Tobias


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Re: All RAM chips destroyed??

Postby Pr0f » Wed Jan 31, 2018 8:59 am

If touching the processor changes the screen output, then it could imply a lose pin in the socket, or it could be a capactive effect altering the oscillations of the crystal adjacent to the ZX8301 chip - the 15MHz crystal. Difficult to see in the picture - but has someone resoldered the resistor near there?


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Re: All RAM chips destroyed??

Postby Cristian » Wed Jan 31, 2018 10:34 am

Pr0f wrote:I think this is the issue 5 board - so no HAL chip, That does mean that the ZX8302 is on the same bus as the memory - so if that's faulty it may be causing problems...

Yes it is exactly an iss5 board.
So a faulty ZX8302 may cause RAM problems on an iss5 board and may work on an iss6 instead?
I thought to try with the ZX8302 taken from the iss6, but I'm afraid to damage the chip!

By the way, I tried to touch the CPU with non-conductive material (plastic) and nothing happens to the white screen. Also I checked the continuity between the CPU socket and the board, so I think we may exclude contacts/pins issues.


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Re: All RAM chips destroyed??

Postby Pr0f » Wed Jan 31, 2018 11:14 am

Cristian wrote:
Pr0f wrote:I think this is the issue 5 board - so no HAL chip, That does mean that the ZX8302 is on the same bus as the memory - so if that's faulty it may be causing problems...

Yes it is exactly an iss5 board.
So a faulty ZX8302 may cause RAM problems on an iss5 board and may work on an iss6 instead?
I thought to try with the ZX8302 taken from the iss6, but I'm afraid to damage the chip!

By the way, I tried to touch the CPU with non-conductive material (plastic) and nothing happens to the white screen. Also I checked the continuity between the CPU socket and the board, so I think we may exclude contacts/pins issues.


Not sure it causes RAM problems so much as if it's data bus pins are providing a loading onto the 8 bit databus that's connected between the 245 2 way buffer chip, the ZX8301 and the RAM chips. As the zx8302 has nothing to do with video generation, timing for the processor, or ROM selection, you could remove it and then test powering on the board - obviously you won't be able to hit F1/F2 keys - as the chip won't be there, but the boot process should take you to that screen I think - it saves putting another one in there.

I had a 'black video' issue on a board I was playing with recently - it's an issue 5 board. The processor was getting warm, and the ZX8301 was stone cold. I got my scope out and looked at the 15MHz singal and it looked a bit dodgy, also the other pins on the ZX8301 had no discernible patterns on any address pins - conclusion - the ULA was dead - swapped in another and bingo - I had video again :-) The 68008 is not designed to work with a clock that doesn't oscillate - not sure if that was why it was getting warm, or if the ZX8301 was loading the pins on the processor.

My gut feeling is that these video ULA's are connected directly to the outside world on the 8 pin connector - so if someone has got a badly wired plug or cable, who knows what damage can ensue.


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Re: All RAM chips destroyed??

Postby martyn_hill » Wed Jan 31, 2018 12:00 pm

Pr0f wrote:My gut feeling is that these video ULA's are connected directly to the outside world on the 8 pin connector - so if someone has got a badly wired plug or cable, who knows what damage can ensue.


Indeed, there is zero protection on the TTL video outputs from the ZX8301 ULA (see annotated schematic). Whether the loading from the 8-pin connection or some other interaction from defective PAL components/MC-1377 could render the observed behaviour is unclear, but a possibility.

ZX8301_VideoConnections.png


Isolating the start-up sequence from the 8302 by simply removing it and powering-up if only briefly does seem like a sensible - and easy - test to try.

On the Iss5 board, neither 8301 nor 8302 are directly connected to the CPU data-bus - both pass through the bus-transceiver. This changed slightly on Iss6, when the 8302 was placed 'outside' 8301's private bus (it never needed to be inside.) That said, they both directly connect to a sub-set of the address-bus (8301: A16..17, 8302: A0..1, A5) and the 8302 _might_ be messing with these.

The fact that touching the CPU changes the display pattern does suggest a grounding issue somewhere...

Replacing the 15Mhz crystal 'can' oscillator by the 8301 is another simple and inexpensive job - the fact that this (along with the other two can oscillators) rises above the average component elevation renders it sensitive to mechanical breaks which are hard to spot (being soldered tight against the board.) The 32KHz oscillator in your photo (used for the real-time clock on the 8302) for example, has obviously been forced-back at some point.

The mod to replace originally fitted EPROMs with the two (MGI) ROMs looks intact, but worth checking those wire-links (JU2..JU4 - look like resistors but with a single black-band indicating zero Ohms) are making reliable contact (JU3, JU4 feed ROMOE, for example). The fact that the ROM start-up commences at all, however suggests that they are good - but no harm in checking.

Coming back to the 'continuous Interrupt' possibility - either shorts on these lines, or either of the 8302 or 8049 holding IPL0/2 and/or IPL1 low could trigger an apparent freeze, though I suspect that interrupts are not enabled until after the memory test (would require checking in the ROM disassembly), so only an NMI equivalent (INT#7 = both IPL lines low) should interfere at this early stage of start-up.

If you have an oscilloscope-type solution, well worth checking the CPUCLK and some of the other lines against your other 'known-good unit'. I personally use both a cheap-and-cheerful digital scope and a 2-channel scope connected to my laptop - search Google and check out:

Hobby Components 8CH USB Digital Analyser (around £13)
Sainsmart DD120 USB 2-CH Virtual USB Oscilloscope (around £60)

:-)


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Re: All RAM chips destroyed??

Postby Cristian » Wed Jan 31, 2018 12:47 pm

First of all, thank you very much for your support.
Let me specify that I use a good quality RGB-Scart cable purchased from "Retrocomputing". Alternatively, I may use a normal antenna cable.
So can I assume it's fairly safe to try with the other ULA?
Anyway, I'll try to remove the ZX8302 first.
Also I'll check the apparently resoldered resistors near ULA (it's a third-hand QL, who knows the possible modifications applied?).
Then I will try to replace the oscillator.
Unfortunately, I don't have any oscilloscope knowledge... for the moment I'll try to fix the problem in other ways.
Your comments are always a big help to me
thanks



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