Faulty machine.

[prime]

Hi all,

Just picked up an Issue 6 UK QL with a fault Machine powers on but the screen is full of mostly white pixels some of which randomly turn on and off. Pressing reset changes the pattern slightly and there is the normal slight click from the speaker.

Unplugging the 8049 and the ZX8302 does not change the problem, but removing the ROMS or the CPU results in a blank screen which seems to indicate to me that the CPU is running and that the ROMS are OK, and the power on tests are getting as far as initializing the ZX8301.

I suspect that I might have a RAM problem I'll open up one of my other machines later and verify that the socketed chips are OK, though none of the RAM chips seem to be getting too warm.

Does this seem like the correct place to start ? Swapping the RAM out should not be a problem as I have a proper de-soldering station, and a ready supply of 4164s

Cheers.

Phill.

[RWAP]

Yes, it does sound like a memory fault unfortunately....

[tofro]

If the ROM enable signal (pin 33, 8301) is not present, the ROM is unable to excite correct code. If,on switch-on, random flickering patterns are displayed, accompanied by excited beeping noises this suggests a faulty ROM....

Pretty much your description, less the noises.... Sounds like the ROMs might be dead.

Tobias

[Wimdev]

I had identical behavior on one of my US-spec QLs. I picked 2 up on eBay a while ago ($10), but neither of them were working. One turned out to have a dead CPU, so that was an easy fix. The other one had this random white pixel pattern that blinked on the screen and changed constantly. Swapped out all the removable chips (including the ROMS) to no avail. Just the other daty I tried inserting a Gold Card to see what that would do. Didn't change anything. While removing the GC, I noticed a very bent pin in the extension connector - it was shorting out. Sure enough, bending it back fixed the problem, and now it works great.

Anyway, you might want to check the connector. No guarantees obviously, but one never knows with these old systems.

Willem

[prime]

Hi all,

I have now verified that all the socketed chips seem to be OK and work when swapped to my Samsung QL, so looks like the fault is elsewhere, I have checked the expansion connector and board for shorts and could not see anything so I guess time to get the desolder gun to work

Good job I re-stocked on 4164s about 6 months back

Cheers.

Phill.

[Derek_Stewart]

Hi,

Try putting Minerva as the ROM, the Ram Test will tell you which ram chip or section on the internal ram by displaying a memory location on the screen where the ram test failed.

This is an ehancement over the JS and earlier Roms.

Derek

[1024MAK]

If you have a Gold card or SGC, it is worthwhile seeing if the system will run with it plugged in, as then you can tell if it is a DRAM cell or internal DRAM decode fault.

Also keep in mind some modes of failure of the multiplexers 74LS257 (IC19 & 20) or 74LS245 (IC21) bi-directional buffer can cause strange effects.

If a DRAM chip is suspected of either dragging a data line down (or up), or a DRAM chip is unable to drive a data line (high or low), a 'scope can sometime help to pin it (sorry) down

Mark

[prime]

It works

After socketing all the RAM, the LS257 MUXs and the LS245 buffer, I discover that the problem was one of the LS257 MUX chips, so I now have another working machine.

Also note to self.....when fitting sockets to a board you have to solder *ALL* the pins to the board

Now I need to order a couple of new keyboard membrains as the machine is missing it's membrain.

Cheers.

Phill.

[1024MAK]

That's the second person with a QL that had a bad 74LS257 chip to post on this forum in the last 12 months.

Well done Phill on getting it going

Mark