CST Thor 1 EPROM problem
Posted: Sun Mar 24, 2019 5:35 pm
Well, I've acquired an old CST Thor 1 (aka Thor PC and Thor 8). It appeared to have been quite well looked after, but inevitably time had taken its toll on the poor thing.
The first thing I did with the machine, was to remove the RTC NiCad battery, which had started to leak. However, the leakage was not too bad, and cleared up quite nicely with some white vinegar and IPA. Thankfully, no PCB track damage appeared to have occurred. With the battery out of the way, I tested the (somewhat alarmingly arranged) PSU, and I found that this seemed to be fine, with the correct output voltages being found on the output side, as measured by my multimeter.
So, I powered up the machine, and was met with a uniform white screen. Hmmm. So, after some reading, I swapped out the JS ROMs on the QL for a Tetroid ROM switcher, set to the Minerva ROM. This resulted in the machine booting up, though reporting only 64k of RAM! The Thor external keyboard interface worked, as did both floppy disc drives (though they needed a bit of lubrication). At boot, the Thor copyright message appeared (after the Minerva splash screen), followed by the machine's serial number, which I noticed was written on the back of the case, and on one of the two Thor EPROMs.
To investigate further, I disconnected the QL PCB from the Thor, and back powered the QL from the Thor-QL connector. Resetting the QL with the Minerva ROM eventually produced some RAM errors on boot. Using the NightFallCrew Minerva Faulty RAM decoder (https://www.nightfallcrew.com/minervaram/index.php), it appeared that RAM IC 16 was the problem. Replacing IC16 did subsequently result in the QL booting with 128K RAM. However, random RAM errors were subsequently reported in RAM ICs 1 and 7, and the machine often didn't complete the boot process. I suspected that there could be a problem with the ZX8301, so I swapped this out for a known good ZX8301. This change seemed to fix the QL, which has since been behaving normally.
I had high hopes when I reconnected the repaired QL to the Thor board. The machine did boot. It reported 128K RAM. No sign of the 512K RAM on the Thor, alas. And, whilst the Thor copyright message appeared on cue, the machine serial number no longer appeared on the screen, but instead some randomly coloured horizontal lines. The machine did not respond to the external keyboard, and would not progress to drop into Super-BASIC when left. I rechecked the QL on its own, and it seemed to be fine. Indeed, I was able to leave the QL connected to the Thor board, and have it boot successfully, if I removed the Thor EPROM board, though in doing this I lost access to the external keyboard and other Thor peripherals. Connecting a Sinclair QL keyboard directly to the QL allowed me to test the QL in-situ, and all seemed well.
I therefore turned my attention to the two Thor EPROMs:
Placing the 27256 Thor 4.00 EPROM (the chip on the left, closest to the main PCB connector) in my TL866CS programmer, I was able to read the EPROM contents without any difficulty, and the data appear to be ok. However, the M5L2764K labelled CST Thor 86/10/1/11 (the computer's serial number) is another matter. This device is not in the supported device list for the TL866CS, but the M5L2764K datasheet states that this chip is interchangeable with the Intel 2764, which is supported. However, when I try to read the M5L2764K using the TL866CS, I find that the device is almost entirely empty (just a long series of FFs), other than for a few bytes right at the end. My conclusion is that the M5L2764K has become corrupted, and that the faulty contents of this EPROM are preventing the Thor from booting, though I suppose that there could be some incompatibility in correctly reading from this EPROM using a TL866CS.
Now, I'm not clear what, aside from the machine serial number, was stored in this M5L2764K CST Thor 86/10/1/11 EPROM - and the Thor Technical Manual doesn't shed much light on this. Looking at the Thor ROMs on Dilwyn's site, it seems that the contents of the 27256 are available there, but not the contents of the M5L2764K.
That's as far as I've been able to get with this machine, so far. If anyone on the forum also has a CST Thor 1, and can give me some clues as to how to get further, then I'd be very grateful for any advice!
The first thing I did with the machine, was to remove the RTC NiCad battery, which had started to leak. However, the leakage was not too bad, and cleared up quite nicely with some white vinegar and IPA. Thankfully, no PCB track damage appeared to have occurred. With the battery out of the way, I tested the (somewhat alarmingly arranged) PSU, and I found that this seemed to be fine, with the correct output voltages being found on the output side, as measured by my multimeter.
So, I powered up the machine, and was met with a uniform white screen. Hmmm. So, after some reading, I swapped out the JS ROMs on the QL for a Tetroid ROM switcher, set to the Minerva ROM. This resulted in the machine booting up, though reporting only 64k of RAM! The Thor external keyboard interface worked, as did both floppy disc drives (though they needed a bit of lubrication). At boot, the Thor copyright message appeared (after the Minerva splash screen), followed by the machine's serial number, which I noticed was written on the back of the case, and on one of the two Thor EPROMs.
To investigate further, I disconnected the QL PCB from the Thor, and back powered the QL from the Thor-QL connector. Resetting the QL with the Minerva ROM eventually produced some RAM errors on boot. Using the NightFallCrew Minerva Faulty RAM decoder (https://www.nightfallcrew.com/minervaram/index.php), it appeared that RAM IC 16 was the problem. Replacing IC16 did subsequently result in the QL booting with 128K RAM. However, random RAM errors were subsequently reported in RAM ICs 1 and 7, and the machine often didn't complete the boot process. I suspected that there could be a problem with the ZX8301, so I swapped this out for a known good ZX8301. This change seemed to fix the QL, which has since been behaving normally.
I had high hopes when I reconnected the repaired QL to the Thor board. The machine did boot. It reported 128K RAM. No sign of the 512K RAM on the Thor, alas. And, whilst the Thor copyright message appeared on cue, the machine serial number no longer appeared on the screen, but instead some randomly coloured horizontal lines. The machine did not respond to the external keyboard, and would not progress to drop into Super-BASIC when left. I rechecked the QL on its own, and it seemed to be fine. Indeed, I was able to leave the QL connected to the Thor board, and have it boot successfully, if I removed the Thor EPROM board, though in doing this I lost access to the external keyboard and other Thor peripherals. Connecting a Sinclair QL keyboard directly to the QL allowed me to test the QL in-situ, and all seemed well.
I therefore turned my attention to the two Thor EPROMs:
Placing the 27256 Thor 4.00 EPROM (the chip on the left, closest to the main PCB connector) in my TL866CS programmer, I was able to read the EPROM contents without any difficulty, and the data appear to be ok. However, the M5L2764K labelled CST Thor 86/10/1/11 (the computer's serial number) is another matter. This device is not in the supported device list for the TL866CS, but the M5L2764K datasheet states that this chip is interchangeable with the Intel 2764, which is supported. However, when I try to read the M5L2764K using the TL866CS, I find that the device is almost entirely empty (just a long series of FFs), other than for a few bytes right at the end. My conclusion is that the M5L2764K has become corrupted, and that the faulty contents of this EPROM are preventing the Thor from booting, though I suppose that there could be some incompatibility in correctly reading from this EPROM using a TL866CS.
Now, I'm not clear what, aside from the machine serial number, was stored in this M5L2764K CST Thor 86/10/1/11 EPROM - and the Thor Technical Manual doesn't shed much light on this. Looking at the Thor ROMs on Dilwyn's site, it seems that the contents of the 27256 are available there, but not the contents of the M5L2764K.
That's as far as I've been able to get with this machine, so far. If anyone on the forum also has a CST Thor 1, and can give me some clues as to how to get further, then I'd be very grateful for any advice!