The 380Z seems to be my system of the month and is causing me head scratching. It’s becoming one of those puzzles where you have to slide pieces around but you can’t because something is always blocking them.
I have tried moving the DRAMs on the CPU card around by one socket. I’m not sure that this made any tangiable difference. The screen rubbish doesn’t have as many ‘C’s and ‘0’s in it but left long enough it changes anyway. Did it allways do that? Not sure.
I would really like to read the EPROMS but they’re TMS2716s and they are different to the run of the mill 2716s and so the EPROM programmer I was kindly lent by a friend won’t ready them without an adapter, which we don’t have.
I have borrowed a logic analyser in an attempt to see what’s going on on the bus. As I mentioned before, in a previous post, the bus terminator card has a row of holes and each signal is brought out to one of them. I have soldered some header pins so I can pop the wires of the logic probe on to key sognals and see what’s going on.
The short answer is nothing. nada, nilque. The long answer is that the logic analyser isn’t playing ball. If I use the ‘scope I can see a clock, memory requests, M1 bipping up and down, activity on the data bus, _RD_ is low and the first eight or so lines on the address bus are wibbling as expected. It’s frustrating the analyser won’t work as that would be a great help.
The 380Z has got as far as displaying rubbish on the screen but no further. It’s quite a predicatable kind of rubbish from a sub-set of characters. Curious.
I have tried running with just the processor, VDU and the passive card that I think is for bus termination. The symptoms are the same. I started removing the chips on the CPU board one by one, using IPA to clean the pins and putting them back. After I had done about a third I just went for lifting the chips, squirting and reseating them.
I did get some different results but I had inadvertently missed a pin on one of the EPROMS. When I put it back where it belonged the machine went back to it’s old behaviour.
It was suggested to me that the EPROMS could have lost their memories by now as their expected life was around ten years so I will try and read them and verift them against some ROM images I have found through the 380Z group on Yahoo.
The bus terminator card has a row of holes for (I think) test pins, on every signal. I’m going to solder right angled header pins into these holes to get easy access to them and start probing about.
I also plan to shift the DRAM chips around by 1 socket and see if the fault changes.
I’ve been having a look at the RML308Z recently. One of the capacitors in the power supply went pop when I first powered it on. This is not unusual for machines of this age and it’s a good idea to inspect the caps before powering on a new machine. I decided to chance it and POP.
It turns out to be a tantalum on the 12v and a visual inspection wouldn’t have shown anything.
It’s very easy to open the case and look at the boards, even to take them out but to get to the PSU is a right pain. There isn’t a PSU in the usual sense. There is a portion of the machine that takes mains electricity and passes it through a collection of seperate bits, all bolted to different parts of the case.
Anything beyond a board swap is a bit tricky.
Anyway. I haven’t got a suitable replacement cap so I snipped of the remains of the old one and re-assembled. A look at the circuit diagram show this is for taking out the ripple and so isn’t vital – I don’t think.
I’ve now powered it on again and it’s nearly working…