What was the original TIM like? Was it still good enough for the cpu to cool properly or did it need replacing? […]
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What was the original TIM like? Was it still good enough for the cpu to cool properly or did it need replacing?
I have been trying to use the motherboard I fixed here, my Taken PCI400-4 which was working after a socket replacement and some loose pins around the southbridge resoldered: Re: Bought these (retro) hardware today
I was trying to test an ISA soundcard on it but it wouldn't boot reliably, getting stuck on / flipping between "C0" and "C1" post codes. Initially I thought it was another loose pin on one of the chipsets so spent ages reflowing the solder on those but to no avail.
Shoulda checked the basics at this point specifically clockspeed, but after some time figured out that the board would boot normally if heated around the CPU / northbridge area which is why I thought it was the northbridge. It would work sometimes when started up but it seemed whenever I left it for a while and it got cold, it would go back to being broken.
Using the hot air with a nozzle I was able to narrow it down to the area around the clock generator, the ARK 915A which has a bash mark on it:
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Checking with the oscilloscope it was showing an unstable ~9mhz on the main cpu clock instead of the 33mhz it's supposed to be. Can't find a datasheet for the ARK 915A or Macronix MX8318 clock generators, but the MX8315 is pretty close. There was no clock on the 14.318mhz crystal so I thought perhaps it was the crystal that was bad, swapped that and no difference 😐
The conclusion seems to be that there's a broken bond wire inside the ARK 915A clock generator since borrowing the MX8318 from my Zida 4DPS has made the board reliable again.
When the ARK 915A clock generator is heated up then it will work with the crystal and do its job properly, but when cold the crystal oscillator is dead so I assume that the XO crystal output pin of the clock generator which drives the oscillator, only has a working connection when warm.
If I heat the ARK 915A then it works and it seems to keep working if it works when it starts up, so a heater atop the chip could work. Mechanical pressure also seems to work so I might try some kind of clip on top of the chip to keep pressure on it.
Or I could look at making some alternative clock generator work, I'd rather not do that and replacements for the ARK 915A or MX8318 both seem to be unavailable. Getting an alternative clock generator working would be a lot of work...
Anyways, now the board works again (albeit at the cost of the 4DPS which I'm not using right now) and it's set up for a Cyrix 5x86 processor I thought I'd try out my 5x86 chips - the IBM one works perfectly and seems to run very cool.
I have another one with the green heatsink and cyrix branding and that works just fine as well. Then there's the one I got on this M919 from a junk lot quite a while back: Re: Bought these (retro) hardware today
I've not tested that chip until now because pin R1 broke off when I was cleaning it, there was only the tiniest little dot of a connection poking through the ceramic. I tried soldering a pin directly to that and making it strong mechanically with superglue, but didn't want to test it because well... I don't think that's going to work.
I tried it this evening and the processor was dead, just "--" on the post card 🙁 As suspected, it did not work. Not possible to solder to something so tiny and have that stay connected to something as big as a cpu pin.
A couple of weeks ago though, while I was trying to go to sleep I was for some reason thinking about this and visualised a new way I could do that repair. I have some scrap PCBs that are 0.8mm thick with 2.54mm spaced through-holes, I used the mini grinder pen to clean off all the traces so now I've got some 6-pin segments of a PCB. Those slide over the pins at each corner of the CPU so that this modified CPU sits level in the socket.
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I've soldered a long single strand of copper wire to pin R1's little silver dot of a connection and checked it's functioning by checking the resistance to ground of this missing R1 pin for Address line 28 and the adjacent address pin, both measure 17mega ohms to ground so the wire is working 😀
To get the PCB with the wire to sit flush there's a channel carved into the backside and a pin from a very dead Pentium 200MMX is soldered into the front-side of the pin repair PCB, which is better for this than a regular CPU pin because there's a larger peg that sits inside the Pentium's organic PCB. That peg gives a bit more mechanical strength and makes alignment easier.
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The little PCB bits are held in place at the corners with superglue. The strand of wire is looped around from the outside of the CPU / pin repair PCB and is soldered to the pin at the front-side, so it now has no mechanical connection to the CPU pin but has an electrical connection.
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Now the CPU works! 😁
edit: ugh now the floppy controller is giving me trouble... not this CPU that's causing it though!!