Watts Up: Episode 12 – Jungle Drops

Sorry, but this is a longish multi-topic post, I have been a busy little beaver the last few days. Which only goes to show that busyness and productivity are not at all related…

Patchwork Revisited
My initial patch-wiring for the fan-power-distribution board had not been quite right – I also needed to bridge pin 4 of the backside connector to the positive terminal of the adjacent 3rd fan socket. It’s astonishing that simple “what’s connected to what” visual inspection can leave such a gap. Literally, gap.

Still, quite easy to fix:fanpatchAt least one would have thought so, but now only the 3rd fan would run. This took 2 whole days to get to the bottom of. It turned out that although my soldering to the pin was solid and good, the heat had separated the pin from the PCB trace through-hole via. Thus a resolder “and let it flow was needed. All three fans running!

The Law of the Jungle
As noted previously, I wanted to step-down the voltage driving the main chassis fans from 12V down eg: to the 7-7.5V that the original APS-39 PSU did. After quite a few calculations, for a triplet of parallel-wired 12V 0.22A DC fans, it seemed like a single 15-Ohm 2W resistor inline with the overall fan-power input would step the voltage down to 6.6V, which would be ideal – I don’t need the fans to rotate as fast as originally, because they do not have to cool the (now empty!) APS-39, just the CPUs and SBus cards. Said resistor would have to dissipate 1.9W, but that is within its’ spec… Needless to say, I had calculated (and sanity-checked, and double-checked) several dropping-resistor scenarios, for both the new PSU fan and the parallel chassis-fans:fandropcalc

However, I discovered the “law of the jungle” rule-of-thumb the hard way; with the above 2W resistor in series with the +12V input (in front of the parallel fan-out), it immediately ran *very* hot, too hot to touch. After a few seconds of running, I cut the power before the magic smoke escaped. With that amount of heat being dissipated, even via a large resistor body, there didn’t seem any way that the darned thing would survive for much more than a minute or two.

Quite a few ‘net suggestions were “always use a dropping resistor rated for *twice* the actual amount of power you will be putting through it”: this suggested that the 2W specification I had calculated according to Ohms’ law and the power law was, quite simply, only enough in theory, not *in practice*. Ouch! you might say.

Given a shortage of other high-power resistors and a shortage of time, the only maneuver available was to run a pair of these 15-Ohm 2W resistors in parallel (ie: 7.5-Ohms equivalent series resistance), which only drops the 12V source to 8.5V instead of 6.6V, but does reduce the power through each resistor – from 1.9W down to 1.6W
CIMG9529CIMG9530Not much of a reduction in power, only 0.3W, but it had an utterly drastic effect on temperature: now the resistors run for hours at just a tad over room temperature; by touch, you can only just tell that they are dissipating any heat at all. Amazing. The fans are a bit faster (louder) than originally, but it will have to do for now, until I can get my hands on a 5W 13- or 15-Ohm resistor.

And here is the dropping circuit attached to the fan-power distibution-board, all screwed-down in the APS-39 chassis, to keep it neat-and-tidy and out-of-the-way.20150125_152627

I know its’ a very minor achievement, but dammit I know you want a truly uninspired meaningless video, so here’s one for you: The Penultimate SS10-Chassis-Fans-on-NV1 Show.

PROM Night
The spare SPARCstation-10 that is recieving all this surgery would not be able to run the 180 MHz dual HyperSPARC CPUs for another reason than the original wilting PSU: the PROM (BIOS) chip is not new enough to support such CPUs. This was a deliberate thing, I had forgotten that I had down-revved the PROM chip to research old CPU compatibility for the Rough Guide to MBus Modules. It had a version 2.7 PROM chip installed, the very earliest version for the SPARCstation-10. Also, the embedded battery in the NVRAM chip (known as “CMOS RAM” in PC circles) had died, due to the system being in storage for so long.

Fortunately, I have a collection of SS10/SS20/HyperSTATION/SPARCplug PROM chips, all the way upto the very latest version, Ross’ 2.25R. Time to swap out the old PROM and NVRAM chips…CIMG9525That should be better:CIMG9526

The Intermittent Dead
So everything should now be peachy, huh? Well, no: sometimes the SS10 boots up OK, but sometimes it doesn’t. Nothing changes betweentimes. Measuring every voltage on every pin of the new PSU, at both the PSU end, and the motherboard end, shows that the voltages are pretty much where they should be: 4.98V, 11.99V, -12V (and the numbers are probably slightly higher – it’s a very cheap digital multimeter). Ideally, the nominal 5V lines should be at 5.05V (they may well be!). I double-checked the PSU sense lines’ level, they are matching the main power lines.

I double-checked the crimp terminations in both plugs, and replaced the couple of maybe-dodgy terminations. Still unpredictable: sometimes boots, but mostly doesn’t. Removing one PSU (still using the 60MHz SuperSPARCs at the moment) did not change matters, neither did disconnecting the fans (and the fan-voltage-dropper circuit). Heck, it was perfectly reliable just 6 days ago, how can it have gone so far downhill, and why? Keyboard beeps are still there, but most times nothing else is.

Damn. I’m getting a bit paranoid now – perhaps these CPUs have failed, or the graphics card, or the memory modules? Methinks it might be time for a swap-fest and prayers.

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