Watts Up, Doc?

A hardware project for RetroChallenge:

The Outer Problem
My surviving SPARCstation-10 PSU is starting to flake – it will run a pair of 133 MHz HyperSPARC CPUs perfectly fine for months on end, but as of June last year it will no longer run a pair of 180 MHz CPUs – it hard-hangs after a few minutes – and it is not the CPU modules that are at fault, as I have tried several modules and they all work in pairs in an SS20, and work individually in the SS10. The SS10 PSU is getting a bit marginal, so the thought is to replace it with an adapted modern higher-rated PSU (it needs to be a slim tiny unit, and need adaption, due to the very specific size and shape of the SS10 case and its’ oddball fan arrangement).

The Underlying Problem
Based on my primitive spot-measurements with a cheap digital voltmeter, it appears that the 5V outputs (now) droop to around 4.5V when pulling more than 100W total – this is a not-uncommon failure-mode of the Sony APS-39 140W PSU.

Buying a new APS-39 PSU is not an option – they don’t seem to be available any more (most of the ones listed online as “new” are in fact “reconditioned” in the small print). Reconditioned units are not recommended, based on my experience of “disassembling” one – you need a chisel and a hacksaw to even get access to the variable-resistors and caps. For the APS-39, “reconditioned” means “we waved a vacuum cleaner in its’ general direction and measured its’ outputs under virtually no load”.

Possible Adaptable Replacement PSU
I have identified current PSUs that could be adaptable – the TDK Lambda NV1-350TT-N or NV1-350TT-N3. For the SS10, the PSU needs to be able to provide 5V@21A, 12V@3A and -12V@0.1A, and *must* provide a 5V active-high HCMOS “power-good” signal. To have any chance of supporting the SS10 “soft-on” and “soft-off” capability, the PSU must also provide a 5V@1A standby supply, and 5V HCMOS power-off and power-on lines of some kind (an external custom level-adaptor/latch circuit would be needed).

These NV1 units are the wrong shape and size to fit directly in place of the APS-39, but are just a bit smaller than a 3.5″ half-height HDD, which opens up the possibility of an alternative location…

The Project Begins
Earlier this year, I managed to source a couple of NV1-350-TT-N units, brand new, from Mouser Electronics at a moderately discounted price, so now the job is to adapt one of them to an SS10 and bring it up running a pair of 180 MHz CPUs, an HDD, Rasterflex graphics and a full complement of memory, and run it continuously for as extended period as time allows.

The NV1 units are 2012 models, rated for 175W continuous maximum total draw and 100W on 5V rail (the 2014 “M” models are rated for 180W continuous, which is why I got the 2012 versions discounted).
The extra total output power headroom (+17.5W@5V and +24W@12V) should allow a CPU upgrade to dual-200MHz CPUs in future, should I ever be able to get my hands on such.

If time (and skill) permits, building an adaptor circuit to get the soft-off and soft-on facilities working, would be a bonus.

The game is afoot!


4 thoughts on “Watts Up, Doc?

  1. For the SS20, there is a chance that the NV1 may fit directly in place of the original PSU – the SS20 PSU bay is wider and taller than the SS10 and the stock fans are much slimmer, and the bay actually has a *horizontal* bottom, rather than the angled one in the SS10, Also, the SS10 HDD and FDD bays are taller (1/2-height rather than 1/3-height) so you couldn’t pull the “locate it in a drive bay” on the ’20. On the other hand, the electricals (plug-wiring. soft-on/off adaptor circuit, etc) are the same.

    If you ever have to do this, watch out that the NV1 units *require* lengthwise fan-cooling if pulling more than 75W.

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