OK, time to fess-up: the 4th front socket on the fan-power distribution PCB is *not* the fan-power input: it actually originally provided a feedback voltage to the APS-39 internals, used by the APS-39 fan-speed control circuit – which is somewhat of a waste of time: anybody who has owned and run a SPARCstation-10 can tell you that the maximum spread of fan-speed adjustment (say between single 40MHz CPU and no drives, to dual-180MHz CPUs and two thermonuclear Segate “upside-down” Barracudas) is approximately 1/3 of a small hamsters fart.
Patch Wire Mod
The fan power input socket is actually a “hidden” socket on the back edge of the board, ie: it is enclosed in the APS-39 metal box, completely inaccessible – and we need an externall-accessible socket, so that the new PSU can be installed/removed without having to desolder anything – just plug-and-go.
Not to worry, the (extremely simple) distribution-board can be patched with a single soldered wire (a single conductor from a 10BaseT ethernet cable) from pins 1+2 of the back edge socket to the positive (red) pin of the front socket. That mod converts the 4th front socket into a usable fan-power input.
This mod preserves the logical wiring of the fans – parallel wired with inline staged voltage-dropping diodes:Those voltages are the ones measured from a running SS10 with it’s original APS-39 PSU. The fans are really 12V, 60mm, 0.22A; but as typical in 90’s computers are deliberately run under-voltage as full-speed is not needed and would be extremely noisy.
Reassembling the APS-39 Box
The fan-power distribution board fits into a recess on the inside base of the APS-39 case, and pokes the socket out of the side of the casebut now that the original internal glue-blobs, sticky-sponge and other crap in the APS-39 has been removed, this one remaining board needs to be insulated from the steel case – encasing the PCB in double-layer insulating tape is a bit over the top, but what the heck! .
Originally, this mini PCB was held in place by the bottom-half main PCB sitting on top of it. That larger PCB also acted as a “spacer” for the bolts that hold the sides of the case on. Now that I have deliberately destroyed and discarded that main PCB, I had to make a spacer which would also clamp the mini PCB in place. Soft plastic recovered from a bin (part of a discarded childs’ toy), and a little cutting and drilling, did the job.
Next up was to clip-and-screw the sides and top of the case together, with the mains cable for the new NV1 PSU routed through the middle, in place of all the original APS-39 circuit-boards, caps, coils, cables and crap; and finally to screw the fans back onto the side.
As you may have noticed, this was so easy I had time to make another cup of coffee, oops! It all almost works – only two of the fans run, so I suspect I might have disturbed the patch-wire when fitting it all back together. Also, the (two) fans are running at full 12V speed – OK for now, but I will need to replace the patch-wire with a 2W 13-Ohm (or 15-Ohm) resistor to drop the voltage down to approx 7V, but thats a job for later, I am waiting for delivery of suitable resistors from eBay…
For the main mains connection, the intent is to remove the 3-pin mains plug and attach the wires direct to the original APS-39 power-socket block (seen in the bottom picture – it fits into the end of the APS-39 case). The other small PCB in the picture is the first-stage AC->DC rectifier from the APS-39, so we’ll be losing that.