Rich Wolos said
   The earlier ones had a sticker DC output of 5v, 28a.  Otherwise same. The earlier ones were stamped REV B, newer ones  REV C. 5v, 30a. Must have been around 1990, we got some that are "old sticker 28amps" with "old leads" that have new part number on taped on, FRU 92F0088, covering up FRU 64F4114, but they forgot to change the REV, still B. 

64F4114 194 watts (from Jan 92 HMM) (Thanks for the +5v Amps, Rich!)
Yellow/Black power leads like on the 95s
+ 5v  28A
+12v 5A
-12v  0.35A
   
 92F0088 215 watts (from 96 HMM)
 Red/Black/Blue power leads.
 +  5v   30A
 +12v  5A
 -12v  0.35A

The easiest way to test the power-supply if it is still working at all is to open the case, look at the right side under the power-supply, locate the 2 plugs from the power supply to the systemboard and carefully unplug them by simply pulling them out. It might be a bit difficult to get them back this way - but if the power-supply is broken you will have to remove it nyways when you get another one, so we can leave that at the moment.

   Now: once you got the two plugs pulled out - look at them. The one "P1" marked connector has 2 blue, 6 red and 1 black cables ... that's the lesser important one at that point. The second "P2" marked is the one we need to look at closer. Do the following: re-attach the machine to the AC, then use a piece of wire or a bended paperclip and connect the white wire with one of the neighboring black ones.
    No danger - the highest voltage on that side of the power-supply is +12VDC ... the white line is "+5V sense voltage" and the black one is GND. As long as the white wire isn't attached to GND the other voltages are 0 VDC at all (power supply de-activated) so there is no risk of accidently shorting any voltage at all.

In case the connection is good and the power-supply itself is functional the power-supply fan should come up - and most likely the machines' harddisks as well, which are directly attached to the power-supply with 4-wire DC-cables. If the power-supply comes up the problem sits somewhere else:
- broken connection between ON/OFF switch and board (low - voltage)
- broken systemboard
- defective processorboard (but at least the green power-LED should light up then)

In case nothing happens apart from a faint "click" inside the power-supply ... now ... get familiar with the idea of getting a spare from somewhere. It often pays trying to get an entire working machine rather than only a power-supply.

Mod. 8590 without harddisks and memory often sold for some 30 - 40 bucks ... some ask 40$ for the power-supply alone. The 9590s may cost some more - the power-supplies however are identical as far as I know. The main difference between an 8590 and a 9590 is the 1.44MB FDD on the 8590 and that some versions come with only a 512K XGA-1 on the planar. Everything else (processorboard, memory, power-supply, harddisks) can be interchanged among the models.

Since you notified this "high pitch whistle" in your first mail I think it will not pass this test and is broken. Usually these power-supplies are pretty reliable - some of them suffer on "long term problems" with dust contamination and related overheating. In case there were an over-current condition (overload on +5V / +12V lines) it should come up in the above-mentioned test - if that malfunction is not caused by the harddisks. Try detaching the 4-pin power-plugs from the harddisk(s) and retry ... if the malfunction persists even with nothing attached to the power-supply it is definitely broken.

To fix these kind of "switching power supplies" you need some more equipment than only a multimeter, a pair of scissors and a roll of duct-tape. Parts inside these supplied are directly connected to mains AC and converted into DC impulses of hazardous voltages and frequencies ... so I would not even recommend trying to open the box if you do not exactly know what you are doing. In case of doubt: leave it as it is. There are a few "power-supply gurus" out which may be able to repair the unit - but they are hard to find.

Planar Power Supply Contacts
This machine is also known for burning the contacts on the systemboard where the power supply connectors go to. They have some problem with too small contact surfaces, the printboard-style contacts oxydate and the current drawn let them get hot. After some time the area around the contacts gets brownish and the machine looses contact and don't work anymore. This *can* be fixed in many cases with re-soldering the contact surfaces with a thin layer of soldering tin or with liquid silver - as used for rear window defoggers on cars. The inside of the power supply plugs however needs to be cleaned with a good TV tuner cleaner spray to remove the oxydation. I have seen this very often in the past years - and there is a slight chance that this is the primary cause for the trouble. In this case it could be fixed.  In the other case you need a new power supply.