VAXstation 4000 Power Switch – Issues and Cleaning

I recently was very happy to take delivery of 2 VAXstation 4000 Model 60 machines from a generous friend I met at DEC Legacy 2011. They’ve taken some time to get to me but are in very good condition and came complete with keyboards, mice and 3W3 video leads so were all my friend (who is due t collect one of them) and I needed to get into the VAX hardware arena. In my case it will probably be the only VAX I own for the forseeable future as I don’t have room for anything bigger!

One issue I ran into with both VAXstations was the power switch, situated at the front-left, was sometimes reluctant to turn off. To understand fully why this is you have to look at the power supply operation itself and how the switch actually switches the machine on. It seemed (and I’m no expert on PSUs so forgive my ignorance) that the power supply is switched by a simple 2-state, 2 pole switch that makes or breaks a circuit depending on it’s positon. However, on inspecting the switch I discovered that the ‘I’ (on) position was actually an open (i.e. ‘off’) position switch and to turn the machine off the switch actually closed and made contact. It would seem the switch operates some sort of latch circuit that controls the power supply, rather than switching the current directly. The upshot of this, of course, was that the symptoms exhibited, the computer came on as soon as mains power was applied and refused to switch off when the switch was moved to the ‘O’ position, were indicative of a poor contact in the switch. Upon contacting the friend that supplied the machines I was informed this was a very common issue.

It seems that DEC’s logic may have been to setup the PSU so that if, over time, the PSU switch began to fail, the failure or poor contact did not render the machine inoperable and also would not if it occurred during the machines operation lead to it switching off suddenly and losing data. Clever but initially a little confusing, but this is DEC we are talking about…

Anyway, it appear s the switch just gets dirty and results in poor contact so the power control latch doesn’t drop when you switch the computer ‘off’. I thought I’d share with you the pictorial instructions I have for cleaning the power switch. It’s quite easy.

Firstly remove the PSU from the VAXstation 4000, it’s relatively simple, but sadly I ave no pictures. Remove the hard drive carrier at the front-centre then unclip the blue-coloured latch under the front of the PSU an pull. On my 4000/60s there is some kind of metal clip on the outside face of the PSU that is attached to the case. I as yet have no idea what it’s for, but it can interfere with removing the PSU so ensure it is out of the way. Some pressure underneath the PSU is required as there is a large multi-plug anchoring it to the main logic board.

Once out of the machine look at the side wit the fan outlets (it also has a longer plate that extends below the PSU). There are 2 Phillips-headed screws that hold the halves of the casing together (indicated below). Remove these.

Power Supply Casing

Next the large multi=plug that connects to the main logic board must be unclipped. There are 2 spiral clips that hold it in position. If these are compressed the plug will pop out of the holes and drop into the PSU.

Un-clip the power supply connector

With the PSU connector unclipped, the part of the casing it was attached to should now come away. It hinges away then can be detached completely.

PSU Cover

With the casing removed you can now see the insides of the PSU. Be careful, some capacitors may not be fully discharged and if the VAXstation has been shutdown recently some components may be hot! I strongly advise against using any metal implements in the PSU or performing any work that might create metallic dust.

Power Switch Location

On inspection, the switch can be seen to have a small port hole in it (this may not be the case on all PSUs!!) through which the switch’s metal armature can clearly be seen operating. It is visible below at the ‘open’ position. Remember that’s I (ON) as far as the PSU is concerned. Testing with a meter will show you that the switch makes the circuit when in the O (OFF) position. Both mine were very intermittent but would make if a gentle pressure was applied to the armature when closed.

Power Switch

Set the switch to I (ON) position and spray a good dose of cleaner into the switch. Exercise the switch between the two positions ensuring the switch opens and closes fully at least 20 times. This should clean the worst of the corrosion and dust off the armature.

Allow all the cleaning solvent to dry from the switch (alcohol cleaners evaporate quickly, especially if the PSU is still warm). Test the switch with a meter and ensure the continuity of the switch in the O (OFF) position is constant and reliable. If it is working correctly, reassemble the Power supply and replace in the VAXstation.

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One Response to VAXstation 4000 Power Switch – Issues and Cleaning

  1. Mark Wickens says:

    Excellent info here. This applies to the VAXstation 4000/90 as well which shares the same power supply (the part number is different but it just means there’s a different label on top of the same PSU). I always suspected there was something weird about the power supply. It looks like a proper disconnect from the mains completely power switch but if you plug one of these into a power consumption meter you’ll find that they draw around 30 watts even when switched off.

    I’ve had one PSU go pop in a ’90 which had to be replaced (it fried the firmware and the hard drives at the same time) and one where the solder joint on the main transformer had dried and started sparking. That one was a quick fix.

    Cheers, Mark

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