ATARI CENTIPEDE RESTORATION – PART 2: POWER SUPPLY UNIT

Welcome back readers, to another entry on this restoration. If you missed the previous part(s), catch up HERE, or use the menu to start from the beginning.

By now, you will be aware that I am hoping to update this particular restoration with shorter, more specific posts, concentrating on one area of business, taking it back to basics, on a (fingers crossed) more frequent basis.

For today, we will be looking at the power supply ‘brick’ unit which was common place across a swathe of Atari cabinets of this era. Those of you that read through my ASTEROIDS restoration will immediately notice the similarities between the units in both cabinets.

The power supply on this cab is made up of a number of ‘parts’ which we will look at in more detail, but includes a heavy transformer, meaning that the unit is usually always found on the floor of the cabinet.

Here you can see it, as it was upon taking receipt of the cab, sitting on the floor in the middle, with the audio regulator PCB immediately behind it and the main game PCB on the right hand side.

Dirty

As you can see, especially on the next photo, it was in a bit of a state, both from dust, dirt and rust, but hey, that’s easily sorted right?

Dirty, Dirty

First up, detach all the wiring looms and connectors – I take lots of photos at this point, numbering the connectors and putting masking tape around each group of wires, writing reminders on the tape as to what each connects to. Then it’s a simple job of unscrewing the unit from the wooden base and lifting it out. Once removed you can see the original colour of the plywood, which has been protected from the grime over the last 40 years.

Dirty, Dirty, Dirty

With the PSU out of the cab and up on the bench, with everything disconnected, it is a simple task of taking the removable stuff off, cleaning everything down with whatever you wish to use, toothbrush, cotton buds, soapy water, Isoprop, you choose, the end result will be the same. Just make sure you give this plenty of dry time, on a sunny window sill or airing cupboard, for plenty of time before you put any electricity through it again.

Pure filth

Once clean and back together again it looks (almost) like new. Now, the original base is zinc coated which is not something that can just be ‘cleaned up’ once the coating has tarnished, with any exuberant cleaning simply removing the coating completely. You can see a couple of spots where this has occurred, which I’m happy enough with, as it is still miles better than what was there before. But the only way to fully ‘renew’ this is to get it re-plated again, which itself can have mixed results from friends I know who have tried it.

Oooh, shiny!

A couple of top tips to zhuzh up the appearance of the transformer block, is to use a fine sandpaper on the copper banding, and then to further impress (and I thank IamJimmi for this) is to remove the four long corner bolts, which frees up the individual plates that make up the block, allowing you to turn over the rusted top plates revealing their, hopefully, clean, shiny underside.

As far as cleaning goes, the same principal can be used on the AR2 and game PCB, which I did and you can see below.

I have then mounted these onto some simple mdf boards which will allow me to wire everything back up and test, out of the machine, but we will cover this in a later episode.

Talking of wires, the loom needs a bit of attention too. It is filthy, hacked, missing parts and in some areas is actually a little dangerous. First things first though, a bath. We can look at the loom and it’s missing bits another time.

Then off with the old, broken plug and on with a new. The cable was also filthy, but it was free from any breaks in the insulation, so it joined the loom for a soapy bath together.

Ok, so with all the tidying up completed it was time to put some power through and check the outputs. It is important to do this first, before connecting any other parts to the psu, as if something has shorted and/or failed, it could lead to a much higher voltage being passed to the sensitive PCBs or monitor chassis.

Now many blogs, before me have covered this and there are numerous videos on YouTube that specifically cover this PSU, or probably any other that you seek to find and are all an invaluable resource to helping you check your own, and are indeed what I used to begin with to check my own.

But I was just copying/mimicking what they were doing without really understanding why/how. So I thought I would try and make sense of it and share it on here. Of coure I could be wrong, so take this with a pinch of your chosen condiment.

The best place to start looking, is at some schematics, even if you cannot fully understand them, they still help to indicate what should be where. Which is why I always look to pick up a full set for this and my other cabs. Plus the Atari booklets really are beautiful in their own right.

Inside the main booklet is an illustrated diagram of the psu, seen from both the top and bottom, with all parts named, etc.

And then inside one of the schematic booklets there is this diagram;

This shows the mains power coming inside [bottom left corner], along with the jumper blocks used by Atari to change the psu to accept the different local area voltages [top right corner], which are not relevant to what I want to cover here. We are mainly interested in the diagram from the middle to the right hand side.

From the mains coming in from the plug, it passes through the filter, which is on the underside of the psu from the first diagram, where the earth ground becomes the chassis ground that is then linked around the cabinet. The chassis ground is illustrated by the little image that looks like a garden fork.

So moving towards the right hand side of the diagram we can see the large connector (labelled J5) with it’s 15 connections, and the various voltages associated to each pin. You can also see that the first 5 pins go through a diamond, labelled CR1, which the PSU diagram tells us is the bridge rectifier pcb found underneath the PSU.

Schematics can be confusing to read as they do not, usually, follow the physical layout of the parts, instead depicted in a way that makes it aasy to display/read. Similar to the London Underground Map in a way.

For instance J5 is a connector with the 15 pins set out in a 5×3 formation, whilst the schems show a single line from 1 to 15. Also the distances as in the London Tube map, are completely fabricated to make the schems ‘flow’.

Is this confusing? What if I tried to draw my own diagram that combined the two?

Looking at this we can (hopefully) visualise where we should be poking our multimeter, a little better. Pins 1, 2 and 3, are set to measure 10.6v DC and have their own secondary ground, from pins 4 and 5. To measure any circuit the circuit must be complete, so we need to measure the first 3 pins individually, pairing each one with either of the ground pins. It matters not which ground is used. It is also important that at this stage the voltage is unregulated (the audio regulator board will address this later in the circuit) and without load so the readings may be a little off, but anything way out means we/you have a problem that needs further investigating, before going any further.

The schems then show that the rest of the pins make their own mini circuits, with 6 paired to 7 to provide 36v AC; 8 and 9 to provide 6.1v AC; 10, 11 and 12 giving 60v AC along with 13, 14 and 15 also. And you can follow the schems further to see where these voltages all get sent.

But the difference in measuring these, apart from changing your multi-meter from DC to AC, is that we are not going to use either of the secondary ground pins at pins 4 or 5 like we did above. The circuit that we are looking to measure is made between the pins themselves. For instance, to measure the voltage at pins 6 and 7, we actually measure just those pins, one probe in each, looking for close to 36v. If you did use one of the ground pins, say probing 4 and 6, then you would expect to see a measurement of half what the schems show, 18v.

I don’t know if that makes any more sense, or confuses you even more, but it certainly helped me understand why I was getting the results I was getting.

But with my voltages all checked and firing right where they should be, I can put the PSU to one side for now and I think we can tie a bow around that and wrap this up for now. Ooh that reminds me, I need to do some online christmas shopping……

Please let me know what you think; any thoughts you have on how far to go with restoration projects; and the game itself, Centipede, surely one of Atari’s more appealing, accesible games, right?

As always, if you like what you find here, please subscribe, read through my earlier posts, hit like, and share on the social media of your choice, via the buttons below. I genuinely would love to hear what you think, good or bad, and will always reply to any comment, kind enough to be left.

Thank you for taking the time to read through, see you all soon for the next instalment.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s