Atari 2600 - doesn’t like switching PSUs

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Lardo Boffin
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Atari 2600 - doesn’t like switching PSUs

Post by Lardo Boffin » Fri Apr 03, 2020 8:51 pm

Hi all

I have an Atari 2600 which I recently got to a usable state.

When I got it the picture was awful and had silly amounts of interference on the screen. It was often worse than this:

C3A7B945-B3C8-4EDF-8BBA-383C89D05CA9.jpeg

I have changed the main power cap (a 2200uF beast), the electrolytic caps, a polyester film cap (part of the power circuit),the voltage regulator and fitted a composite mod board.

While doing all of this I noticed that the picture was (mostly*) rock solid / very good when using an original ZX81 PSU. If I use a modern switching PSU the results vary from a band of interference pixels going up the screen through to a completely broken / flickery / whole screen moves vertically effect.

* - if I have the video cable (which is screened) anywhere near the PSU cable or any source of power the screen starts to get vertical bands going up it again.

So I can get a perfect (well, for composite) picture when I use a transformer based PSU and position the cables carefully. But if I use a modern PSU the whole thing falls apart.

So I have a couple of questions for the electronics experts!

1) What, if anything, can I do about the interference from cables near the video cable?

2) What, if anything, can I do about the hatred of switch mode PSUs? I am not a major fan of using 30 year old transformers.

I noticed this in an RGB mod kit:

80799DAD-0255-4921-A006-5B85FFD98220.jpeg
(from http://etim.net.au/2600rgb/installation-6switch/)

Is the extra cap some sort of power smoothing process? It is not present before the mod.

Thanks for any suggestions!
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Re: Atari 2600 - doesn’t like switching PSUs

Post by danielj » Fri Apr 03, 2020 8:54 pm

What voltage switched are you using?

d.

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Re: Atari 2600 - doesn’t like switching PSUs

Post by 1024MAK » Fri Apr 03, 2020 9:21 pm

Traditional mains transformers are an order of magnitude safer than the typical high frequency transformer used in SMPSUs. Even the transformers manufactured for Sinclair (yes, they meet U.K. electrical safety standards).

Have you tried a different make/design of modern SMPSU?

If the frequency that a SMPSU uses for the DC to AC to DC conversion happens to be in the range that it mixes and ‘beats’ with another frequency (used by the item it is powering), you will end up with undesirable interference.

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Re: Atari 2600 - doesn’t like switching PSUs

Post by Lardo Boffin » Fri Apr 03, 2020 10:28 pm

I have tried 3 different switched PSUs, all three outputting 9V.

One from here https://www.thefuturewas8bit.com/shop/s ... irpsu.html

One from here https://www.oldskoolconsoles.com/produc ... wer-supply

And one that came with the Atari. It looks pretty cheap and nasty.
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Re: Atari 2600 - doesn’t like switching PSUs

Post by johnkenyon » Sat Apr 04, 2020 5:45 pm

A common fix for SMPS noise is to clean the SMPS output by feeding it into a linear PSU.

For example, for a device expecting 12v regulated, feed 19v from a laptop PSU into a 7812 regulator+heatsink.
Note that the laptop PSU will already have sufficient output capacitors to strip low frequency ripple, so any input capacitors on the linear PSU will be to reject high frequencies - you could do the maths or simply throw in a 100uF 25V electrolytic, 100nF polyester and a 10nF polyester across the input (keep the capacitor leads short and close to each other.

For 9v, find a 12V SMPS, feed either use a 7809 regulator, or get a 5V low dropout regulator and then connect the "common" pin to GND/0v via a 3v9 zener diode (anode to ground) to give you a 8.9v regulator (you'll find the low dropout 9v regulators on the same shelf as pasta, toilet rolls and rocking horse droppings)

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Re: Atari 2600 - doesn’t like switching PSUs

Post by 1024MAK » Sat Apr 04, 2020 5:49 pm

In that case, I suspect that there is either a design limitation with the Atari 2600 or a problem with your unit.

Is the analogue section of the video circuitry in a screened metal can / box?

I suspect noise (‘interference’) from the SMPSU is getting into the video circuitry. You can try to use a filter between the SMPSU and the Atari 2600.

However the values I give are a pure guess, as unless you have (or have access to) an oscilloscope, it’s not possible to do anything else...
C5FDB701-B570-4243-9B9D-F54F53945748.jpeg
I give absolutely no guarantees that this will work, as it probably will not.

I’ve shown various values of capacitor, but it would also be a good idea to put a 1nF and a 10nF in parallel with the 100nF capacitors.

Regardless, you should keep the SMPSU and it’s lead as far as possible away from any and all other leads.

Personally it may just be easier to renew the 1000uF capacitor in the ZX81 PSU, use a 1000uF 25V capacitor.

The diodes should be fine and the transformer will be safe as there is a thermal fuse either as part of the transformer or mounted next to it.

Mark

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Re: Atari 2600 - doesn’t like switching PSUs

Post by Lardo Boffin » Sat Apr 04, 2020 8:11 pm

Thanks Mark and John.

Is it a 1000uF electrolytic cap in the ZX81 PSU? It certainly sounds an easier way forward than the other route!

Most of the PCB is in a metal box but some of the circuitry is outside it. I will pop the case off and take a photo. Part of the composite mod is outside the box. I may have to drill a hole in the metal box so I can put it all inside and still get the cable out.

I have just bought another 2600 so will find out if that behaves in the same way.

This is the PCB with wires for the comp mod:

4265BF93-9112-4CF4-B53F-3D196944549D.jpeg

And this is a generic photo from the web showing it in the metal case, actual photo to follow...

CFC3F3D4-12E7-4DAC-A6FB-823D6E911066.jpeg
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Re: Atari 2600 - doesn’t like switching PSUs

Post by 1024MAK » Sat Apr 04, 2020 9:41 pm

Lardo Boffin wrote:
Sat Apr 04, 2020 8:11 pm
Is it a 1000uF electrolytic cap in the ZX81 PSU? It certainly sounds an easier way forward than the other route!
Not always :(
There were at least four different suppliers, plus sometimes UK1200 or UK1400 PSUs are found in UK700 cases...
Anyway, I’m aware of the following:
  • UK700 with a single 1000uF capacitor,
  • UK700 with two 1000uF capacitors connected in parallel,
  • UK700 with a single 2200uF capacitor,
  • UK700 with a single 4700uF capacitor (not very common as far as I can tell).
All the original capacitors have a voltage rating of 16V, but for replacements, I recommend using capacitors rated at 25V.
No harm will result if you fit a larger rated capacitor (higher capacitance or higher voltage) as long as it actually fits in the case. As modern capacitors are normally smaller this is not normally a problem.

There are only three screws that need undoing to open the case, but some also have some double sided sticky pads on top of the transformer to prevent it from rattling around. This means even with the screws removed, sometimes the top of the case does not want to pull off.

One day I mean to properly document all the UK700, UK1200 and UK1400 Sinclair PSUs that I have.

Mark

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Re: Atari 2600 - doesn’t like switching PSUs

Post by Lardo Boffin » Sat Apr 04, 2020 11:13 pm

I managed to get the composite mod board inside the metal box by changing where the wires run and not having to drill which is a bonus!

It seems to have seriously minimised interference but, and this is possibly just my imagination, slightly reduced picture quality with a little more blur. Maybe the comp mod board is now picking up some noise from the PCB?

Before:
A892F8CC-15FD-45C0-B3E3-571A5DCC68C3.jpeg
After:
70FEDC38-5DD4-4C2F-A96B-4867FAFBA04E.jpeg
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Re: Atari 2600 - doesn’t like switching PSUs

Post by Sampull-MC » Thu Apr 09, 2020 9:20 pm

Now. I'm no expert on Atari, but I know my way round them a fair bit, and being a collector of vintage consoles, I need to know how to fix them.
Lardo Boffin wrote:
Sat Apr 04, 2020 11:13 pm
It seems to have seriously minimised interference but, and this is possibly just my imagination, slightly reduced picture quality with a little more blur.
Composite or RF, the 2600 is never going to be particularly sharp. The technology is far too outdated to be capable of that.
It very well could be your imagination, but if not, I'd just get used to it.
Lardo Boffin wrote:
Sat Apr 04, 2020 11:13 pm
Maybe the comp mod board is now picking up some noise from the PCB?
Very possible. Try making a little card box to go around it covered in Aluminium tape or foil. see if that helps any.
Just make sure it doesn't short anything!

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Re: Atari 2600 - doesn’t like switching PSUs

Post by Lardo Boffin » Fri Apr 10, 2020 10:48 pm

I’ll give that a go thanks.

I got another 2600 today and the RF picture on this one is better than the composite picture on the one I have had troubles with. It also works fine with a modern switching PSU.

This is the new one with RF. The image looks better in the flesh.

4853F4C6-C198-4B23-946E-4376634118F1.jpeg

I can only assume therefore that there may be something wrong with the first one?
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Re: Atari 2600 - doesn’t like switching PSUs

Post by Sampull-MC » Sat Apr 11, 2020 1:19 am

Lardo Boffin wrote:
Fri Apr 10, 2020 10:48 pm
I can only assume therefore that there may be something wrong with the first one?
As I say. I'm not the go-to guy for Atari, but like with some Commodore 64's, different revision motherboards have (a) different components, affecting the video output. And (b) varying degrees of sharpness/contrast preset from the factory. Some might be sharp enough to make the Nintendo Switch jealous, others, not so much.
All I can really say is, their era is dead. RF is dying. All we can do is preserve what's left for the future. What we have is what we got. And some things, we'll just have to live with

There *could* be something wrong with it, but I have no ideas as to what, exactly.

But I must say, that 2600 is impressively sharp. Nice find there.
Last edited by Sampull-MC on Sat Apr 11, 2020 7:43 pm, edited 1 time in total.

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Re: Atari 2600 - doesn’t like switching PSUs

Post by 1024MAK » Sat Apr 11, 2020 6:23 am

Atari produced many different revisions of the PCBs inside it’s products, including the 2600...

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Re: Atari 2600 - doesn’t like switching PSUs

Post by Lardo Boffin » Sat Apr 11, 2020 9:06 am

My problem Atari is a revision B but I have not opened the new one yet (not sure I am going to either as it is working so well).
My original plan was to refurb the new one and sell it on to get some money towards a Tim Worthington RGB mod. The output on this is so good as RF I think I may just keep this one and not do the mod on my original one!
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