Atari 2600 Jnr PSU Video Interference

[ManicMiner1977]

Hi there,

 

I have a composite modded Atari 2600 Jnr. I have noticed a horizontal band that passes up the screen slowly.

 

I have heard that cheap PSUs can cause this issue.

 

Any advice on a good PSU that will remedy this?

 

Best regards

 

Gavin.

[SYSTEMSQUIRREL]

Yes cheaply made modern switch mode power supplies can cause this issue especially on AV modded systems. What you need is a old school transformer based power supply like this one

https://console5.com/store/power-supply-adapter-for-atari-2600-and-atari-dedicated-systems.html

 

If you’re capable of basic soldering you can also just use any old AC transformer you have kicking around that has a ouput of 9-12V DC and capable of supplying at least 500ma. You would just have to replace the tip with a 3.5mm 1/8 mono plug with the tip wired positive.

Edited August 7, 2023 by SYSTEMSQUIRREL

[+Stephen Moss]

On 8/7/2023 at 8:29 PM, SYSTEMSQUIRREL said:

If you’re capable of basic soldering you can also just use any old AC transformer you have kicking around that has a ouput of 9-12V DC and capable of supplying at least 500ma.

While that may work, it is likely that when using a transformer that can supply at least 500mA you will find that the extra power dissipation in the voltage regulator that creates would cause the voltage regulator to go into thermal shutdown. That would then require you to need to find room to add a suitably sized heatsink to compensate, otherwise the console will cycle between operating for a short period and not working for a period as the regulator heats, enters thermal shutdown, cools, retruns to correct opperation, heats and again enters thermal shutdown and so on.

 

The orignal PSU transformer would be something like a 12V AC @ 300mA, which after rectification = around 15.5V DC output off load. Consequnetly, when in use taking 500mA from the transformer drops its output voltage to around 7.5V AC which after rectification = around 9.2V DC output resulting in around 2.1W disspiated in the voltage regulator.

Whereas a transfomer that can supply at least 500mA of current would result in a higher input voltage, which in turn results in around 3.1W disspiated in the voltage regulator for a 9V transformer (50% increase) or around 5.2W (140% increase) if using a 12V transformer.

[+-^CrossBow^-]

5 hours ago, Stephen Moss said:

While that may work, it is likely that when using a transformer that can supply at least 500mA you will find that the extra power dissipation in the voltage regulator that creates would cause the voltage regulator to go into thermal shutdown. That would then require you to need to find room to add a suitably sized heatsink to compensate, otherwise the console will cycle between operating for a short period and not working for a period as the regulator heats, enters thermal shutdown, cools, retruns to correct opperation, heats and again enters thermal shutdown and so on.

 

The orignal PSU transformer would be something like a 12V AC @ 300mA, which after rectification = around 15.5V DC output off load. Consequnetly, when in use taking 500mA from the transformer drops its output voltage to around 7.5V AC which after rectification = around 9.2V DC output resulting in around 2.1W disspiated in the voltage regulator.

Whereas a transfomer that can supply at least 500mA of current would result in a higher input voltage, which in turn results in around 3.1W disspiated in the voltage regulator for a 9V transformer (50% increase) or around 5.2W (140% increase) if using a 12V transformer.

The OEM 2600 PSU is rated for 500mA output? Same for the VR inside it and yeah I've had a few that read at least 12v without a load some nearly 13v. A heatsink is required on the 7805 inside the consoles with later made 6 switch and 4 switch etc... using a large ground plane on the main board as that heatsink to dissipate the heat.

 

So the PSU only putting out 300mA is new to me when again, they are rated on the labels for 500mA? A stock 2600 with a pitfall 2 cart or flash cartridge like the Harmony actually draws about 320mA on average.

 

 

[+Stephen Moss]

20 hours ago, -^CrossBow^- said:

The OEM 2600 PSU is rated for 500mA output?

Yes, but that does not necessarily mean the transformer used is rated for 500mA.

 

20 hours ago, -^CrossBow^- said:

So the PSU only putting out 300mA is new to me when again, they are rated on the labels for 500mA?

The 300mA value I mentioned does not refer to the limitation of the amount of output current you can take from the PSU (Power Supply Unit), but referres to the possible rated output current of the transformer being used in the PSU (it may be 8V @ 400mA), but whatever it is for those types of PSU typically the design will use a transformer with a current rating less than that of the specified current rating of the PSU. I just used 12V @ 300mA as an example bacause they are typical voltage and current values for transformers and the math worked out quiet well with regards to a suitable example to help illustrate the point I was making.

 

There are three primary ways of making a 9V DC power supply for a console...

1) Use a 9V, 500mA transformer, bridge rectrifier, large smooting cap and a voltage regualtor with heatsink, known as a Regulated or Stabilzed supply.

The upside is that the output is always 9V DC whether it is off load or on load as the output voltage is independant of the amount of current drawn, up to the rated current value of the PSU. The downside is the end product is much more expensive to produce and larger.

 

2) Use a transformer with lower than required current rating and a bridge rectifier to create pseudo voltage regulation, and then place the actual voltage regulation componets (voltage regualtor, smoothing cap and heasink) inside the console where you have more space.

The upside is that the resulting PSU is smaller and cheaper to make as the size and cost of transformers increases with their current output. The downside is a lot of power is then dissipated in the transformer as the extra current draw over the reated value results in voltage losses (IxR) in the transformer winding = Power dissipation = Heat, and thus why those types of supply can get damn hot after a while.

 

Those losses caused by the excessive current draw is the reason why the output on such supplies may be something like 12-15V DC off load with no current drawn but then drops to a lower value dependant upon the amount of current being draw from the transformer, i.e. sticking with 300mA as an example, up to 300mA there will be no voltage change, at 350mA it may drop from say 12V to 11.5V, but will only drop to approximately 9V when the rated current amount of current for the PSU (i.e. 500mA) is being taken from the transformer.

 

NOTE:

1. That is how this type of PSU is designed to operate, the transformer used is carefully selected by electrical engineers to do the job required while not overheating to the point it catches fire or explodes. If you do not know what you are doing you should never attempt to take more current from a transformer or PSU than its rated value.
2. This genralized explanation and that of my previous post above should not be used as a template/guide for attempting to take more current from a transformer or PSU than its rated value. Anyone choosing to use them as such despite this warning accepts that they and they alone are responcible for any damages/physical injury/death that may result from doing so.

3) Make a switching supply, they need a lot more components, particualrly for noise filtering if you want a good one, which makes them bigger and more expensive (i.e. laptop PSU). However, the advantage is the outout voltage is stable with changes in current draw as with option 1 above and they run cooler than the liner regulated supply of option 1.

 

20 hours ago, -^CrossBow^- said:

A heatsink is required on the 7805 inside the consoles with later made 6 switch and 4 switch etc... using a large ground plane on the main board as that heatsink to dissipate the heat.

I am aware of that, however the size of the ground plane heatsink will be designed to dissipate the expected amount of power going into the regulator from an OEM PSU. What I ment was that it may need augmenting with additional heatskinking if an incorrect choice of replacment PSU/Transformer used in a home made PSU were to result in significantly more power dissipation in the 7805 than the existing heatsinking was designed to cater if thermal shutdown of the 7805 is to be avioded. 

[SYSTEMSQUIRREL]

9 hours ago, Stephen Moss said:

Yes, but that does not necessarily mean the transformer used is rated for 500mA.

 

The 300mA value I mentioned does not refer to the limitation of the amount of output current you can take from the PSU (Power Supply Unit), but referres to the possible rated output current of the transformer being used in the PSU (it may be 8V @ 400mA), but whatever it is for those types of PSU typically the design will use a transformer with a current rating less than that of the specified current rating of the PSU. I just used 12V @ 300mA as an example bacause they are typical voltage and current values for transformers and the math worked out quiet well with regards to a suitable example to help illustrate the point I was making.

 

There are three primary ways of making a 9V DC power supply for a console...

1) Use a 9V, 500mA transformer, bridge rectrifier, large smooting cap and a voltage regualtor with heatsink, known as a Regulated or Stabilzed supply.

The upside is that the output is always 9V DC whether it is off load or on load as the output voltage is independant of the amount of current drawn, up to the rated current value of the PSU. The downside is the end product is much more expensive to produce and larger.

 

2) Use a transformer with lower than required current rating and a bridge rectifier to create pseudo voltage regulation, and then place the actual voltage regulation componets (voltage regualtor, smoothing cap and heasink) inside the console where you have more space.

The upside is that the resulting PSU is smaller and cheaper to make as the size and cost of transformers increases with their current output. The downside is a lot of power is then dissipated in the transformer as the extra current draw over the reated value results in voltage losses (IxR) in the transformer winding = Power dissipation = Heat, and thus why those types of supply can get damn hot after a while.

 

Those losses caused by the excessive current draw is the reason why the output on such supplies may be something like 12-15V DC off load with no current drawn but then drops to a lower value dependant upon the amount of current being draw from the transformer, i.e. sticking with 300mA as an example, up to 300mA there will be no voltage change, at 350mA it may drop from say 12V to 11.5V, but will only drop to approximately 9V when the rated current amount of current for the PSU (i.e. 500mA) is being taken from the transformer.

 

NOTE:

1. That is how this type of PSU is designed to operate, the transformer used is carefully selected by electrical engineers to do the job required while not overheating to the point it catches fire or explodes. If you do not know what you are doing you should never attempt to take more current from a transformer or PSU than its rated value.
2. This genralized explanation and that of my previous post above should not be used as a template/guide for attempting to take more current from a transformer or PSU than its rated value. Anyone choosing to use them as such despite this warning accepts that they and they alone are responcible for any damages/physical injury/death that may result from doing so.

3) Make a switching supply, they need a lot more components, particualrly for noise filtering if you want a good one, which makes them bigger and more expensive (i.e. laptop PSU). However, the advantage is the outout voltage is stable with changes in current draw as with option 1 above and they run cooler than the liner regulated supply of option 1.

 

Let’s not get all anal retentive and dramatic here the vast majority of transformer based ac adapters that I was refering to are only going to be at the most 1 amp which is within what a 7805 regulator with a heat sink is rated to handle. Perhaps you right however maybe you shouldn't push it too much over 500ma considering the heat sinks on a 2600 console are on the anemic side not to mention that in almost all cases these regulators are going to be running with the original dried up thermal compound.

[MaximRecoil]

On 8/11/2023 at 4:14 PM, SYSTEMSQUIRREL said:

Perhaps you right however maybe you shouldn't push it too much over 500ma considering the heat sinks on a 2600 console are on the anemic side not to mention that in almost all cases these regulators are going to be running with the original dried up thermal compound.

It doesn't matter how much current the power supply is rated for, as long as it's at least 500mA (which is what the factory one is rated for). It could be rated for 1,000 amps, or a "zillion" amps, and it wouldn't make a difference, because the Atari 2600 draws less than 500mA of current regardless of how much current the power supply is capable of delivering. Power supplies don't shove current down the wires. The device being powered by a power supply draws the current it needs. Think of what goes on in a car. A car battery can deliver hundreds of amps; some of them over 1,000 amps; enough to arc weld fairly thick steel plate. Yet the tiny little lights in your dash, the small dome and other interior lights, and lots of delicate electronics are powered by the battery and the alternator; the alternator is often rated for over 100 amps by itself.

 

You can do an experiment to illustrate how it works. Connect a small 12v light bulb directly to your car battery. It will light up to its normal brightness and only draw a tiny amount of current, which you could measure with an ammeter. Then take that same light bulb and connect it inline between the positive post and the positive battery cable that goes to the starter (or between the negative post and negative battery cable that goes to the engine block) and try to start the car. The bulb will blow quickly, because now the starter is drawing hundreds of amps and the only path is through that small light bulb.