Page 1 of 1

Basic Troubleshooting - ST basics - The Power Supply

Posted: Fri Jul 24, 2020 9:36 am
by rubber_jonnie
For all its complexity, the Atari ST has some fundamental areas we can focus on when we start to troubleshoot a sick machine. It is assumed that you have basic skills with a multi-meter before attempting the troubleshooting described here and can use one to carry out both continuity checks and voltage checks.

The first area to check is power. Note that the Atari ST has many different power supplies, many which are documented here: https://www.exxoshost.co.uk/atari/last/psu/index.htm

Whilst you may see the power LED illuminated at the bottom left of the keyboard, that doesn't mean that the PSU is delivering the power needed.

Before we go on, there is a warning:

THE POWER SUPPLY HAS MAINS VOLTAGE PRESENT WHICH IS POTENTIALLY FATAL. EVEN WHEN POWERED OFF, THERE IS MAINS VOLTAGE PRESENT ON THE WIRES BETWEEN THE MAINS SOCKET AND THE POWER SWITCH. WITH POWER ON, DO NOT TOUCH ANYTHING IN THE PSU WITH BARE HANDS OR UNINSULATED METAL TOOLS, AND AVOID TOUCHING THE ALUMINIUM HEATSINKS MARKED WITH RED ARROWS, AS YOU RISK AN ELECTRIC SHOCK. IF YOU ARE UNSURE OF WHAT YOU ARE DOING, DON'T DO IT WITHOUT FIRST SEEKING ADVICE.

Quick checks you should do before pulling the PSU out are:

1. Check the fuse in the power cable. If this has gone, there may be a more serious reason, however it's always worth checking before pulling the machine apart. Replace if necessary.
2. Check the continuity of the power lead from the mains plug to the IEC plug that pushes into the ST. It's unusual for these to fail, but it can happen. Replace if necessary.

Once you've done this and decided to remove the PSU for further investigation, there are a few things to check before you go ahead and test under load on the bench.

1. Check all the components for signs of failure/damage. This includes obvious failures and heat damage. Failed & heat damaged components should be replaced, but with the caveat that failed components may have done so as a side effect of another component failure, and replacing may not resolve your problem. It is just good practice.
2. With no mains lead plugged in and the PSU power switch in the off position, check continuity from the PSU mains connector to the power switch, and from the other side of the power switch to the PCB. If there is no continuity, you will need to repair/replace the section that has failed.
3. With no mains lead plugged in and the PSU power switch in the on position, check continuity from the PSU mains connector to the PCB. If there is no continuity, you will need to repair/replace the section that has failed.
4. With no mains lead plugged in and the PSU power switch in the on position, measure the resistance across the contacts. If any set of contacts has a high resistance, you should replace the switch.

I personally have seen failures in the above 4 areas, so it is worthwhile checking before going any deeper.

To deliver the correct power means delivering the correct voltage and correct current. Voltage is easy to measure, but first let's look at the PSU and break it down into functional units.

PSU.jpg
PSU.jpg (98.54 KiB) Viewed 136 times

The PSU is roughly divided into a high voltage section, to the left of the red line, and the low voltage section to the right of the red line. This is simplifying it considerably, but at least gives an idea of what you're dealing with.

It should be noted that there is a fuse present indicated by the yellow arrow, which if blown will mean there is no power to the ST. It is quite unusual for these to blow, but if they do, ensure you unplug the PSU before attempting to change it due to the proximity to the mains connections at the switch. Note that the fuses are generally a specific type, and you should only replace them with the same type of fuse.

When testing the PSU, it is advised that you do not connect it to your ST. Whilst unusual, a catastrophic failure of the PSU could kill your ST completely. Instead, since the PSU should not be run without a load, use a dummy load like the one below:

PSU_Load.jpg
PSU_Load.jpg (87.25 KiB) Viewed 136 times

This is simply made from an old CPU heatsink, and the load resistors available in exxos' store here: https://www.exxoshost.co.uk/atari/store2/#0070

Having connected our PSU to the dummy load we can turn our attention to checking the voltages. At the power connector, the red wires indicate 5v, blue is 12v and black is ground (GND). With the power switch turned on, use a multi-meter to check these voltages, paying particular attention to the 5v output.

The Atari ST has a small requirement for 12v, but if it is not present, the machine will continue to operate, so it is important to ensure the 5v supply is good. Looking at the data sheet for the 41256 RAM chips used in the ST, it states a range of between 4.5v and 5.5v to work correctly. The other chips required for operation such as the CPU, ROMs etc will have similar ranges.

At the bottom of this range, you will almost certainly see instabilities, so if your voltage is below 5v then you should consider remedial work to restore the output to what the ST needs.

There are several steps that can be taken in order to correct the voltage levels:

1. Some PSUs have a voltage adjustment pot. If you are going to adjust this DO NOT DO IT WHILST CONNECTED TO THE ST. Connect the PSU to a load that will not be damaged by accidentally adjusting the voltage too high.
2. Carry out a capacitor replacement. Some people argue this is not necessary, however given these components are 30+ years old, they might appear fine, but may be dried out internally. This requires soldering, so if you are not skilled, have somebody do this for you who is. Remember the PSU capacitors have a positive and negative polarity and they can fail spectacularly if incorrectly connected.

The two methods above will resolve most PSU problems, but do rely on the PSU working and delivering some sort of voltage that is close to the rated output.

Since this article only intends to cover basics, what do you do if the PSU does not have any output at all?

There are several options open to you if this is the case:

1. Have an electronics expert repair your existing PSU.
2. Obtain a replacement Atari PSU from an online seller. If possible, find one that has already been refurbished/recapped and is ready to go.
3. Obtain a modern replacement. This can be from Exxos https://www.exxoshost.co.uk/atari/store2/#0030, Centuriontech https://centuriontech.eu/product/stpsu-st/or something more general purpose, like a Meanwell RD-50A. The Meanwell can be had from many sites and is an excellent alternative, especially since it will accept a wide range of input voltages and is inexpensive https://www.powersuppliesonline.co.uk/r ... upply.html.

Here is an example of a Meanwell RD-50A powering a Mega ST 4:

Meanwell_Mega4.jpg
Meanwell_Mega4.jpg (105.21 KiB) Viewed 109 times

The main thing to consider is that any modern replacement should have at least the same power output capabilities as the original Atari device. All of those mentioned above fit the bill.

It should be noted that some basic skills are required for item 3, as you will need to remove your old PSU circuit board and wire in the new device. Again, soldering is required so if you are not capable, you should have an expert do this for you.