Fuse blows every time I turn on the Atari

[Roberto]

Hi guys, I have two Atari 1040 STFs, which are powered up and used regularly, but one of them (let's call it Atari A) decided to blow its fuse today. So I thought, perhaps, a grid overvoltage; I changed the fuse of equal voltage and Ampere but turning it on again the fuse blows. So I tried to replace the defective PSU with the (surely working) one from Atari B, and I found that it works: the problem is definitely the PSU from Atari A. I read somewhere that it could be the bridge rectifier (confirm me which is the one circled in red in the picture?). How is it measured? Is it a component that I find in the Exxos store? Perhaps some of you will remember that I have both a multimeter and an oscilloscope.

PS: I had just changed all the capacitors of the two PSUs.

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[rubber_jonnie]

I'd use the multimeter to check the path of the power through the fuse (A schematic will help here) so you can identify the components beyond the fuse, then work out which component is faulty in the mains voltage side.

[Smonson]

How is it measured?

Hi Roberto, a bridge rectifier is nothing more than 4 diodes in a square shape. You can measure it with your multimeter on "diode" mode by checking pairs of pins like so:

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You will be able to find the markings in red near the pins in question. Good luck with the PSU!

[Steve]

You say that you just finished recapping them? That's where I'd start trouble shooting. I recapped a PSU once, it started blowing fuses, I realised I accidentally got the polarity wrong on one of the capacitors

[rubber_jonnie]

Looks like you have an ASP34 rev3 as found on this page: ST PSUs

@Steve's suggestion is a good one, and you should also check for shorts too.

The cap orientation looks ok compared to a pic from the link I sent above, but definitely worth double checking.

[Roberto]

Hi guys, thanks for the advice. I followed the leads with a multimeter and saw that indeed there was something wrong in the bridge rectifier (one of the diodes was shorted), so I changed it and now the fuse doesn't blow anymore. So I checked the (+5 Volt and +12 Volt) outputs from the PSU connector and to my surprise I noticed that when turned on it reads 5.6 Volt immediately; after a second the value rises to 5.7, then to 5.8 and from there it goes up (until I don't know where because I immediately turned off the PSU). Same thing happens on the side of the blue wire (which should be around 12 Volts), and instead there is the same voltage as the red wire, and it behaves exactly like the red wire, going up in voltage as the seconds go by.

I took my measurements without load, therefore with only the PSU powered.
I realized that there is a capacitor that I haven't changed (for both PSUs I have), I don't remember why. It's the one on the bottom left with the red adhesive band and now that I see it's a bit swollen.

Have you ever experienced similar behavior?

[exxos]

I took my measurements without load, therefore with only the PSU powered.

You cannot test them without a load, it will be unstable.

[Roberto]

You cannot test them without a load, it will be unstable.

I would not use my Atari as a load. What could I use? Any device that runs on 5V (for red) and 12V (for blue)? Like a light bulb?
PS: I tested the other PSU (I have two identical ones) without load, and it gives me approximately stable 5V and 12V...

[exxos]

I would not use my Atari as a load. What could I use? Any device that runs on 5V (for red) and 12V (for blue)? Like a light bulb?

Load resistors or light bulb which will pull 500mA or 1A is fine on the 5V.

PS: I tested the other PSU (I have two identical ones) without load, and it gives me approximately stable 5V and 12V...

Then your meter isn't fast enough to register the voltages correctly. PSU will shutdown without a load and drop to 0v and jump to about 6V or more, then shutdown. Then will keep doing that, it's a protection feature for overvoltage. You need a scope to measure voltages properly.

[Roberto]

I would not use my Atari as a load. What could I use? Any device that runs on 5V (for red) and 12V (for blue)? Like a light bulb?

Load resistors or light bulb which will pull 500mA or 1A is fine on the 5V.

PS: I tested the other PSU (I have two identical ones) without load, and it gives me approximately stable 5V and 12V...

Then your meter isn't fast enough to register the voltages correctly. PSU will shutdown without a load and drop to 0v and jump to about 6V or more, then shutdown. Then will keep doing that, it's a protection feature for overvoltage. You need a scope to measure voltages properly.

It's true, I had never thought about it. Actually the multimeter doesn't exactly give the voltage in real time, because it has a sort of "buffer"; the unbelievable could really happen to a voltage measured by a multimeter.