TRS-80 Repairs – Blown Model III/4 Power Supply

Fixing a TRS-80 or Osborne Power Supply

Michael Lohmeyer – 2013

NOTE: The below has not been verified. If you follow the below, you proceed at your own risk.

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It is common for a Model III or Model 4 to pop (combined with noxious smoke) when being powered on. Fear not, most likely it is just the power supply input cap, what is referred to as the X cap. These are metalized polyester film caps and like many capacitors, they have a limited life before they blow in spectacular fashion. The good news is, your power supply is probably OK. The cap did not blow because of a problem with the power supply. It likely blew because it was just old. Replace the cap, and the problem should be solved.

The picture to the right shows examples of these caps. All six capacitors in the picture are bad, but only the two large caps at the bottom of the picture actually blew. The smaller caps are getting ready to blow as shown by the buldging or cracked cases.

TRS-80 Models III, 4, and 4P typically use one or more Astec power supplies. Two different Astec power supplies are shown below, the upper picture with a blown cap (picture borrowed from here), and the lower pictures with a cap that is close to blowing. Note the cracks in the plastic case on the cap that is about to blow. The caps are marked with red arrows. The blown cap is marked with the yellow arrow. It is typically the larger cap that blows.

To fix the power supply, all you need is a replacement cap, and a soldering iron. The hard part, however, is finding the replacement cap.

These are special UL listed AC voltage X caps. The X has special meaning for UL and fire safety. So if you replace them, make sure you get the right type cap. The specific features you are looking for are:

For the large cap:

0.1uF, 275VAC, X2 (or X1 300V will work)

For the small cap:

0.01uF, 275VAC, X2 (or X1 300V will work)

These will almost always be some form of metalized polyester or plastic film capacitor. There are many types and sizes, but the specs above are the critical things to look for. Getting the same dimensions is preferred, but not absolutely required. Note it’s 275 VAC, not VDC. The capacitors are specifically rated for 50 and 60 Hz AC power supplies. That is why they are called X caps.

Here are some sources that should work (please let me know if any links are broken).

0.1uF, 275VAC, X2 cap

0.01uF, 275VAC, X2 cap