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Macintosh II PSU repair

Posted by: johnnyilblasco on 2026-05-26 01:23:53 Hi guys, I’m trying to repair an Astec 699-0389 from a Macintosh II. The primary section of the power supply is completely blown, probably due to leakage from capacitor C5. I definitely need to replace the bridge rectifier TR1 and VDR1. My doubt is about the TR1 and VDR1 (under the burned) area components: what should I replace them with? I should probably also check R3, which seems to be fine, but I don’t know its original value... The rest of the power supply seems to be in good condition, but it still needs to be checked once the primary section has been repaired. Do you have any suggestions ? Schematics in attachments

Posted by: Durosity on 2026-06-20 15:50:48 Did you get anywhere with this? I have one with a similar issue.

Posted by: franklyn on 2026-06-21 12:34:21 TR1 is a Triac http://www.frankshospitalworkshop.com/electronics/data_sheets/triac/T0805.pdf DB1 is a bridge rectifier maybe 600V/20A. Look for the correct form factor but the old one is good maybe. You can test it, there are only four diodes in it. It’s no problem with a multimeter.

Posted by: franklyn on 2026-06-21 12:38:43 Check all the diodes on the small add-on board.

Posted by: Durosity on 2026-06-21 13:06:59 Check all the diodes on the small add-on board. Useful to know, thank you!

Posted by: johnnyilblasco on 2026-06-23 10:08:12 Yes, I confirm. TR1 is a T0805NH triac, probably replaceable with a BT137-800. My problem is VDR1... I don't know the value of this varistor.

Posted by: franklyn on 2026-06-24 03:29:42 ... the VDR opperate at the half of 314 Volt max see the schematics and the are 0,25W and 0,4W ones and it feeds the Triac. Varistor - Wikipedia en.wikipedia.org

Posted by: franklyn on 2026-06-24 03:34:47 Yes, I confirm. TR1 is a T0805NH triac, probably replaceable with a BT137-800. ... if the Pinout is the same?

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