Atari TT PSU Repair

Repairs & upgrades.
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mgarnett
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Atari TT PSU Repair

Post by mgarnett » Wed May 08, 2019 8:19 am

Hi All,

I turned on my TT the other day and there was a loud bang from the machine which was the fuse blowing in the PSU. It blew up quite violently, glass shards everywhere. I took the supply out and examined it and I could see that there was some charring on the PSU connector that connects to the motherboard localised around the yellow wire, which I believe is the +12 volt supply.

I am going to try and fix the PSU and have ordered a new capacitor kit and will replace those when it arrives. I’ve also found that the diodes in the bridge rectifier at D1 and D2 are bad (continuity in both directions). I will next be checking the switching transistor and other suspects to try and bring it back to life.

I am unsure what caused the fuse to blow, but seeing as it looks like something on the 12 volt rail, I am assuming it might be the SCSI hard drive that has caused the failure. It was having difficulty spinning up on a few previous boot ups, but that’s my only reason for suspecting it.

Is anything else powered by the 12 volt rail? Do you think anything else in the machine might be dead, or would the fuse have protected the components?

If anyone has any advice about anything else I need to look out for, then I’m all ears!

Cheers

Mark

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exxos
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Re: Atari TT PSU Repair

Post by exxos » Wed May 08, 2019 9:11 am

PSU's are protected against shorts etc so low voltage side can't really cause a PSU to explode (bad caps can explode in low voltage side though). Normally its parts in the high voltage side (mains) such as rectifiers, main switching transistors, capacitors etc.
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mgarnett
Posts: 7
Joined: Mon May 06, 2019 7:44 pm

Re: Atari TT PSU Repair

Post by mgarnett » Wed May 08, 2019 9:36 am

Thanks Exxos,

I’ll replace all the caps, the two diodes, and the switching transistor if it tests bad. I’ve also ordered a new PC111 opto chip as well just in case....I wanted to try and rule out everything before powering it back up and testing it so as not to blow it up again!

Cheers

Mark

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exxos
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Re: Atari TT PSU Repair

Post by exxos » Wed May 08, 2019 9:39 am

A TIP:

Run a 50watt lamp via the mains input to the PSU. Its a trick we used to do. If the lamp lights up brightly, PSU has shorted out (prevents it from exploding normally) if lamp is DIM, then PSU is probably running OK.
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxoshost.co.uk/atari/last/storenew/ - All my hardware mods for sale - Please help support by making a purchase.

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