Quadra 700 Astec PSU - Bad caps or something else?

[Daniël]

As you might have recently seen, I've acquired a Quadra 700 that seemingly had a dead power supply. I've since done some more testing, and I'd like to know whether I just got a case of bad caps, or something more.

In the machine, when powering up, the speaker would make some pops, each spaced out a second or so, with the LED flickering with each one. I later noticed the PSU fan would also twitch during those pops.

I've taken out the power supply, and tried running it on its own by shorting pin 9 and 10, which makes this PSU turn on. It will loudly click every few seconds, with the fan trying to spin up inbetween clicks.

The voltage rails don't really get to go to full power, they swing around 3V (-3V on the -12V rail) while the PSU is forced on.

Reading on 68kMLA, I did see mentions of a diode that could cause the power supply to fail powering on, hence why I'm wondering if it's truly bad caps.

The PSU is a Astec AA15831. Has anyone here ever had that PSU with the same or similar behavior, and have you been able to diagnose/fix it? I'd love to get the original PSU going again, but if it's going to get too complex, I might do what others have done and retrofit it with a modern PSU. Thanks in advance for any help, tips and suggestions!

 

[Daniël]

Bump!

 

[jessenator]

I haven't experienced that particular problem, but with a PSU that old, new caps is good due diligence and might solve the issue. Not sure if there's much difference between the AA15831 and the AA15830, but you could take a look at the cap list here:



I'm sure those Zener diodes aren't terribly expensive, but not knowing the exact spec, I can't say for sure.

 

[Daniël]

I haven't experienced that particular problem, but with a PSU that old, new caps is good due diligence and might solve the issue. Not sure if there's much difference between the AA15831 and the AA15830, but you could take a look at the cap list here:



I'm sure those Zener diodes aren't terribly expensive, but not knowing the exact spec, I can't say for sure.

I've taken apart my AA15831 to check the diodes, I didn't find anything out of the ordinary with a quick multimeter diode check. However, the 15831 and 15830 are quite different, so I have taken the opportunity to snap photos, which I hope to use to make a comprehensive cap guide, to contribute to your thread

 

[techknight]

This is called cycling. This is the result of the power supply is going back into shutdown from startup. 

This happens from 1 of 2 conditions. Overcurrent, or overvoltage. If the secondary diodes are not shorted, the most likely cause is a bad opto-isolator in the feedback network. the SMPS controller isnt sensing feedback from the output stage so it goes into a lockout. the lockout capacitor timing circuit discharges, and the SMPS controller attempts to restart. Wash, rinse, repeat. 

If its a capacitor problem, applying heat to the power supply would rule this out. Such as using a heat gun or a safer hair dryer. Once the power supply is good and warm, it will startup if you have capacitor issues. 

Otherwise, if not, your trouble is in the feedback network if your secondary rectifier diodes are good. 

 

[Daniël]

This is called cycling. This is the result of the power supply is going back into shutdown from startup. 

This happens from 1 of 2 conditions. Overcurrent, or overvoltage. If the secondary diodes are not shorted, the most likely cause is a bad opto-isolator in the feedback network. the SMPS controller isnt sensing feedback from the output stage so it goes into a lockout. the lockout capacitor timing circuit discharges, and the SMPS controller attempts to restart. Wash, rinse, repeat. 

If its a capacitor problem, applying heat to the power supply would rule this out. Such as using a heat gun or a safer hair dryer. Once the power supply is good and warm, it will startup if you have capacitor issues. 

Otherwise, if not, your trouble is in the feedback network if your secondary rectifier diodes are good. 

Turns out it wasn't just the PSU that is bad. I've tested the PSU on a spare IIsi board I have for a project, and it won't power it on either, but no clicking. I then tried my DIY ATX adapter (that worked on said IIsi), and found that the splitsecond the Quadra 700 powers on, the ATX PSU just throws itself into a protection mode, not powering on regardless of what's attached for a few minutes. Turns out, the Quadra 700 has two shorts to ground, on the 5V rail and the 12V rail! The 12V rail short was fixed by removing a 22uF 16V tantalum capacitor at C48, that had gone open. However, despite some initial troubleshooting involving carefully lifting up caps attached to 5V on one side with the soldering iron, and looking for obvious bridges, I've not yet found where the 5V short is. I fear it's going to be an annoyingly painful process to figure out what's the short, but I'll have to do it anyways. But that's for later, I'm going to lose my sanity if I keep going at this for hours on end.

 

[techknight]

Yea tantalum caps suck. They dont last either. At least they dont leak. But they do short. 

 

[miko-Tokyo]

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Sorry. I'm using Japanese. You need the help of google or DeepL to read English.

I can't get information on older Macs in Japan.

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Right now, I am working on repairing the power supply unit of my Macintosh IIci,cx.

However, I can't get it to work by replacing the capacitors; I can only hear the relays continuously when I input 6v to pins 9 and 10 and switch it on.

Also, if you plug it into a working logic board, nothing will work.

Can you tell me what exactly I should do about this part?

>This happens from 1 of 2 conditions. Overcurrent, or overvoltage. If the secondary diodes are not shorted, the most likely cause is a bad opto-isolator in the feedback network. the SMPS >controller isnt sensing feedback from the output stage so it goes into a lockout. the lockout capacitor timing circuit discharges, and the SMPS controller attempts to restart. Wash, rinse, >repeat. 

I have an ASTEC power supply, probably an early and late model.

Can anyone give me some advice?

Translated with www.DeepL.com/Translator (free version)

 

[miko-Tokyo]

This is a picture of the capacitor after replacement.