Quadra 650 only turning on when caps are discharged?

[jbenam]

Hi everyone, just got hold of a wonderful Quadra 650 in great shape. I am having some issues powering it on, though.

After receiving it and cleaning it up, I turned it on, saw that everything worked and went to bed. Next morning it wouldn’t turn on anymore. Skimmed over some stuff about the 5V trickle charge and asked a friend to bring over his Quadra 700 power supply - everything worked immediately. Thought it could be a caps issue since it worked once, so I got to work and recapped the power supply.

Turned it on and lo and behold! Everything worked - “fixed it” or so I though. Turned the 650 went to do other stuff and came back tonight to the same exact symptoms as the first time.

That got me thinking - it’s kinda like it only starts up when caps are completely depleted?

I intend to keep the original power supply so I was wondering if any of you guys could enlighten me on the 5V trickle issue (saw a PDF that suggested replacing a diode with two 1N4148s?) - if it helps, before recapping I tried powering on the power supplying shorting pin 9 and 10 and it didn’t work, not sure if it will start the fan now, will try tomorrow.

Thanks for looking!

 

[jbenam]

Bump, I have since determined that TRKL raises to 5V with discharged caps (which allows me to turn it on if I press power very quickly) but that it quickly drains to 0V and stays like that after a few seconds.

From reading around on https://www.loneadmin.com/iicipower.php

It seems the fault lies with some diode that “holds” the voltage. I am now probing stuff around with a multimeter but I haven’t got much technical background with diodes so I don’t know if they should have a forward or reverse bias and/or if the voltage drops I am seeing are normal.

I have tracked back from Pin 10 (TRKL) and it goes up the small upright board shown in the third picture.

Sadly http://shobaffum.com/iici/trickle.pdf doesn’t apply to my stupid power supply as it’s for the Astec one and I have the Delta variant.

Has anyone else fixed this issue before? I wonder if someone can just point me to the components in question :/

Thanks for reading!

Very frustrating not being able to enjoy my new Quadra because of this stupid issue…

Some pics:

 

[jbenam]

Ta-dah! We fixed it!

In a very rare occurrence for these kind of threads, the original poster doesn’t just disappear but comes back with a fix, for everyone else to enjoy - even if we didn’t get any help even after asking repeatedly

So it turns out that the issue was with the two bleeder resistors next to the filter caps.

My initial assumption was right - TRKL only got up to 5V if caps were discharged.

Since I don’t have much time these days I gave the PSU to a good friend of mine (@jexus), who kept riffing with me on possible solutions.

Luck had it that he had an identical power supply and his one didn’t exhibit the issue.

While working on mine and probing things he got zapped - I did too, which I found very weird but I didn’t think much of it - and it turns out that even hours later the filter caps still had around 80Vs in them. His didn’t though - he checked them and they fell back to 0V immediately.

“That must be it, the bleeder resistors must be borked” we thought. He got some bleeder resistors he had around and lo-and-behold, the soft power on worked again!

So apparently the PSU design expects the filter caps to be completely drained before pulling TRKL high (I guess that the logic was the following: filter caps discharged = connected to the mains but Mac is turned off, so ready to be soft-powered on) and without TRKL high the soft-power on circuit wouldn’t proceed.

So it turns out it was just a matter of broken two resistors.

One last interesting tidbit - my unit apparently either had 220 Ohm or 270 Ohm resistors (can’t quite understand what the second band was supposed to be - either brown or a dark purple) but my friend’s came with 22kOhm ones. We put those in just to be safe.

Maybe the issue that plagues this specific power supply was wrongly fitted bleeder resistors at the factory (subsequently fixed in a revision) which being undersized would go kaput in a few years time? Who knows.

Anyway, case solved! Hopefully this info will help people repair this PSU instead of chucking the internals out and slapping an ATX PSU in it.

A shoutout to @techknight who was the only kind person to actually reply to my plea on Twitter. Thanks again, mate!

 

[cheesestraws]

Thanks for the update! This kind of information is definitely worth disseminating. Glad you posted it.