Recapped PowerMac 8100 PSU and now unstable

[LaPorta]

Hello everyone,

This is my first thread, and I am new here. I am part of the 68kMLA forum, and I was referred over here by one of the members there who is a member here as well.

I have a power supply from a Power Mac 8100 (Delta Electronics, Model # DPS-225AB Rev: A0, Apple Part Number 614-0040). It worked fine prior but, being close to thirty years old, it was time to recap. I have done a lot of capacitor replacement, but am by no means an expert at all. I looked up the replacement parts and ordered on Mouser. All electrolytics were replaced with same capacitance and voltage rating (or higher voltage if the identical could not be found). After reinstalling, all seemed to be well: machine starts normally, everything runs. However, after about five minutes, the entire PSU shuts down all of a sudden. Dead. This is a soft power on power supply: it has no on/off switch, and it supplies a +5V trickle current to the motherboard for soft power on/off signaling. If I try to start it right when it dies, I hear a switch/relay click inside and then right off again. I can do it about three times till it won't do anything else. Only if I unplug the supply, let it sit for 15-20 minutes, then try again will it restart again, only to last for five minutes.

I checked output voltages, and all grounds are connected properly, all +5V read 5, all +12V read around 12, and all -12 are around that too. They all are consistent: the output is dead on until the supply suddenly shuts off.

At this point, I do not know enough to troubleshoot further. Anything else to try, and any more information I can supply to assist in figuring this out?

[stj]

Re: Recapped PowerMac 8100 PSU and now unstable

pictures?
what caps did you fit?

[LaPorta]

Re: Recapped PowerMac 8100 PSU and now unstable

Attached are three photos of the supply prior to recapping. They were not the best photos, mostly for my reference while I was working. There is a horizontal board, vertical board, and also a tiny board that is sandwiched in there that I had to remove to replace two or three more.

I'll also attach a list of the caps I used to replace the ones on it with.

[sam_sam_sam]

Re: Recapped PowerMac 8100 PSU and now unstable

Can please post some pictures of the front and back of the board now after you recap it

Did you recap the daughter board or boards if it had more than one

Did you replace all the capacitors except the main filtering capacitor

If you did not replace all of the capacitors why not

Thanks

[LaPorta]

Re: Recapped PowerMac 8100 PSU and now unstable

All were replaced - including the large filter ones. Will have to take more photos now to show the whole thing.

[LaPorta]

Re: Recapped PowerMac 8100 PSU and now unstable

Attached are the photos. There is the small 120VAC in/out repeater board. The rest of the supply is two main boards at 90 degrees to each other. There are also two small daughter boards, one that had two caps that I replaced (the photo that looks like a cave). The other has what looks like small control circuitry. The undersides of the boards are very clean as well. Again, this worked fine until after the recap. Now it just shuts off after a few minutes.

[sam_sam_sam]

Re: Recapped PowerMac 8100 PSU and now unstable

Ok I have a question for you about the capacitors that were chosen what criteria did you use

I checked one of them and you used EB series I would have use FR unless that series that you select was on the board originally there if use the wrong series it possible to have stability issues please show a picture of the the capacitor that you used a different series than what was originally on the board


Panasonic Electronic Component FR FC
Rubycon
United Chemi-Con KY KZE

[LaPorta]

Re: Recapped PowerMac 8100 PSU and now unstable

Thank you for the reply. Please forgive me, as I do not know the difference between series and would appreciate being tought these differences. What would cause the instability? And, which one is it that is? If you can tell me from the list that I posted, I can figure out which capacitor on the board it was and post the requested photo...and possibly get a new part if required.

EDIT: Is that the 100µf, 50V ones? I can find them this evening per my notes on the board.

[sam_sam_sam]

Re: Recapped PowerMac 8100 PSU and now unstable

Thank you for the reply. Please forgive me, as I do not know the difference between series and would appreciate being tought these differences. What would cause the instability? And, which one is it that is? If you can tell me from the list that I posted, I can figure out which capacitor on the board it was and post the requested photo...and possibly get a new part if required.

It has to with the ESR value of the capacitors if the ESR value is low is a problem and a ESR to high can also be a problem depending on what series of capacitors that were used

Just take pictures of the ones that you used a different series than what was used on the battery I have to leave for work now so I can tell you anything else right now

[LaPorta]

Re: Recapped PowerMac 8100 PSU and now unstable

Ok so the series determines what the ESR ranges, I got it. Regrettably, I do not have the original parts. I had saved them for a while but no longer.

Would you then suggest similar capacitence/voltage rated parts, but instead of the FR series? I can always order/locally source some to try.

Also: where do I find the ESR on it's dat sheet? I likely chose this part simply because of it's high stated life.

[stj]

Re: Recapped PowerMac 8100 PSU and now unstable

one of the original output caps is clearly a chemicon LXF

[LaPorta]

Re: Recapped PowerMac 8100 PSU and now unstable

one of the original output caps is clearly a chemicon LXF

Thanks...how does that help us with this?

[stj]

Re: Recapped PowerMac 8100 PSU and now unstable

find a datasheet, and you will know the specs required/expected

[stj]

Re: Recapped PowerMac 8100 PSU and now unstable

replaced by LXY
http://www.chemi-con.co.jp/e/catalog...num_unify.html

panaonic FC looks damned close.

[LaPorta]

Re: Recapped PowerMac 8100 PSU and now unstable

You guys were on point: I changed out that 100µf with one that was closer in spec and the thing now stays on!

However, I discovered something that I did not know previously. I was having issues with the machine locking up, freezing, having bus errors, etc, within a few minutes of it starting up. I thought it was the logic board previously. However, now that I have a spare logic board and power supply to test with, I have discovered that the issue only happens when this power supply is connected. The spare PSU does not cause this issue. All I can figure is that the power output is "dirty" somehow to the digital circuitry, causing these random errors and freezes.

The question is, how do I diagnose something like that? I have a DMM and a scope, but I am just learning how to use it. Where would I track this down? Should I compare rails between the original and spare PSU? What would I look for?

[stj]

Re: Recapped PowerMac 8100 PSU and now unstable

scope the 5v pin on any eprom or ram chip - it should be close to flat.
that's where you look for ripple - at the load.

if you can find it, you can also compare the 12v signals - but i doubt that could cause crashes.

[LaPorta]

Re: Recapped PowerMac 8100 PSU and now unstable

I did as you suggested. Both PSUs supply a constant +5V DC to the 5V pin of one of the built-in RAM chips. With the known good PSU, the 5V is constant no matter what time course adjustment. However, with the PSU that is giving me issues, I can detect what appears like a sine-wave interference within the 5V signal becoming noticeable at a scale of 100 ns/division. This is not visible at all on the good PSU. So, I would assume this tiny wave does not interfere with gross function (power on, etc), but at some point causes garbage either in the RAM of Processor processes. Any way to know what this is or how to track it further?

[stj]

Re: Recapped PowerMac 8100 PSU and now unstable

maybe it's coming from mains.
check any ceramic or film caps connected between the mains supply and the output-ground.
does it use any RIFA resin-cased caps?

[LaPorta]

Re: Recapped PowerMac 8100 PSU and now unstable

There are a number of ceramics and what look like thermistors in the input section. There is also a board with transistors, etc. on them that is tied into the circuit. The input from the mains is connector J1. The last photo is the actual input board. There are two disc caps on there, they appear to bridge between the hot and neutral to ground, respectively.

The gray box cap, CX1, in line with the connector J1, has a listed value of 0.1µf on its casing, but tests with both my ESR tester and DMM at 0.58µfm with ESR of 2.2ohm. Not sure if this could be artificially high due to it being in-circuit, I can take it out and test separately. Would it being way more than 10% out of whack possibly cause this?

[stj]

Re: Recapped PowerMac 8100 PSU and now unstable

not sure about that 2.2ohm one - it sounds high but i'd needto check a good one to compare