Recapped PowerMac 8100 PSU and now unstable

[LaPorta]

Re: Recapped PowerMac 8100 PSU and now unstable

I took that box cap out and it tested fine. Now, I have removed the blue disc caps and tested them out of circuit with the DMM, and they all test within tolerance for their specified capacitance. They are all far too small to test ESR. I tested the large filter caps, both published at 680µf, both with 0.17 ohm ESR, one 601µf, the other 592µf, so all around good. I also took out and tested the thermistor within the circuit...It appears to be rated for 7 ohm (it is 15SP M007), and tests at 11 ohm.

I can test more ceramics as I go, but I m running out of ones to test at this point.

[stj]

Re: Recapped PowerMac 8100 PSU and now unstable

caps across the line usually have a parallel discharge resistor too.
that will make it read higher.

[PeteS in CA]

Re: Recapped PowerMac 8100 PSU and now unstable

Re CX1, measuring its capacitance in circuit will include any other X caps in parallel with it. So, for example, if there's a .47uF that is also across the AC line your .1uF cap will measure ~.57uF.

[RJARRRPCGP]

Re: Recapped PowerMac 8100 PSU and now unstable

Sounds like maybe the PSU temp is too high. (or a thermal sensor problem)

[LaPorta]

Re: Recapped PowerMac 8100 PSU and now unstable

I’ll do that. One thing also to remember is this problem existed prior to my changing out the caps. The problem existed 20 years ago when I acquired the machine, I just never got around to it till now!

[stj]

Re: Recapped PowerMac 8100 PSU and now unstable

no, check the 2 big filter electrolytics next.
i know you replaced them - but i dont know what with

[LaPorta]

Re: Recapped PowerMac 8100 PSU and now unstable

So I pulled that cap and some of the blue ones and the other larger gray box one as well. My ESR meter couldn't read them because they are just too low capacitance (0.1µf, etc), but my DMM at least verified their capacitance to be good. As for the high ESR reading, that was in-circuit and it was misreading as the capacitance reading was off as well.

Should I just shotgun replace some of these? I'll keep pulling the filter caps up to and including those around the massive ones.

[sam_sam_sam]

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

I would second that

[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

[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

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

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

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

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

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

panaonic FC looks damned close.

[stj]

Re: Recapped PowerMac 8100 PSU and now unstable

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

[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

one of the original output caps is clearly a chemicon LXF

[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.

[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