Author Topic: Obvious PSU fault  (Read 3049 times)

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Offline 1980s_john

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Obvious PSU fault
« on: May 10, 2017, 09:49:17 pm »
Hi,

I went to a remote site today and found this PSU - no prizes for spotting the failed cap! I had to scrape out the remains from the chassis.

I haven't seen one fail quite so spectacularly, luckily there was a spare PSU on site I could swap in. I don't think I will repair this one, but maybe time to check the spare working PSUs and re-cap one ready for next time.

Regards,
John
 

Offline tautech

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Re: Obvious PSU fault
« Reply #1 on: May 10, 2017, 09:59:24 pm »
IMHO all those e caps are past their best before date.  :scared:
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Offline floobydust

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Re: Obvious PSU fault
« Reply #2 on: May 10, 2017, 10:07:52 pm »
I see a lot of 1984 date codes, hmmm over 30 years old that's pretty good life. The cap died of old age or maybe a transient or semi failure, it's only rated for 10V.

Those blue Philips electrolytics are horrible, I hate them they had poor lifetimes and fail short or open  >:(
Axial parts are obsolete hard to get. If the customer still wants to run old gear like this, I'd recap the boards.
 

Offline MagicSmoker

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Re: Obvious PSU fault
« Reply #3 on: May 10, 2017, 10:25:48 pm »
Rifa made top-notch capacitors back in the day, but that day was, indeed, 30+ years ago. Axial elkos are getting harder to find, but there should still be sufficient stock at the usual legit distributors like Farnell, RS, Digikey, etc.

Not sure of your skill/experience level with electronics, but usually the value of elkos isn't too critical, so it would be perfectly reasonable to replace all capacitors of, say, 470uF to 1000uF (aka 1mF) with 1000uF units. In other words, don't go searching too hard for that 680uF/16V capacitor...



 

Online james_s

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Re: Obvious PSU fault
« Reply #4 on: May 11, 2017, 01:01:40 am »
It's pretty easy to replace axial caps with radial if necessary. It can even be done fairly tidy if one is careful, splice on a longer lead, slip some heatshrink over it and fold it back against the body of the cap. A small zip tie on the far end provides mechanical support.
 

Offline T3sl4co1l

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Re: Obvious PSU fault
« Reply #5 on: May 11, 2017, 06:19:06 am »
I wonder what kind of functionality they squeezed out of that discrete control circuit.  I don't see a current shunt resistor.  It may simply be self resonant.  In which case it probably gets quite hot if there's an accidentally shorted output.  Who knows. :)

Tim
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Electronic design, from concept to prototype.
Bringing a project to life?  Send me a message!
 

Offline Wirehead

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Re: Obvious PSU fault
« Reply #6 on: May 11, 2017, 07:52:52 am »
Seems it's always the rifa's that exlode... Same thing with their X2 caps.. Smokin' formula  >:D
"to remain static is to lose ground"
 

Offline WaveyDipole

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Re: Obvious PSU fault
« Reply #7 on: May 11, 2017, 08:28:01 am »
Well at least it failed open.....

For a moment there I was sure that writing on the side read 'AILING POWER CARD', but I guess that was my imagination.  I guess if those RIFA caps are getting to a point where they are failing spectacularly like that due to ageing, it would seem sensible to do some pro-active maintenance and replace them.

 
 

Offline DTJ

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Re: Obvious PSU fault
« Reply #8 on: May 11, 2017, 08:37:15 am »

For a moment there I was sure that writing on the side read 'AILING POWER CARD', but I guess that was my imagination.

I got a laugh out of that too.


Out of interest what is the PSU out of?
It reminds me of mid 80's telecomms line terminal gear.
 

Offline cvanc

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Re: Obvious PSU fault
« Reply #9 on: May 11, 2017, 12:46:20 pm »
Those blue Philips electrolytics are horrible, I hate them they had poor lifetimes and fail short or open  >:(

I totally agree.  The old blue Philips electrolytics that say MADE IN HOLLAND are trouble, especially in warm environments.
 

Offline WaveyDipole

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Re: Obvious PSU fault
« Reply #10 on: May 11, 2017, 02:42:32 pm »
Curious since I have been led to believe that those Philips blue caps are actually very stable, although admittedly 'warm environments' (unlike the UK for most part) might make a difference. I would expect those on board transistors put out quite a bit of heat so 30 years of service doesn't seem too bad a lifespan.....

I recently replaced a couple of axial caps in an oscilloscope with equivalents from Vishay. There did seem to be less manufacturer options as well as capacitance values available but some can still be found. Unfortunately they do tend to cost a premium in comparison to an identically rated radial.
 

Offline floobydust

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Re: Obvious PSU fault
« Reply #11 on: May 12, 2017, 01:44:31 am »
The blue Philips electrolytics, anytime I encounter one, it has failed. The rubber bung cracks; hopefully they've dried out- but many short. Replaced hundreds on Neve recording consoles.

This PSU is a weird design- all those high power semi's (TO-3, stud mount diodes) with no heatsinking. When is a 20A diode not a 20A diode? After a second...
 

Offline GreyWoolfe

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Re: Obvious PSU fault
« Reply #12 on: May 12, 2017, 11:56:17 am »
John, I have seen a bunch like that.  I got stuck repairing some "refurbished" ELO touch screen LCD monitors for another program in the company.  They were purchased as refurb but apparently done by the 3 Stooges.  I found quite a few where there was only the base left and a scattering of material inside the monitor.  Almost all the joints in question were cold/no solder joints, lifted pads from the rework and other foolishness.  I haven't received a shipment of bad monitors for a few years now so, hopefully, those have been upgraded and sent to the scrap heap.
That which doesn't kill you still requires a co-pay.
 


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