[Coco] Died from old age?

Chad H chadbh74 at hotmail.com
Mon Dec 22 15:17:15 EST 2014 

Indeed.  I still hadn't located the Service Manual for my model but decided to check the stuff I could, starting with the easiest and got lucky after only the second capacitor pull.  It would have saddened me to lose this unit, despite the fact I have spares, as it's my original from 1988 that my mom had bought for me when I was just 14 years old.  I've done a LOT of tinkering with it over the years, added sockets for the RAM and CPU which have been replaced, and installed a small 40mm cooling fan.

My first concern was that maybe the cooling fan I had installed had maybe overloaded something, but I have external resistors inline with the fan that drop the 12V down considerably as the fan was too fast and noisy at full power.  Also, when the power failure was occurring, the fan ramped up speed drastically! I suspect this may be due to other parts that would normally put a load on the circuit, now were not and it was getting more power than usual.  Also, the circuit is still connected and everything is still running with the original capacitors. 

Keeping my fingers crossed for now.  Already located replacement 4700uf capacitors online for under a buck so hopefully I'm good.

- Chad

-----Original Message-----
From: Coco [mailto:coco-bounces at maltedmedia.com] On Behalf Of Luis Antoniosi (CoCoDemus)
Sent: Monday, December 22, 2014 2:00 PM
To: CoCoList for Color Computer Enthusiasts
Subject: Re: [Coco] Died from old age?

The coco's are so simple that is actually fun finding these problems ;)

Luis Felipe Antoniosi



On Mon, Dec 22, 2014 at 2:57 PM, Zippster <zippster278 at gmail.com> wrote:
>
> Good to hear another machine has been brought back to life.  :)
>
> - Ed
>
>
> > On Dec 22, 2014, at 1:49 PM, Chad H <chadbh74 at hotmail.com> wrote:
> >
> > My ESR meter is home-built, arduino based. I've tried testing caps 
> > or
> resistors inline before and then removing them and got different 
> results so I just got in the habit of removing the part.  I pulled 2 
> caps this afternoon and tested them.  The microfarads was close to the 
> labeled capacitance and the ESR was .24 on the first one.  The 2nd I 
> pulled was the big 6.3v 4700uf cap.  The microfarads read ok but the ESR reading was a "?"
> so I was suspect of that one.  I didn't have any spare caps that large 
> laying around but I did have a couple of old 63v 2200uf caps I had 
> pulled from a old stereo receiver that were OK.  I installed one of 
> those and the CoCo came alive! Yay!  I tested the original 4700uf cap 
> afterwards and the ESR reading was "0".  From my experience with 
> capacitors, the ESR on good capacitors seems to be slightly more 
> elevated as the microfarads increases.  I know lower is better but 
> "0"?  Still suspect of that, but I reinstalled it and the CoCo came 
> on??  Repowered it several times and have been leaving it running, so far so good.
> >
> > At this point I think my problem is one of 2 things..
> >
> > 1)  Poor solder joint at one of the capacitors which re-soldering
> corrected (though I didn't notice any breaks under my magnifying 
> lamp.)
> >
> > 2)  The 4700uf Capacitor is 'Marginal' at best and should be replaced.
> >
> > I think I'm going to order a replacement for the 4700uf capacitor 
> > and
> compare the readings from the old one with the new one, then replace it.
> >
> > Thanks for the ideas guys!
> >
> > -----Original Message-----
> > From: Coco [mailto:coco-bounces at maltedmedia.com] On Behalf Of Gene
> Heskett
> > Sent: Monday, December 22, 2014 11:42 AM
> > To: coco at maltedmedia.com
> > Subject: Re: [Coco] Fwd: Re: Died from old age?
> >
> > On Monday 22 December 2014 11:35:56 Chad H did opine And Gene did reply:
> >> -------- Original Message --------
> >> From: Chad H <chadbh74 at hotmail.com>
> >> Sent: Monday, December 22, 2014 10:34 AM
> >> To: ooogalapasooo at aol.com
> >> Subject: Re: [Coco] Died from old age?
> >>
> >> Well I have a ESR meter so I shouldn't have any problem pulling 
> >> capacitors and testing them.
> >
> > Why should you have to pull them?  The only ESR meter I have any
> experience with uses only 85 mv of AC voltage at 100 khz to make its 
> measurement with.  This is so low that it does not turn on any 
> parallel semiconductors it may be attached to circuit wise, so the 
> caps are checked while still in circuit. Because of its time and part 
> saving measurement ability when measuring surface mounted caps in 
> DVC-PRO broadcast VCR's, it saved me at least 2/3rds of the time to 
> repair and put back in service for many of the electronics bits & 
> pieces of a busy TV station for several years.
> >
> > Its called a Capacitor Wizard, and sells for about 2 bills, made in
> Omaha originally but I think the guy has moved to KC sometime it the 
> last
> 20 years.  FWIW, caps suffering from ESR are often easily, but 
> temporarily fixed, by giving them a charge or discharge at a high 
> enough rate to form a micro-weld across the ESR causing aluminum oxide 
> at the internal to external terminal connection points.  Quite a few 
> of the other so called ESR testers do use enough voltage to "heal the cap", for a day or so.
> >
> > IMNSHO, this particular device is the only one that measures them 
> > all,
> and does it accurately and in circuit. ANYTHING else just wastes your 
> time lying to you.
> >
> > Expensive enough at $200 or so that I do not personally own one, but 
> > I
> know where one is that I can use anytime I need it.
> >
> >> I just don't know about the DIP
> >> switches.  I also need to identify the thing underneath the crazy 
> >> heat sink.  I think its some sort of power transistor.  Here are pics of
> >> the board ... tinyurl.com/ko5dxy7.   Still looking for a service
> >> manual for this model.
> >
> > That is a more or less std Si, NPN power transistor in a TO-3 case.
> Never had one fail, but a strong magnifying glass may disclose a 
> micro-crack in the solder around the pins on the bottom of the board.  
> Its cheap solder, so suck it or solder-wick it off & replace it with 
> decent, modern silver bearing solder.  Use a temp controlled iron, 
> needs 700F to remove that junk, but only about 625F to 650F for modern 
> silver bearing solders.
> > Rosin core of course.
> >
> > Lead free solders need more heat. 750F maybe. But I haven't used any 
> > of
> that roll I bought with a new soldering iron 2 years ago, and now the 
> iron has failed & no help from the american dealer in Lincoln NE, its 
> out of warranty and I feel screwed $200 worth.  But guess what? I have 
> a 54 year old weller WTC that I pulled off the shelf, and it still 
> works perfectly at whatever temp is stamped on the button on the back 
> end of the tip.  Old faithful.  Newer ones aren't so good, with fairly 
> rapid heater failures when subjected to on in the morning, off 
> whenever the bench lights go off for the night duties.  That one 
> worked under such conditions at KOTA-TV for several years, and is my emergency goto iron.
> >
> >
> > Cheers, Gene Heskett
> > --
> > "There are four boxes to be used in defense of liberty:
> > soap, ballot, jury, and ammo. Please use in that order."
> > -Ed Howdershelt (Author)
> > Genes Web page <http://geneslinuxbox.net:6309/gene>
> > US V Castleman, SCOTUS, Mar 2014 is grounds for Impeaching SCOTUS
> >
> > --
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> > Coco at maltedmedia.com
> > https://pairlist5.pair.net/mailman/listinfo/coco
> >
> >
> > --
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>
>
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