[Coco] Died from old age?

[Chad H]

Gene, are you saying that the voltage increase during the power failure means I likely have a failing power transistor?  The big one with the heat sink I presume.  I never touched the solder joints for that and they look really solid underneath the board.  Should I just look up the number on it and find another that meets the same voltage , etc?

Sent from my Galaxy S4

-------- Original Message --------
From: Gene Heskett <gheskett at wdtv.com>
Sent: Monday, December 22, 2014 08:54 PM
To: coco at maltedmedia.com
Subject: Re: [Coco] Died from old age?

On Monday 22 December 2014 15:17:15 Chad H did opine
And Gene did reply:
> 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!

This is a clue Chad.  Think about it.  The supply is unloaded so the
voltage rises.  And virtually the only 4 ways the supply can become
unloaded are the bolt and two solder joints on the big transistor, or the
transistor itself.  And this is one place where an exact replacement is
NOT needed as almost any of the modern ones available is at least 10x
better than the one the shack put in there nearly 30 years ago.  The
technology marches on.

> I suspect it 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.

Sounds like, although those may be a mite dusty from a long time on the
shelf & might test low for capacitance, but again with voltage on it, it
could slowly rise in capacitance.

> - Chad

[huge snip]

Cheers, Gene Heskett
--
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