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Re: IIgs Monitor Repair

Polymorph wrote:

On Dec 27, 8:41 pm, "Michael J. Mahon" <mjma...@aol.com> wrote:

<snip>


If the physical size is compatible, capacitors with higher
voltage ratings are no problem, and often electrolytics with
up to twice the capacitance are OK too.  (Electrolytic caps
have pretty wide tolerances on capacitance, so exact values
are usually irrelevant as long as the minimum requirement is met.)



OK, I knew you could use a higher voltage capacitor, but I didn't know
you could use one with a higher capacitance. Thanks for the tip! The
problem is, I went to Jaycar today (our local electronics supplier)
and found that they do not stock a 150uF 450V electrolytic cap - the
highest they stock is 100uF 400V, so I didn't bother buying anything
yet until I can source the right caps.


You may need to go mail order for a larger selection.


I had another idea anyhow, as I have another IIgs monitor that is
working OK, I thought I might try removing caps from it and use them
in the problematic monitor - trying one at a time until (*hopefully*)
the problem goes away. That way I should be able to isolate the
problem cap and replace only it. It will involve more work, but might
be the best way of debugging the problem? Again, experts thoughts are
welcome - I'm most definitely out of my comfort zone (please remember,
I'm a software guy, cutting his teeth in hardware) and I'm mostly
winging it with educated guesses.  :-)


Actually, substitution is a time-honored and excellent method of
isolating a failing component.

A couple of notes:

1. Sometimes a component has failed as a consequence of stress
   induced by *another* failure--for example, a resistor may have
   "smoked" because of a shorted capacitor.  In cases like this,
   substitution of a good component can result in damage to the
   good component, if the source of the problem is not fixed
   earlier.

2. Many components have a predominant failure mode that makes the
   substitution much simpler.  For example, electrolytics tend to
   fail "open" (or low-capacitance), which means that substitution
   can be easily tried by "bridging" (connecting in parallel) the
   possibly failed capacitor with a known-good capacitor of an
   appropriate voltage and capacitance (in this case, from 1/2 to
   2x the substituted capacitor).

   If the circuit then works better, the bridged capacitor is likely
   faulty.  (Watch polarity, of course!)

-michael

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Home page:  http://members.aol.com/MJMahon/

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