Re: IIgs Monitor Repair

[Polymorph <polymorph69@hotmail.com>]

On Dec 28 2007, 6:35 am, "Michael J. Mahon" <mjma...@aol.com> wrote:
> 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!)


Apologies for the delay in reply (I was on holidays!).

I have spent some time today trying to repair the problematic
AppleColor IIgs monitor. As I mentioned in a previous post, I have
been taking caps from a working monitor to replace the high voltage
caps in the defective monitor one at a time, but I have still not
found the cause of the problem.

The caps I've replaced are:

* 150uF / 450V  Schematic reference: C906
* 22uF / 200V  Schematic reference: C613
* 330uF / 180V  Schematic reference: C951
* 100uF / 100V  Schematic reference: C409

I will continue to keep replacing the high voltage caps >= 100V. Is
this a reasonable value to use as "high voltage"?

If the problem is still present after replacing all the high voltage
caps, is it worthwhile continuing replacing the lower voltage
electrolytics, or should I start suspecting something else as the
failure? Michael, you mentioned some non-electrolytic capacitors as
possibly being responsible. How would I find the ones you described?
Bear in mind the schematics for the IIgs monitor are available (I have
downloaded them) from Tony Diaz's info site - http://apple2.info/. So
hopefully you (or someone else) can describe what I should be looking
for, or better yet, point out on the schematic the likely ones.  ;-)

Any help would be appreciated.

Thanks,
Mike