Polymorph wrote:
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!).
Likewise! ;-)
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"?
As I noted earlier, you may find it much easier to simply use one or two known-good electrolytics to "bridge" across the terminals of each suspected capacitor until you find the one that makes a (positive ;-) difference. A 100uF 450v capacitor is a good "high range" test capacitor, and a 22uF 450v capacitor would be a good "low range" one. This saves a lot of unsoldering and soldering and works well in the vast majority of cases. I should point out that it is a good idea to discharge the test capacitor after each "bridging" operation, since residual charge could disrupt or damage another circuit. Of course, discharging is good after you're done, too! ;-)
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?
Using the bridging technique really lowers the penalty for checking out most of the other capacitors, and electrolytics are still the most likely failure candidates.
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. ;-)
I'll take a look and see. What exactly is the symptom, again? -michael NadaPong: Network game demo for Apple II computers! Home page: http://members.aol.com/MJMahon/ "The wastebasket is our most important design tool--and it's seriously underused."