[sdiy] Re: Learning electronics (was Polyevolver)

[Bob Weigel]

I wasn't thinking flameproof resistors would be likely all over a moog 
rogue and hence the thought there :-).  Perhaps a couple in the power 
supply.  Some mixing boards use them on each module which is a nice idea 
to effectively fuse each section.
   Re: glassy looking capacitors it looks like you spliced my thoughts 
together.  The poly caps aren't necessarily the little 100nf ones used 
for bypass caps near chips.  They ARE failure prone somewhat when run 
near voltage rating in amps and so on... btw and are often yellow 
colored.  I can't recall what they use for dielectric in those at the 
moment. Not polystyrene though I'm fairly certain. More likely mylar I 
beileve? Not sure though off the top.
    And then the last lines are about the little glass 'diode' looking 
caps.  Anyway not sure what those use either but.......if you have them 
in an audio circuit....any cap that is that small (with same value and 
tolerence rating....something that went without saying.  In your 
example, the ceramic cap probably wasn't the same tolerence with respect 
to slop or temperature :-) )you can *likely* substitute.  An RF circuit 
I'd be more concerned.  But small caps like that all have frequency 
response characteristics that are fine for audio and microphonic 
characteristics and field exposure aren't usually a factor in synths.  
Obviously in any application that is timing sensetive you have to have a 
capacitor that is the same tolerence range (or even select them in some 
cases for value match).  That's why like I say many oscillator circuits 
use polystyrene caps. Extremely stable.
    But life is short...and I've substituted in many applicaitons 
figuring that the mfg. PROBABLY selected the cap I'm seeing because they 
got the BEST DEAL on a WAD of them :-).  In those kinds of cases, it's 
fine to substitute a more expensive capacitor usually if you happen to 
have it on hand..  But...you should be a fairly experienced to tech to 
be able to make those kinds of decisions in general of course.  -Bob

Roy J. Tellason wrote:

>On Wednesday 20 April 2005 07:05 pm, Bob Weigel wrote:
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>>The resistors with 'different background colors' are...in all likelihood
>>capacitors also :-). 
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>Unless you're talking about the difference between brown body and blue body 
>(flameproof) resistors.  But yeah,  those light green ones are indeed caps.
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>>Some caps have been produced which have a more
>>glassy looking finish and are shaped and striped in similar form to
>>resistors.
>>   Some glass looking capacitors are high stability caps that are often
>>used in oscillator circuits and the like.  I have a big bag of 390pf's
>>in stock I seem to recall.  Usually fairly fine wires coming out of the
>>glass case with a label wound inside.
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>The ones I've bumped into the most were 100nF,  and used as bypass caps for 
>logic and such.
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>>Actually the dielectric material used is polystyrene in these though.
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>I thought it might be.
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>>They don't fail too often unless they get smashed :-).  In most cases
>>where they are used you can substituted a ceramic or mica type,
>>especially in an audio circuit.
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>I wouldn't substitute ceramic for places that didn't use it in the first 
>place,  or vice versa.  Too many circuits are going to be sensitive to the 
>parameters of ceramic caps that change wrongly for the circuit in question.
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>One time I was playing around with a 567 chip,  just to see what it could do.  
>Worked all the formulas and such and did the same with a 555 to give me a 
>tone at the same frequency,  and it didn't work.  Then I took out the ceramic 
>caps and put other types in there and it worked.
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>S/H is another area where the parts might be sensitive.
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