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Polystyrene caps and noise pick-up

a.palfreyman

a.palfreyman

pfm Member
Some 'styrene caps are end-marked for outer foil, but some don't appear to be. I've had a play with some unmarked ones by placing the oscilloscope across the end wires but after swapping the connections they seem to be similar in how much mains noise that is picked up.
Is it possible to determine which end is outer foil if they are not marked, or do these not have a full wrap of one foil and so are equally susceptible to noise?
 
Good question, and I don't know the answer. Does your test give the right answer on the marked up ones?
 
Try holding the body cap between your finger and thumb whilst doing the 'scope test.
 
I founds philips styrene barrel caps could be identified, never managed it with lcr, milflex or Siemens styrene caps.

The squared cased lcr seemed less prone to melt when soldering as well....
 
Sorry PD, don't think I have any marked ones.
Yep, tried that Mike, but was inconclusive... the noise pick-up seemed to vary with the rotational position of the cap. You soon know if you stray onto the wire to the 'scope input though...:eek:
The HF filter caps on the input to my modded NCC200 pick up a fair bit of noise using the 'poked finger' test, but in use it is very quiet.
 
'Acquired' a pair of 330pF marked-end 'styrenes (from the test-lab at work :eek:) so will see if I can determine if these are better etc. My Maplin soldering iron is a good source of rectifier switching noise...;)
 
The marked 330pF caps show good screening with earth probe at marked end and not the other. I've retested the unmarked 470pF caps and they are not quite as good, but better 1 way around. No correlation to the printed values though. 3 are best at one end and 1 best at the other. I'll whip the 'styrene filter caps out of my amp and see what's what.
 
The ends on these are mid-wrap...:rolleyes:
Anyway, before I remove them I'm going to try the 'scope on them in-situ (amp off) to see if I can determine anything before going any further. Might be lucky...
 
Good Morning All,

Today's stupid question but if said polystyrene capacitor was adjacent to such as a wire wound resistor or an inductor.............

Regards

Richard
 
Dont know TBH. On the ncc boards it goes straight to the base of either the input transistor tr1 or the VAS current source transistor tr6 in the form of a hf filter. I suspect that tr1 will be more critical.
I'm going to give my amp some use over next couple of days to see how it settles and then see if I can determine which way round the caps at c14 are.
To temper this, see @Arkless Electronics comments above.
 
Well I found that three across the two boards were slightly noisier with the finger prod test that the fourth as 1 produced no notable increase in noise at the outputs where the other three did. Switched them around so all good. Then out of curiosity I tried my finger next to the signal input spades... this was actually a lot worse for mains / rectified noise pick-up.
 
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