Wow, thanks for the responses. Worked on a data recovery job on old hdd's and came back to a pleasant surprise.
Well, nobody makes new polystyrene capacitors any more, so your best bet is to replace with polyester or polypropylene film capacitors, which are available in 1% and 2% tolerance from your large suppliers. As the polystyrene is likely rated for 100V, use a 250V capacitor in place of it, which should work.
actually they are rated for 50v, but im all for extra headroom. I have got polystyrene caps from mouser in the past, but not 2%. And I couldnt find any on their site so same thing essentially. Unobtainium.
It's assuming the meter you used for measurement is absolutely ideal and has no tolerance at all.
well im not exactly proud to admit but used a tc1, I would have used the lcr meter (the patient under operation) though yeah hehe. Though with the tc1, its fairly accurate, and I had the 2nd cap measuring 462pf. I performed multiple tests on each and there was no variation respectively.
Polystyrene caps rarely fail if not subjected to electrical or physical abuse, neither of which seems likely in your case. I would not be concerned about those measured values and I doubt that replacement(s) would make any difference.
thats what I was under the impression of, they seem to have a highly held reputation from my research. Though since my cap is rated at 50v and its an lcr meter I feared maybe a previous owner hooked a 63v charged cap up to test or worse... but a suspicious mind conjures its own demons. I hope you are right. Im just going off what I saw and since the resistor was spot on I went to whats holding its hand.
Polypropylene, definitely. Polyester/Mylar is low (well, modest) Q. C0G ceramic is perhaps an even better choice. Precision values may be special order, but 5% at least are widely available, and -- I just haven't looked, but 1 or 2% probably isn't crazy either.
If you can't find accurate values, but you do have a trusted meter, you can calibrate them yourself; select a low value (or choose a lower nominal value like 430pF), and connect smaller caps in parallel to make up the total. For values in this range, variable capacitors are also an option.
Tim
thanks tim. Well said. I really like this advice. I actually thought of that same ingenuity shortly after posting, if I was that desperate to get exact capacitance. but yeah good call
if im in a pinch I might do this. I actually have some variable caps as well, definitely worth keeping in mind.
It looks like OP used multimeter for measurements but expects measurement accuracy of NIST lab.
just used what I could, though I didnt see discrepancy to make me question what I saw
Given your story, IMO those capacitors are very unlikely to be the cause of your problem.
Perhaps tell us what model LCR meter you have, the problem, what you've done and then post it all in the repair forum?
its an elenco lcr-1801, ive spent hours trying to find schematics and yeah somehow theres none on the web. Despite many other companies rebranding the seemingly same products like the bk precision 875a. I could make a post in that realm though since its been over years I worry I might miss some points in history. Though ive removed many components that could be culprits. Now I have nearly every mode zeroing when before 50% had auto zeroing fault. Only 200uf has ghost readings now. It uses a maxim max130. If you wanted an idea of construction mr. Modem head has a blog post about the 1801. Though his board isnt the identical revision to mine.
Polystyrene caps have one lead as the outside foil, they usually have a polarity stripe - if it's an issue for hum and noise pickup. Testing the cap lying on a metal surface can give different readings than free air.
For audio gear, it's more important the L+R capacitance values match for stereo imaging.
Well, nobody makes new polystyrene capacitors any more ...[]
That is malarky. Mouser carries Xicon TW polystyrene caps, PS and 23PS series for many years now.
I still use them in some audio circuits and an integrator.
great to hear from you floobydust. These dont actually have a stripe but they are clear so I can see the legs internally. My tests on said caps were done freely hanging off a cardboard box. But on the other side of the board under those caps is a spring to contact the foil shielding in the back cover, and that spring pin through hole stands up around 1mm between those 2 470pf's. So assembled they would be very close proximity to a sheet of metal. Yeah mouser does have polystyrene caps. Though not 2%. Not even 2% film caps. Non stocked, custom order, and iirc smallest order was like 1000 pieces.
Again thanks everyone. You all are awesome.
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