Polystyrene cap percentage replacement ? - Page 1
Forum section: electronics / beginners

author: algorithm
author_rank: Regular Contributor
timestamp: April 11, 2024, 08:46:28 pm
content: So im trying to repair an lcr meter. Theres no schematics.In the largest uf range it reads capacitance with nothing on test (auto zeroing failure)After running around the boards all sorts of ways, mapping out the rotary dial to modes, I started to probe parts and observe the screen to see if anything made a difference.Upon probing a leg of a resistor it seemingly zeroed !The resistor was a 562k. Took it out of circuit and it tested good.Put it back in and problem was still there.Further inspection revealed that resistor traced directly to a 470pf polystyrene cap.I pulled both identical caps from board to have comparison.The first and 1 connected to said resistor reads 458pf.The other measured 462pf.Initially I thought they were alright, until I read they are rated 2%.Now I worry that 1st one is a problem. Though barely right 470-2%=460.Though I couldnt find any seller of 2% polystyrene. So do you think I should replace it?If so, whatshould I substitute? I have polypropylene, monolithic ceramic, mylar, etc.Ive been trying to repair this lcr meter for years technically so it will be a huge relief to get resolution.Im unaware how crucial polystyrene would be in said application so curious to read what others think.Thanks for any and all input

author: SeanB
author_rank: Super Contributor
timestamp: April 11, 2024, 08:51:00 pm
content: 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.

author: wraper
author_rank: Unknown
timestamp: April 11, 2024, 08:51:34 pm
content: It's assuming the meter you used for measurement is absolutely ideal and has no tolerance at all.

author: wraper
author_rank: Unknown
timestamp: April 11, 2024, 08:52:55 pm
content: Quote from: SeanB on April 11, 2024, 08:51:00 pmhttps://eu.mouser.com/c/passive-components/capacitors/film-capacitors/?dielectric=Polystyrene%20%28PS%29

author: themadhippy
author_rank: Super Contributor
timestamp: April 11, 2024, 09:00:53 pm
content: QuoteReally?https://www.hificollective.co.uk/components/polystyrene-capacitors.html

author: wasedadoc
author_rank: Super Contributor
timestamp: April 11, 2024, 09:01:08 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 11, 2024, 09:05:30 pm
content: 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

author: wraper
author_rank: Unknown
timestamp: April 11, 2024, 09:11:05 pm
content: It looks like OP used multimeter for measurements but expects measurement accuracy of NIST lab.

author: bdunham7
author_rank: Super Contributor
timestamp: April 11, 2024, 09:38:53 pm
content: 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?

author: floobydust
author_rank: Super Contributor
timestamp: April 11, 2024, 10:15:42 pm
content: 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.Quote from: SeanB on April 11, 2024, 08:51:00 pmThat 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.

author: algorithm
author_rank: Regular Contributor
timestamp: April 11, 2024, 11:51:03 pm
content: Wow, thanks for the responses. Worked on a data recovery job on old hdd's and came back to a pleasant surprise.Quote from: SeanB on April 11, 2024, 08:51:00 pmactually 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.Quote from: wraper on April 11, 2024, 08:51:34 pmwell 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.Quote from: wasedadoc on April 11, 2024, 09:01:08 pmthats 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.Quote from: T3sl4co1l on April 11, 2024, 09:05:30 pmthanks 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 callif im in a pinch I might do this. I actually have some variable caps as well, definitely worth keeping in mind.Quote from: wraper on April 11, 2024, 09:11:05 pmjust used what I could, though I didnt see discrepancy to make me question what I sawQuote from: bdunham7 on April 11, 2024, 09:38:53 pmits 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.Quote from: floobydust on April 11, 2024, 10:15:42 pmgreat 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.

author: wraper
author_rank: Unknown
timestamp: April 11, 2024, 11:58:26 pm
content: Quote from: algorithm on April 11, 2024, 11:51:03 pmSo you have more trust in that thing than in 2% capacitors?It does not even have accuracy specs AFAIK.

author: xvr
author_rank: Frequent Contributor
timestamp: April 12, 2024, 12:19:00 pm
content: > Upon probing a leg of a resistor it seemingly zeroed !> The resistor was a 562k. Took it out of circuit and it tested good.> Further inspection revealed that resistor traced directly to a 470pf polystyrene cap.> I pulled both identical caps from board to have comparison.> The first and 1 connected to said resistor reads 458pf.> The other measured 462pf.But should be short-circuit. Not guilty.

author: CaptDon
author_rank: Super Contributor
timestamp: April 12, 2024, 01:25:15 pm
content: You stated on the highest UF scale it shows some capacitance with the leads open. Like how much 1UF on the 10,000UF scale? How bad is it percentage wise of full scale? Auto-Zero may have never been able to fully zero on that scale? Perhaps it is nearly at the threshold of not being able to auto-zero on all the capacitance scales and the main bridge needs re-balancing therefore that one scale itself is not faulty?

author: algorithm
author_rank: Regular Contributor
timestamp: April 18, 2024, 11:05:12 pm
content: Quote from: wraper on April 11, 2024, 11:58:26 pmwell its just all i have that can test less than 1nf... My fluke only reads to 1nf. But after replacing a film cap on a obscure german industrial machines circuit, the tc1 could differ from my fluke by like 2nf. So if that polystyrene cap was 2nf more.

author: algorithm
author_rank: Regular Contributor
timestamp: April 18, 2024, 11:07:19 pm
content: Quote from: xvr on April 12, 2024, 12:19:00 pmi wish i knew

author: algorithm
author_rank: Regular Contributor
timestamp: April 18, 2024, 11:11:01 pm
content: Quote from: CaptDon on April 12, 2024, 01:25:15 pmits range is 200uf. On display i see between 100uf, to 47uf to 15uf. Seems to vary everytime i try to test something out of circuit and put it back in... Last time it was 99uf and before that it was 47uf. I hoped those readings woukd point out the part obviously but it didnt for me.

author: algorithm
author_rank: Regular Contributor
timestamp: April 18, 2024, 11:22:43 pm
content: So somethings ive done. Replaced the 4 electrolytics, removed all diodes and transistors to test out of circuit. Removed all 062 op-amps to test. (1 062 was bad) removed lots of resistors. Im just fighting a losing battle it seems. I just removed 2 resistors correlated with the questionable ranges rotary traces... Color code as follows: black, gray, white, gray. 2nd resistor:gray, white, gray, gold... (Laughs) i would have assumed they were both the same but black cant be tolerance band. But wtf are they supposed to be? I cant even find an 8g ohm resistor for sale, or 8.9g. the resistors test 900ohms and 90ohms. So are the bands misprinted? Times like these...

author: wraper
author_rank: Unknown
timestamp: April 18, 2024, 11:51:26 pm
content: Quote from: algorithm on April 18, 2024, 11:22:43 pmThey are probably not even resistors.898 Ohm and 89.8 Ohm, bands in opposite order for the first one. Also please stop removing all random parts. It's not how repair is done and all that you are doing is causing damage and diminishing chance of success.

author: algorithm
author_rank: Regular Contributor
timestamp: April 19, 2024, 01:01:50 am
content: Quote from: wraper on April 18, 2024, 11:51:26 pmIf I removed a part it was because testing in circuit seemed wrong. I havent damaged anything. (As mentioned said resistors are directly connected to the rotary traces for 200uf mode, and according to band code I thought it was a g ohm)Can you elaborate on the resistor bands making sense? Seemed that either way around it was a g ohm. I tried to post photos of resistor but it didnt work.

author: wraper
author_rank: Unknown
timestamp: April 19, 2024, 01:16:57 am
content: grey - 8, white - 9, grey - 8, black - multiplier x1. When you are soldering parts they tend to drift.

author: algorithm
author_rank: Regular Contributor
timestamp: April 19, 2024, 09:13:24 pm
content: Quote from: wraper on April 19, 2024, 01:16:57 amI finally got what you meant after nearly 24 hours lol. So no tolerance band. Is that some artifact of the 80s? I dread to think of my mistaking other resistors in the same no tolerance band boat in the past. I cant say ive ever seen that before but maybe I did and was just that dumb. Weird though, a 4 band 5 band...On a similar note I noticed other resistors that the band printer messed up on on the lcr 1801. It was like 1 of the 5 bands went crooked overlapping an adjacent band marking partially. Luckily it was still discernible but I dont recall seeing that either. Though did remind me of hand painted bands from way back.Ill keep that in mind and reserve desoldering with certainty. In my defense though after 2 years of this being a work in progress to repair to say im desperate would be an understatement.. I would really like to use this lcr meter. Would be nice to be able to measure 0.1pf.Well if I get any further ill start a new topic in repair.