OK to use NOS (late 80s) Mica & Polystyerene caps? (application case included)

[ExtraThiccBoi]

Hi everyone,
i'm from Europe and sourcing Mica capacitors is quite an expensive job, the cheapest ones from a reputable retailer are Reichelt ones (blue CY marked caps), usually between 1 to 4 euros a piece. In my own country, i can source old capacitors made by Tesla and they are very cheap, around 25 cents per cap. Is it safe to use these Mica and polystyrene caps even though they have been sitting in a warehouse for decades?
My use cases are bypass and coupling in audio circuits.
Specifically, i am having an issue with this little fella right here (see attachment).
Originally, this cap was a thoroughly cooked 68pF Mica cap (it sits right next to preamplifier BJTs which run quite hot by design). I changed the cap for Wima 68p film, which made the dc offset in that amplifier circuit jump up to approx. 100mV from previously unstablish 20mV.
I am not very sure if Wima is the right candidate for that application or if i should change it back for a mica cap.
Thanks for any help!
Dave

[dmills]

At a value like that I would be reaching for a C0G/NP0 ceramic disk, no need for mica or film in that application, the class 1 ceramic dielectrics are fine at that sort of voltage and frequency range.

That said, if you have mica (and can make it fit) it is not exactly prone to major issues with ageing.

Regards, Dan.

[wraper]

Originally, this cap was a thoroughly cooked 68pF Mica cap (it sits right next to preamplifier BJTs which run quite hot by design). I changed the cap for Wima 68p film, which made the dc offset in that amplifier circuit jump up to approx. 100mV from previously unstablish 20mV.

It's nonsense. First of all it was stupid to replace it to begin with. Secondly, there is no way DC offset could change because of this cap. You either mechanically disturbed some trim pot when taking out PCB or excessively heated some transistor or resistor when soldering, which caused change of parameters. EDIT: or you replaced some other component you did not say about.

[IanMacdonald]

20yo nonpolarized caps will be perfectly OK. The main degradation probs are with electrolytics or waxed paper. Mileage with electrolytics varies, good brands will be perfectly OK when even 40yo, poor brands failing much sooner. Waxed paper are only found 1960's or earlier gear and should always be regarded as suspect.  (The paper becomes acidic over time, leading to leakage current) Mica caps from the 1920's are still in use today.

[ExtraThiccBoi]

It's nonsense. First of all it was stupid to replace it to begin with. Secondly, there is no way DC offset could change because of this cap. You either mechanically disturbed some trim pot when taking out PCB or excessively heated some transistor or resistor when soldering, which caused change of parameters. EDIT: or you replaced some other component you did not say about.

Hmm, interesting. I have replaced them after hearing from other owners of these amplifiers (6500 and 6600, same basic design) that these mica caps tend to go bad and can cause all sorts of issues. Especially DC offset issues. I had DC offset issues as well, so i decided to change it. Ill get one of the the NOS micas and replace it again. Btw, the old mica caps were all black charred and one of them had cracked epoxy.

[ExtraThiccBoi]

20yo nonpolarized caps will be perfectly OK. The main degradation probs are with electrolytics or waxed paper. Mileage with electrolytics varies, good brands will be perfectly OK when even 40yo, poor brands failing much sooner. Waxed paper are only found 1960's or earlier gear and should always be regarded as suspect.  (The paper becomes acidic over time, leading to leakage current) Mica caps from the 1920's are still in use today.

Well, i pulled out electrolytics from that same amplifier and all of the panasonic capacitors measured fine, even the large filter ones had values (cannot check leakage) still great and none of them was physically leaking.
Ill get some of those micas then, thanks for the help!

[wraper]

It's nonsense. First of all it was stupid to replace it to begin with. Secondly, there is no way DC offset could change because of this cap. You either mechanically disturbed some trim pot when taking out PCB or excessively heated some transistor or resistor when soldering, which caused change of parameters. EDIT: or you replaced some other component you did not say about.

Hmm, interesting. I have replaced them after hearing from other owners of these amplifiers (6500 and 6600, same basic design) that these mica caps tend to go bad and can cause all sorts of issues. Especially DC offset issues. I had DC offset issues as well, so i decided to change it. Ill get one of the the NOS micas and replace it again. Btw, the old mica caps were all black charred and one of them had cracked epoxy.

It can cause DC offset only if faulty (leaky). Otherwise it does not matter if it's ceramic, film or mica as long as part is not faulty. It cannot affect DC parameters. Of course it's possible that faulty cap was driving DC offset to desired (lower) side, and changing it made offset bigger. I suggest to just adjust offset with offset adjustment trim pot which most likely is present on the PCB.

[ExtraThiccBoi]

It's nonsense. First of all it was stupid to replace it to begin with. Secondly, there is no way DC offset could change because of this cap. You either mechanically disturbed some trim pot when taking out PCB or excessively heated some transistor or resistor when soldering, which caused change of parameters. EDIT: or you replaced some other component you did not say about.

Hmm, interesting. I have replaced them after hearing from other owners of these amplifiers (6500 and 6600, same basic design) that these mica caps tend to go bad and can cause all sorts of issues. Especially DC offset issues. I had DC offset issues as well, so i decided to change it. Ill get one of the the NOS micas and replace it again. Btw, the old mica caps were all black charred and one of them had cracked epoxy.

It can cause DC offset only if faulty (leaky). Otherwise it does not matter if it's ceramic, film or mica as long as part is not faulty. It cannot affect DC parameters. Of course it's possible that faulty cap was driving DC offset to desired side, and changing it made offset bigger. I suggest to just adjust offset with offset adjustment trim pot which most likely is present on the PCB.

Yep, i will adjust the offset back to the value given by service manual. I suspected the cap could be driving the offset nuts. I'll give the thing an overnight run when i finally assemble it back (pulled it apart to check for bad resistors or transistors as you suggested).

[LaserTazerPhaser]

...Mica caps from the 1920's are still in use today...

Transmitting mica caps?