Hi guys,
I'm new to electronics, I'm building a synth project and I need
to know if what's supposed to be a polystyrene cap can be replaced by
a polyester cap. Can anybody help?
I believe that you can make this change without much concern. Both technologies are very stable vs temperature which is probably your main priority. There may be differences in breakdown voltage behavior or other characteristic, but I've done this substitution and had no problems.
Besides, polyester (aka Mylar, a DuPont tradename) caps are easier to find in catalogs. Polystyrene caps tend to have lower values overall, in the tens and hundreds of picofarads where the polyester caps are available in higher values, 1 nF (.001 µF) through .47µF (470 nF).
IIRC, polystyrene melts at about 240C, so soldering requires some care. It has very low dielectric absorption, making it especially desirable in sample-and-hold circuits.
Ooooh, that's right, Ron/Roff! It's amazing the number of late-1960s-era kits with melted polystyrene caps. They used to use a lot of them back then in solid-state receiver circuits when kit builders were still soldering PCBs as though they were working with terminal strips and tube sockets!