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| SMT mylar capacitors |
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| Alex Eisenhut:
Going through my TRS-80 Model 102 parts list, I saw some caps are Mylar. They are 1206 parts. I've never seen Mylar caps that small in SMT and in any case, I've only ever seen them as little SMT stacks of sheets, not a monolithic white chunk. They don't seem to be used in anything particularly demanding, C71 C72 and C73 in the level shifting networks in the attached schematic. VDD is +5V and VEE is -5V in the 102. I don't see anything special going on here, it's simply 5V CMOS logic going in from the right, and feeding these MC14584 hex Schmitt inverters which are powered at +/-5V, hence the level shifting. Of course, I am upset that regular SMT 1206 caps are not white. 1) Can anyone tell me if Mylar caps are needed here? This is the machine's RS-232 serial port. 2) Can anyone tell me if there are 1206 39nF capacitors that are white? |
| coppercone2:
well they won't work as ultrasonic transducers and disturb other things on the PCB. They also might have slightly more parasitics so they won't produce as big of an inrush as ceramics. Looks like an old circuit, maybe the ceramics had a bad reputation then. I would still consider it high end to use them in this circuit, gilding the charge pump. |
| Yansi:
There are none mylar/foil SMD caps in the photo. The reason for using them (mylar/foil) is mostly linearity. Most SMD ceramics above about 1000pF are using highly non-linear materials, making them extremely difficult to use for anything else than supply decoupling. Of course higher capacitance NP0/C0G ceramics exist, but price goes significantly higher. Also as you have already pointed out, there is no reason for a foil capacitor in that part of a circuit. 1206 mylar caps likely do not exist, not 39nF at all. Will be more like of a size of a typical THT foil cap, but without the leads. |
| coppercone2:
for a max232 everyone is gonna by default use ceramic capacitors, the data sheet says they are acceptable and people just think 'its a fucking digital logic thing fuck it im not spending any money on that' but, if you have capacitor nonlinearity, a consequence is that EMI profile might change over temperature range and across other drift parameters significantly with a charge pump circuit. |
| Alex Eisenhut:
I guess I should add that the BOM in the service manual lists C71-C73 as "Mylar". |
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