7
\$\begingroup\$

I'm busy repairing a guitar amp from '79 and stumbled upon these. They look like resistors, but they are definitely caps. I confirmed that with the PCB schematics and by measuring the ones with larger values, but I don't have the equipment to measure the ones in the pF range (e.g. the one in the middle should be 470pF). So I thought about replacing them, just in case. From what I found via Google, I suppose I can use ceramic disc caps as replacement. Is that correct? And is there a way to determine their maximum voltage rating?

Edit: Not sure if I may post the complete schematics, so I added images of the areas around the caps in question. The ones on the photo are C6, C8 and C5.

unknown caps schematics pcb layout

\$\endgroup\$
  • 2
    \$\begingroup\$ Showing the schematic would help. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 19 '16 at 18:13
  • 2
    \$\begingroup\$ Are you reeeeeeeeaaaaaaaally sure those are caps? Did you unsolder one and check with a multimeter? They might also be inductors as well. \$\endgroup\$ – AndrejaKo Oct 19 '16 at 18:17
  • 1
    \$\begingroup\$ In that range they have to ceramics...but they sure look like resisters. In any case, I see no reason to replace them. Ceramics, like diamonds, are forever. \$\endgroup\$ – Robert Endl Oct 19 '16 at 18:35
  • 1
    \$\begingroup\$ Can you add a picture showing more area ... maybe 5 times more?... those look like resistors. it's possible the circuit is wrong... \$\endgroup\$ – Spoon Oct 19 '16 at 18:53
  • 1
    \$\begingroup\$ No reason to replace them, ceramics don't age (mostly). Aluminums age, replace those. The colors seem to agree with the schematic values. The 10 pF part is brown-black-black. That must mean something. \$\endgroup\$ – Robert Endl Oct 19 '16 at 19:10
9
\$\begingroup\$

They're just axial ceramic capacitors. They used to be quite popular and I've used them in some designs, some (many) years ago.

You can replace them with (preferably) NP0 or X7R (if you can't get NP0 in the right value and voltage rating) ceramic capacitors. Axial leaded parts are still available from disties, though without the nifty color codes.

But if they're not shorted they're probably fine and I would suggest not messing with them.

\$\endgroup\$
  • \$\begingroup\$ That was something I figured out as well. Never heard of those before though.... tried to understand the colour codes but haven't figured it out yet though. Figured out that the middle colour is the multiply and the first two are the digits. \$\endgroup\$ – Handoko Oct 19 '16 at 19:22
  • \$\begingroup\$ Thanks! Can you also tell me how to figure out the voltage rating? As I understand it that should be in the color coding, but the calculators I tested didn't work with the color code, the gold and silver stripes proved problematic. \$\endgroup\$ – Tox Oct 19 '16 at 19:23
  • \$\begingroup\$ From what I found out of c5. Brown = 1, black = 0, multiply of black is 10 which would mean 10pF. Though, I can't figure out those last two colours which are gold and silver I guess \$\endgroup\$ – Handoko Oct 19 '16 at 19:26
  • \$\begingroup\$ Yup, that's gold and silver. One band is for the precision and one for the voltage rating, I guess. \$\endgroup\$ – Tox Oct 19 '16 at 19:32
  • 1
    \$\begingroup\$ I don't think the voltage rating is in there, though some US standard parts did that. Those parts look more like Japanese standard parts. It's more likely to be the temperature coefficient, which is not too important in your application (no tuned circuits). \$\endgroup\$ – Spehro Pefhany Oct 19 '16 at 19:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.