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 Need to know about this ceramic disc capacitor
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 Return to top of page · Post #: 1 · Written at 5:29:45 PM on 1 October 2017.
Dalpets's Gravatar
 Location: Adelaide, SA
 Member since 17 August 2017
 Member #: 2154
 Postcount: 10

I'm refurbishing a Heathkit IG-102 rf signal generator. It uses a number of RMC .02 μF +80-20 Z5U ceramic disc capacitors. Unfortunately a couple are missing from someone's previous mod. It has a green banded top.

Do you know what voltage this cap is?

If it is now unavailable is there a modern counterpart that I could use in its place?

Thanks for any help.


 
 Return to top of page · Post #: 2 · Written at 8:26:23 PM on 1 October 2017.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5341

You can still get 0.2μF caps, but they are not common anymore. In the standard series numbering scheme used today, the nearest would be 0.22uF.

As for voltage, it depends on where it's used.

What part of the circuit are these caps used in?


 
 Return to top of page · Post #: 3 · Written at 12:23:39 AM on 2 October 2017.
Dalpets's Gravatar
 Location: Adelaide, SA
 Member since 17 August 2017
 Member #: 2154
 Postcount: 10

A picture of the relevant part of the circuit has been sent for approval as an attachment to this post.

The location of the 2 caps of concern are on tag strip 'S', one between lugs 2 &3, the other between lugs 3 &4.

The power supply 'N' is 105-125 VAC.

Capacitor Wiring


 
 Return to top of page · Post #: 4 · Written at 4:54:13 PM on 2 October 2017.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 5624

Diagram uploaded to Post 3.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 5 · Written at 6:20:01 PM on 2 October 2017.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 843

Wouldn't it be great to have such diagrams on everything we work on!

Some ceramic caps are used to compensate for temperature drift. Might be worthwhile checking if the green band means that these are temperature compensating caps, though it seems unlikely as they appear to be in the power supply circuit.


 
 Return to top of page · Post #: 6 · Written at 7:01:17 PM on 2 October 2017.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 5341

The schematic would be more helpful in this case.

The tolerance range for that cap is quite wide +80 to -20% (0.36μF to 0.16uF.) so the value can't be very critical to the circuit.

If it's across the mains then using a modern type X capacitor is recommended, such as this one:

https://www.jaycar.com.au/220nf-250vac-metallised-polypropylene-x5-capacitor/p/RG5238


 
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