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-   -   A3000 - VC190 (22-47pF) and VC470 (3-11pF) (http://eab.abime.net/showthread.php?t=98361)

WStyle 07 August 2019 19:07

A3000 - VC190 (22-47pF) and VC470 (3-11pF)
 
Hello,

I need two new variable capacitors for my Amiga 3000.

VC190 value range is 22-47pF
VC470 value range is 3-11pF.

Does anyone know, which is the correct setted value of this two capacitors ?

And maybe where to get this two capacitors ?

Thanks and regards.

cdh 12 August 2019 00:56

You can find these cheap on ebay, for example:
https://www.ebay.com/itm/45-Pcs-9-Va...s/223444787706

I recommend using the blue (5pf) for your 3-11pf range replacement, and yellow (40pf) for your 22-47pf range replacement.

As an aside, be aware it is tricky to tune VC190 using an oscilloscope to measure the resultant crystal frequency because your probe is going to add some capacitance to the circuit. If you have a 10x mode, that will help somewhat.

WStyle 12 August 2019 12:07

Hello,

thanks ! Ordered the Blue and the Yellow ones.

But I dont have a Osciloscope :-(
What to do with VC190 without a Osciloscope ?

cdh 17 May 2020 08:07

Without an oscilloscope, the best I can suggest is to do repeated cycles of
1) Set the RTC to the current time
2) Wait X minutes
3) See how far your RTC is off
4) Tweak VC190
until the difference is small enough that it's good for you.

Mathesar 17 May 2020 21:28

Quote:

Originally Posted by cdh (Post 1337807)
As an aside, be aware it is tricky to tune VC190 using an oscilloscope to measure the resultant crystal frequency because your probe is going to add some capacitance to the circuit. If you have a 10x mode, that will help somewhat.

That, and most scopes are not that accurate when it comes to measuring frequency. For the kind of accuracy you need for an RTC clock you really need a dedicated counter. That said, my cheap ass Rigol DS1052E actually has a counter built-in. Dunno how accurate it is though but I used for some relative measurements once and that went quite well.


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