changing microfarad values in replacing capacitors in an amp

[haskellbob]

I want to replace the electrolytic capacitors on a circuit board for the power amplifier section of an old Heathkit AR-15, but I can't find 250mfd, 25v electrolytics anymore. The closest value I can find is 220 mfd.


This capacitor seems to be a filter, as itisolates the input, the Base, of the first transistor in the power amp circuit, which receives the signal from the pre-amp, from ground.

is that 30 mfd difference going to cause problems?

Thanks for da help.

[vtech]

In this application, (coupling) it is not going to make a difference as 250 MFD has not been a common value for some time & not readily available at corner parts house though still avail thru places like Mouser. Also take in to account the 10~15% value tolerance common with electrolytics in general.

However, as a rule in most applications with electrolytics, best practice is to choose the next higher value if unable to find the original--in this case 330MFD 25Volts(higher voltage is also OK) is common & easy to find.

[haskellbob]

Thanks. That was very helpful -
although I haven't been able to find a 250 mfd even at Mouser...

[vtech]

[torbjorn]

Most older electrolytic caps have a very wide tolerance range - typically -10, +50 % or -20, +80 % for old parts from the 60's or 70's, while the manufacturers use to state a symmetrical tolerance range - for example ± 20 % today.

So if you have some caps marked 220, 250 and 330 uF, their tolerance range will surely overlap each other.

In most applications, it does not harm if the caps have a larger value than the original ones, that gives a better margin for capacitance loss due to aging. If i had to repair some older equipment with 250 uF caps, i would probably replace them with 330 uF.

[haskellbob]

Thanks very much for your help and for the information.

Bob