2. amps-preamps
  3. 160455-109191

Replacing Amp Caps

I am currently trying to replace the caps in my Bedini 100/100 amp and I am having quite the time trying to find a replacement that will work. I am hoping that someone with more knowledge can give me direction on which way I can go. The amp recently stopped outputting a signal and after all the usual internal checks, with an amp as old as this, cap replacement should be on the list of things to do.

The values and associated numbers on the OEM caps are as follows:


National Cap

85DX

+22000MFD 65VDC

85C USA 8137

 

Screw Terminals

D=2.5"

H=4.0"

Apparently 65VDC caps are something in the distance pass, of the possible replacement values is either 63V or 70V
is there a rule of thumb of which direction this value can go? Since it is for an amp, is there a brand/style I should be looking at? I can find little to no information on National Cap, do I assume they are no longer in business and move to another brand?

I have been searching for five days and fully understand I am out of my element and looking for all the help I can getand open to all suggestions.


SteveA
stevea11757
erik_squires
16,892 posts
 
In a power supply you should strive to keep the mfd about the same. Exceeding this amount by too much can stress the power transformer by increasing the surge current on turn-on.  Stay between the rating and maybe 10-15% more to be safe.

However, the V and temperature ratings can always be safely exceeded. A cap with a higher temp rating is particularly good as it will have a longer life span.

The V rating refers to the maximum before the capacitor breaks down. So, you can replace a 63V cap with a 300V cap safely. The temp is kind of the same thing, but is related to operating temperature and hours. By keeping the ambient temperature the same, but increasing the cap temp you are essentially increasing the rated lifespan. Kind of. Caps will vary. :)


Best,

E
Post removed 
kijanki's avatar
kijanki
4,477 posts
 
Steve, Rated voltage has to be equal or higher, but as you can guess if it is too high something has to give.  It is usually size or ESR.  There are many factors that would be helpful, like ESR and max ripple current.  There are tons of capacitors listed in Digi-Key or Mouser, but only few of them are in stock.  I found one that might fit and will be available in the mid November (order now).  It has 2.5" dia but is only 3.125" tall.  It can be more compact because technology always improves.  You have to check mounting (spacing and screw dia)  

https://www.mouser.com/ProductDetail/KEMET/ALS36H223L3C075?qs=b8LFrSqGEF8HOXdlWVz1Bw%3D%3D

Arrow list only the same capacitor, but no stock.  There was nothing in Newark.
Post removed 
kijanki's avatar
kijanki
4,477 posts
 
Be careful with high capacitance values that you might not need.  By getting much more capacitance you are also adding much more inductance, making sluggish power supply.  Yes, you can always bypass it with lower inductance smaller caps, but then you create parallel resonance circuit (L+C), that might ring.  Inductance can be reduced by placing many smaller caps in parallel or by using more expensive low inductance caps (like slit foil), but then you have to modify existing circuit.
More to discover