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.
Always always DC voltage rating greater or equal to original. The screw terminal spacing must be exact. The physical dimensions can be different as long as it fits (don’t neglect top clearance to cover). The farad value can be 24mfd, which should be a common size. You can go bigger but you have to know the current rating of the rectifiers to see if they can handle the resulting higher inrush current, as well as being mindful of the effect on the transformer (higher stresses can do damage).
Warning: accidentally reversing the polarity of filter caps can cause an explosion (literally!) so triple check the original orientation with the new. It is always recommended to use a variac to power on the amplifier with about 10 volts AC and check the dc polarity across the caps to make sure they are correctly installed. l
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. :)
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)
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.
Great information from everyone, thank you. I will continue looking and see what I can locate in stock. While I have a spare amp I am currently using I would like to try and do this in a timely manner.
The manufacturers names I have seen and assume there will be minor SQ differences between them?
At least I have something to start looking for with more than just an assumption on the caps values to go with.
Just curious gs5556, why the caution as to terminal spacing. They were joined with an aluminum bar (took pics before disassembly) wnd used crimp connectors to terminals. I was figuring that I could either drill out the bar (or go across the street to a fabricator and get a new piece and drill it out) or just use a heavy gauge jump wire instead of the bar. The spacing is one of the few specs that I was having difficulty matching.
And yes, this is not the end of the troubleshooting, I was planning on insulating the wires going to the caps and powering it up to check voltages coming out of the transformers to see if there is an issue there.
Should have looked at a picture of the amp under the hood... the ground bar between the negative/positive can possibly fit if the new caps are adjusted by rotating slightly. I would make paper templates of the top of the new cap using the dimensions from the spec sheets and superimpose on the existing caps. This way you would know which brand will work.
Also, if there is a cap clamp on the bottom plate, a different diameter won’t fit. You may have to get the correct size and drill new screw holes to accommodate.
Yes, there are mounting clamps on the bottom plate. I had already figured that they would most likely have to be re-tapped if the new diameter was smaller. Obviously a same diameter cap would be the most ideal situation but from my searches thus far, that did not seem terribly likely to match.
dannad, Thank you - very interesting. I wasn't expecting that. So my reservation about inductance is unfounded. I would still be a little afraid of lower ESR. If we reduce voltage ripple using larger capacitor (and lower ESR), we have to be sure that this larger capacitor also has higher max ripple current, since shorter charging time requires larger charging current to deliver the same amount of energy (at the same load).
dannad, I would assume, that larger capacitor has larger ripple current. The thing that worries me is that, for instance, rectifier was chosen with certain peak currents in mind*. If they are deigned with certain margin, then by using lower ESR cap we effectively reduce these margins. Unless I can be sure that designer picked unnecessarily large rectifier I would be afraid of big changes. If we're talking 10% or even 20% lower ESR that should be OK, but people often put largest capacitance that fits.
*larger, but narrower charging pulses might have the same average value, but RMS value will be way higher, increasing rectifier's temperature. It will also increase transformer's temperature - not only because of higher rms (copper losses) but also higher frequency component (narrower pulses) and related core losses.
Looked at 22mfd 2.5 inch diameter caps. The United Chemi-Cons are 105mm in length and should fit nicely. Also, they use 10/32 threads so your original screws may work.
Dannad, I cannot see you simulation, but load and not the source should be a constant current. Because capacitor voltage is pretty constant you can just use resistor as the load.
Those model numbers do not have a mounting stud. Going higher in capacitance by 50% should not be a problem and, as a plus, the higher voltage plus capacitance will result in a lower ESR.
At just 2000 hours for the Cornell-Dubilier getting the above with the higher max temp should extend its life considerably. Any advantage to the other that I am not aware of?
BTW- Mouser has just refunded a previous purchase of a different cap that I was going to use. But due to not reading the data sheet correctly I ended up with a smaller cap with a mounting stud. Emailed about a RMA and got a reply from Customer Service that told me not to bother and return and was crediting me the purchase price. Talk about customer service, will definitely be making the purchase there for the replacements.
0- Do NOT skimp on price, buy the BEST if you want to use them forever (relative).
1- The higher the voltage the rating, the safer the bet. This also increases the ripple current rating which makes it more resilient. This is VERY important for higher power amps. It also protects for AC mains spikes which may occur and endanger exceeding the voltage rating, even for a very short time, if you buy them right at the limit voltage.
2 - Temperature, temperature & temperature. The higher the rating (105 degrees C) the better the capacitor will be constructed and the longer it will last. Temperature is the greatest enemy of an electrolytic capacitor and higher ratings are much better.
Check these out, even tho they are obsolete, they can still be bought here & there. They last 20,000 hrs at 105 degrees C... almost unbeatable.
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