PSU Filter cap value does not match schematic - Page 1
Forum section: electronics / beginners

author: Mp3
author_rank: Frequent Contributor
timestamp: August 14, 2021, 07:11:40 am
content: Check out this very badly bulging 22,000uf 16v capacitor.The circuit to provide high current unregulated 8V DC is made up of two schottky diodes from 7V AC to this large capacitor. This capacitor then powers a pair of 7805 regulators.The service manual lists the value of this capacitor as 33,000uf.But both the units I have here have a 22,000uf capacitor in this position.I did some googling and I saw a lot of other units also have 22,000uf but they later changed this to 33,000uf at some point....I foundthis discussionwhere a couple people say they replaced a 22,000uf with 33,000uf; but I am skeptical about taking this kind of advice at face value.... especially because the guy saying its "totally fine" to change this cap's value has some bad ideas about what makes a "good" cap. Since it is nothing but a current source for the 5V regulators, I would believe it if it'll be safe to swap it for a 33,000uf, but i wanted to ask before i do this if there's some possible danger, I don't know the resistance of the circuit or intended ripple current so I can't calculate if it will make a big difference to go from 22k to 33k.I already bought a 33,000uf and will install that if it's most likely safe... but I worry that it could put un-needed stress on the rest of the circuit if i increase that capacitance more than i need to.I attached a picture of the bulging cap! very exciting...

author: agehall
author_rank: Frequent Contributor
timestamp: August 14, 2021, 07:21:29 am
content: In this case, going to a larger cap is not an issue at all. The cap is there to smoothen the power output and you can think of it as a battery. The larger the cap (i.e. more capacitance), the more power you can store and the more “slack” you can pick up.

author: james_s
author_rank: Super Contributor
timestamp: August 14, 2021, 07:44:25 am
content: This sort of thing is very common. Either somebody will design something using one value and then during production due to either cost or maybe they have a bunch of parts to use up they decide to use a different value capacitor instead, or they designed it with one part and then later revised it due to some performance deficiency in the field, high failure rate, discontinued part, etc. As was already mentioned above, it's totally fine to upsize a capacitor like this. Electrolytic capacitors have notoriously loose tolerances in the first place and you can usually replace one with a value that is 50%-100% higher without any problems at all, especially in a bulk filter application like this. If you increased by some ridiculous amount like stuffing a 150,000uf capacitor in there you could have problems with inrush but going from 20k to 33k uf will be no problem at all.

author: Jan Audio
author_rank: Frequent Contributor
timestamp: August 15, 2021, 04:19:00 pm
content: Look in the servicemanual for a list of revisions to see the changes.Maybe they have.

author: bdunham7
author_rank: Super Contributor
timestamp: August 15, 2021, 04:37:14 pm
content: Quote from: Mp3 on August 14, 2021, 07:11:40 amIs the cap actually bad or is it just the plastic disc on top has popped up? If you can pop it back down, it's a completely external issue that has nothing to do with the cap. Are there any performance issues?It would be unusual to have a circuit like this that would have any problem with increasing the filter cap by 50%. What advice in that thread did you take as 'bad ideas'?