[tonyptony]

I have a phono preamp that requires a rebuild. The main power caps are starting to bulge, so I figured I'd just replace them. The problem is I can't find a direct replacement for these power caps. I need some help in identifying the best direct replacement caps for this rebuild.

Here are the details:

Cosonic 24037 - aluminum electrolytic, 1000uF, 63V, 105C rated, ripple current unknown, radial lead

I've emailed Cosonic, but their catalog has nothing that tells me what I could use for a direct replacement. Of course if anyone knows what this cap's characteristics are and can suggest an equivalent that might sound better, I'd be happy to hear about it.

[avahifi]

Gee, just go to DigiKey or Mouser and search capacitors and then fill in the appropriate numbers and you will come up with dozens of replacements.  Lots of main stream vendors make them.  Select for high ripple current and105°C rating.

Go to this link and scroll down, the mind boggles!

http://www.digikey.com/product-search/en?pv13=65&pv63=491&pv69=80&pv16=11811&FV=fff40002%2Cfff80009&k=capacitor&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25

Note that the physical size of lytics has been shrinking a lot over the years so you likely will find a 2000µF 63V radial lead cap available that will fit the board space for your old 1000µF 63V parts.

If you are looking for jack and the beanstalk types, the search will be harder and much more expensive.

Frank Van Alstihe

[tonyptony]

Thanks Frank. Coincidentally this is just what I did about an hour ago. I also used the size filter as the phono preamp is in a pretty small chassis. The max I can fit in this space is 1.240" long and .630" wide. Searching through Digikey for caps that meet the electrical and size requirements yields:

Panasonic NHG and FC series
Nichicon PW, VY and VZ
Rubycon PX
United Chemicon KMG

Not sure which would be best. Since this is a MC preamp I suspect very low noise and distortion would be a nice thing. Not sure if that would outweigh something with like a 2.7A Ripple Current (the Nichicon PW series).

[avahifi]

Assuming that you are replacing PC card mounted lytics, make sure you check the lead spacing specifications to make sure the replacement caps will fit on your circuit board.  I assume that you know that radial lead means the lead wires come out of one end, axial lead parts have the leads coming out each end and take a lot more PC card real estate and are pretty obsolete parts these days.

All of the brands you mentioned will work just fine, I would select the one with the highest ripple current rating, all things equal. Also look for the load life rating.  I would suggest a 2000 hour part at least.  I see nothing special about the Cosonic brand parts in your preamp.  They appear to be just another Taiwan brand without normal US distribution.

I also suggest you set the filters for 2200µF at 63V and see if you can find a small case version of this higher value that will fit.  Higher capacitance in the supply lytics means lower ripple after the diode bridge and thus probably a lower noise floor.

Whoops, of course you should know that electrolytic capacitors are polarized, with a plus and a minus lead.  They must be installed with the correct polarity or they will immediately short out and fail, and often blow up like a Star Trek control panel under alien attack.

Frank

[tonyptony]

Thanks Frank. Yep, all this is familiar to me. I just wasn't sure if a very high Ripple Current capability mattered all that much in a phono pre that probably draws very little current. Nice tip about trying for a larger value, though. Thanks again.

[tonyptony]

The input to the preamp is 90VAC. The DC voltage is 45VDC. I guess there's a good reason why 63V rated caps are being used here. I could probably get away with 50V caps but it may be safer to just stick with the same rating as what is being used.

[tonyptony]

Frank, is it correct to assume that the degredation of these PS caps could explain the hum I am now getting from the preamp?

[avahifi]

Certainly defective power supply capacitors will cause the preamplifier to hum.  However it is not the only reason.  There may be other defective components, open connections, or even external causes.  However, if the original capacitors are bulging, it is time to replace them.  Putting the preamp on a test bench is the best way to troubleshoot, a signal generator and scope tells you a lot more then you can identify just by looking and listening.

Of course the problem may be that it just does not know the words. 

Frank

[tonyptony]

Ha ha! Well, I have a Tek 2465 that I use for such things, but I'm always borrowing a signal generator. I really should get one off eBay or something.

Hmm, where do I get a signal generator that can put out a clean, low noise signal at less than 1 mV? If I'm going to test a MC phono preamp I need a really low voltage source.

[el34]

You may already have one... your computer, software to generate waveforms (I use Audacity) and sound card/interface.  You're only limited by the quality of the sound interface and the sampling rate.  Pad the output level down with a voltage divider.

[tonyptony]

Nice tip el34! I do have Audacity but figured I'd get too much distortion / noise driving a signal out of my PC or laptop, but I hadn't considered running a higher level signal out and padding it down.

I did replace the PS caps, as it turns out. I used the Nichicons. Hum has been eliminated but I agree that a check on a bench is probably a smart thing to do at this point. It does sound like it's fixed, though.

[el34]

I've only used this for basic frequency response testing and amp bias balance so far. To do a phono eq curve check, It would be more complicated, switchable output padding would be required when changing frequency bands assuming equal amplitude source waves.  The padding applied would then need to be taken into account when graphing.  It might be a viable alternative to the reverse RIAA filter approach.

[el34]

Oh yeah, congratulations on your repair!

[tonyptony]

Thanks!