SMPS Reservoir Cap query

Hi All,

My trusty old Linn AV5103 processor/preamp PSU has stopped working. It was the start up cap and it works ok now I have replaced it. However as it's getting on a bit and the PSU PCB is in an enclosed case with no ventilation it runs very hot and all the caps are probably nearly dried out. I want to replace all of them but having trouble sourcing the reservoir caps. They are Jamicon 390uf/200v 85deg C. The only ones I can find that willl actually fit (in the UK) are all 105 deg C Types. The problem is the rated ripple current. The Jamicons are rated at 1.8A and the max I can find in 105deg C is 1.34 amps. The manufacturers data doesn't show the ripple at 85 deg. Anyone have any idea of how the caps are up-rated or solved this problem?
 
Administrator
Joined 2007
Paid Member
Although this is hard to answer without more details I'm going to say that if these are true reservoir caps on the mains input side of an SMPS then the exact value of cap (the 390uF) may well be not that critical and 330 or 470uF would be viable alternatives. Only you can make that determination. Given the low voltage rating (200 volt) suggests they must be in series if they are a reservoir cap after the mains bridge.

The fact it is a preamp/processor means the current draw is ultimately not that high and so again, I suspect ripple current isn't so much of an issue.

If you have a circuit diagram then that would help enormously.
 
Hi Mooly, Thanks for the prompt reply. Yes, there are two in series and they are on the input side. I don't have the circuit as Linn don't release them but will have to do a bit of searching. I looked at using a 470uf and still have the problem of physical size. Will have a look at 330uF caps. The only thing I can find that is virtually a direct replacement (slightly less ripple current rating) is a Cornhill Dubliier 85 degrees type but I'll have to get them from Farnell's US arm, making it an expensive recap. And i'd rather use 105 deg types anyway.
 
They are 25mm dia x 25mm high and there is virtually no clearance above with the lid on. It's actually a Linn Slimline Brilliant PSU. I suppose I could leave the top off it and fix the board in the rest of the case somehow but I prefer to leave things as original as possible. I'm trying to find a photo of it so someone may recognise it.
 
Administrator
Joined 2007
Paid Member
Oh... that's very small indeed.

If the caps are in series with nothing connected to the centre junction (apart from say balancing resistors) then could a single 385 v or 400v single cap be fitted safely on its side. You need at least a 385v for safety for UK mains.
 
Hi all. Thanks for the info. I hadn't looked on Mouser, just the usual UK suppliers as I thought they didn't deliver to the UK.

Anyway a bit more info for anyone else that may have this in the future:
The are several different versions of the slimline PSU, some with 2 reservoirs and some with 4 and it's very different from the round version as I have found the circuit for that. The secondary and startup caps are easily found as they are 100uF 35V but need to be 8mm dia to fit. Ripple rating is 500mA.
The reservoir caps in my unit are used for the voltage switching, one is used on 115v and both in series for 240V with the selector switch being on the back of the unit. Ive attached a pic I found.

I'll probably go with the 470uf's and hopefully the bridge is rated enough to take the extra surge on switch on, though will remove it later to look up the spec as it's not visible in circuit.

Thanks for all the help up to now.
 

Attachments

  • IMG_2801.jpg
    IMG_2801.jpg
    42 KB · Views: 43
Administrator
Joined 2007
Paid Member
Components ordered!
(y) The bridge should be fine. For curiosity you could see whether a surge limiter is fitted (series resistor) from the AC input to the bridge. A few ohms would be typical. Whatever is or isn't fitted though, the difference between approx 195uF (two series 390uF) and approx 235uF (two series 470uF) is absolutely minimal.

The data sheet shows recommended series limiter values but I would not go altering anything.

The smaller reservoir and smoothing caps on the secondary side always benefit from being top quality.
 

Attachments

  • 1kab-e.pdf
    70.2 KB · Views: 42
Thanks for that. I just wanted to check the bridge to make sure as you never know! I'm a bit out of touch with SMPS's and their idiosyncrasies as all we tended to do when they failed was swap them. There is no series resistor, just the usual input filtering with a surge protector. I never use cheap and unknown brands and have ordered Panasonic for the filter caps and Nichicon for the Reservoirs and I am going to change 3 large electrolytics on the audio board whilst I'm at it.
 
Administrator
Joined 2007
Paid Member
Nichicon and Pana are top quality, they will be fine.

Most SMPS failures are caused by dried out caps, dry joints and also high value resistors such as start up resistors going 'high' in value. When I think back to all the TV and VCR supplies I've had on the bench in years gone by... scary, it must be a few thousand clocked up :eek:

Just take it slowly, make sure the caps are discharged before unsoldering them and also note the polarity before removing them. Board markings have been known to be wrong or a bit ambiguous.