SCSI Enclosure Recap Warning

[Phipli]

Hey folks. A few days ago my external CD enclosure failed. It took me a while to troubleshoot because a) I hate power supplies b) I find it hard to troubleshoot them because they're spicy c) I'm not very good at power supplies.



What I found was that the manufacturer had used super cheap capacitors, and unlike what we usually see, all the small caps were ok, but the large power smoothing cap had lost almost all of its capacity.

The part is a 100uF 400V capacitor. I replaced it with a Nippon SMH series part. I replaced the other caps while I was at it because they were budget brand.

I suspect the PSU had kept soldiering on until the day I switched it on and a transient knocked out the UC3842AN - this generates the PWM used to drive the MOSFET that drives the second transformer.

Public Service Announcement

I know these enclosures are common, so if you have one, I suggest replacing the large PSU cap with something better. They're not too expensive and it saves risking other part failures.

 

[AndiS]

You would not believe, I'm currently working on the exact same enclosure - cleaning, a bit of retrobriting and so on. It still works, so I did not plan to do a recap, having enough other things to do.

So thank you for the warning! I will definitely swap at least this cap before I turn it on next time. And probably the others too while I'm at it.

 

[Phipli]

You would not believe, I'm currently working on the exact same enclosure - cleaning, a bit of retrobriting and so on. It still works, so I did not plan to do a recap, having enough other things to do.

So thank you for the warning! I will definitely swap at least this cap before I turn it on next time. And probably the others too while I'm at it.

Be interested to know what capacitance your capacitor measures at out of circuit, if it is anything like the one I pulled. This is apparently a 100uF cap :

 

[s_pupp]

Did any of the smaller value caps need to be low ESR caps?

I did not consider this when I recapped a dozen or so SCSI power supplies (including a couple of those pictured here) 10 years ago, and it struck me recently that maybe I should have done so. What does the community think?

 

[pizzigri]

That is very weird I was under the impression that Nippon Chemicon was a very good brand.

 

[Phipli]

Did any of the smaller value caps need to be low ESR caps?

I did not consider this when I recapped a dozen or so SCSI power supplies (including a couple of those pictured here) 10 years ago, and it struck me recently that maybe I should have done so. What does the community think?

I used low ESR caps for the output side of the PSU as is generally advised, and I used a high ripple tollerant cappacitor for the main fat smoothing capacitor.

Hum. Your "not low ESR" caps, do you know what part number they were? Sometimes generic caps still have a fairly low ESR.

I'm not an expert but I think issues can be ripple / ringing and lower efficiency? But I'm talking out of my bottom. @GRudolf94 will likely have a more authoritative answer.

 

[Phipli]

That is very weird I was under the impression that Nippon Chemicon was a very good brand.

That's the replacement, not the original!

 

[pizzigri]

Argh I totally misunderstood! I have actually one of these I kept to maintain the 8100 look, but never used since I do not own a 8100 haha I apologize

 

[s_pupp]

I used low ESR caps for the output side of the PSU as is generally advised, and I used a high ripple tollerant cappacitor for the main fat smoothing capacitor.

Hum. Your "not low ESR" caps, do you know what part number they were? Sometimes generic caps still have a fairly low ESR.

I'm not an expert but I think issues can be ripple / ringing and lower efficiency? But I'm talking out of my bottom. @GRudolf94 will likely have a more authoritative answer.

Many thanks for the info. I unfortunately did not label my receipts with the device under repair back then (a practice I have since adopted), and so cannot easily look up the part numbers; it looks like re-recapping is in my future.

 

[GRudolf94]

You want low ESR caps on any PSU secondary side filter because that's what allows them to fill in high frequency noise - if the rail sags, and if the cap's internal resistance was high, it wouldn't be able to supply enough current to raise that rail back to nominal. Else your output rectifier will work harder and run hotter, and your device will get noisy power.

 

[MindWalker]

I might have one of those enclosures as well, might need to check on that! I had vertical SCSI hard disk enclosure that released some cap-fumes just a couple of months ago! Worked fine after a recap.

I have another external SCSI CD-R drive by Yamaha. It's my go-to external cd-drive as it seems to work with everything (including my Sparcstation). It's an early CD-writer with drive mech that has a fan (in addition to the enclosure's fan). While cleaning the drive and replacing the noisy fans, I noticed that the +5V was slightly low (even as low as 4.65V or something) so I recapped that PSU as well - it's now much more precise and doesn't flactuate like it did before.