DEC H7822 power supply

[Maciej W. Rozycki]

Hi Peter,

> I still have the leaky electrolytics I removed from the POWER-ONE PSU in my
> Cisco IGS a while back.  I stored them with their leads up and goo seems to
> be still oozing out of some of them despite their inactivity and orientation.
> These ones are marked Nichicon PL(M) 4700uF/63V, 2200uF/16V and 330uF/35V and
> also have markings like H8950, H9018 and H8946 - maybe these are date codes?

 Right, my notes indicate Nichicon PL parts might be problematic too, e.g. 
one at 4700uF/10V on 5V output of the H7826 PSU.

> I also removed the smaller ones like 47uF/35V PF(M) H8952 for example but it
> is less clear to me whether these were leaking too or just got leaked on by
> the others.  There were only a few of them so I decided they were better out
> than in.  They all have similar coloured brown sleeves like the faulty ones
> in the DEC power supplies too.

 I can confirm now Nichicon PF 47uF/35V parts to be the source of an issue 
with my Bel Power.  All four leaked.  Thankfully I was able to fully 
revive that PSU (now in 24/7 operation).

> There are also the leaky 10uF/35V axial electrolytics in my LK201 keyboard.
> Those are in orange sleeves and marked "ESZ", whatever that is.  They have
> date codes like 8612.  I thought this might be a widespread problem but
> so far I have only found it in one keyboard.

 These are likely standard type parts.  I've yet to come across a low 
impedance axial capacitor type.  I may have missed something of course.

> Looks like I need to go back and recheck everything I thought wasn't leaking
> last time I checked :-(
> 
> Thing is, to check them, they have to come out of the case and that involves
> at least some change in orientation, except for contortionists...

 I doubt a temporary reorientation of parts you want to replace anyway is 
going to cause any trouble in that short amount of time.

> > Heat dissipated by the cap itself under high ripple current never helps 
> > and will surely speed up cap deterioration.  After all its service life 
> > halvens with each 10°C temperature rise even with non-faulty parts.
> >
> 
> When I was shopping for replacements, I was a bit alarmed to find that
> the maximum specified "endurance" (whatever that is) I could find was
> 5000 hours.  This isn't much more than a long life incandescent light bulb.

 Mind that it's at 105°C.  If you keep such caps operating at 65°C (which 
is still rather hot), then endurance raises to 80000h (~9 years continuous 
use).

 Anyway try to chase replacements specified for at least 10000h at 105°C.
Nichicon HE/UHE and Panasonic FR seem suitable replacements for Chemi-con 
SXF, Nichicon PL, surpassing old parts in terms of ESR/impedance/ripple 
and dimension-wise.

> All the leaking ones in DEC H7821 and H7822 PSUs I have come across so far
> are 1800uF/25V Chemicon with brown sleeves.  I think mine are all SXF
> but I am not 100% sure of that.  There are lots of other electrolytic
> capacitors in these power supplies but I've only looked closely at the
> larger ones.  All of the reservoir capacitors attached to the mains bridge
> rectifiers that I have seen look fine.  Maybe I need to go back and check
> the smaller ones though :-(

 I don't have any H7822 PSU.  Your experience with the H7821 is the same 
as mine though (and I still need to figure out what's wrong with one which 
still doesn't drive its power-good line active after recapping).  I used 
Nichicon HE P/N UHE1E182MHD as the replacement for those.  There does 
appear to be COVID-related shortage of this part (600 expected at Mouser 
15/03/2023, ugh!), which used to be readily available in large quantities 
several years ago.  However Panasonic FR P/N EEUFR1E182 is available in a 
small quantity (and is better).

> I think I came across some LXF ones that seemed to be ok, I can't remember
> where though.  I probably need to go find these and check them again :-(

 I came across LXF parts in one H7826 only and they were clean, but I 
chose to replace them as a precaution anyway as I've got stuck with trying 
to repair a couple of broken H7826 PSUs already still not working after 
cleaning the mess and replacing broken caps (mind that I'm a software 
engineer with enough hassle to sort out on the software side already).

> Here's a thought.  Apart from the keyboard, all the ones I have seen that
> are leaking are filtering the outputs of switch mode power supplies.  I
> wonder does the higher frequency of the ripple they are dealing with have
> a bearing on this?

 I've seen leaks from SXF parts on the primary side too with the H7826, so 
it is not that they only fail on the secondary side.  Also it is lower 
frequency ripple that's more problematic, see frequency correction factors 
for ripple current in datasheets, because impedance is higher at lower 
frequencies.  That's an inherent property of capacitance.

> I replaced the five leaking capacitors on the upper board in my H7822,
> disconnected the input to the lower board and moved the LED connection to
> the upper board.  The machine powered up nicely afterwards, the fans spun
> and the green LED came on after a short delay.  However, the diagnostic
> LEDs all come on and stay on so it appears the CPU is being made aware that
> the lower board is not functioning, even though I don't need it.  I should
> to do some comparisons with a H7821 and see if I can work around this.  Or
> maybe I could fit some not quite to specification electrolytics from the
> junk box on the lower board, just to keep it happy?

 The symptom is exactly like with my broken H7821.  Check the power-good 
signal (brown wire with the H7821, possibly likewise with the H7822).  It 
should be driven high at the TTL level.

  Maciej