NAP 160 puzzling cap explosion...

[smee]

A puzzling thing has happened to my trio of CB NAP 160s - There's been an occasional smell of dying electronics and most recently a bit of noise through my speakers, so my assumption was the reservoir caps were on their way out in at least one of the three (they're all '82 or older, and not serviced to my knowledge). Upon opening them up though, I find that the reservoir caps actually look ok, but the preamp PSU's have all suffered an explosion of the RH tant cap on the PCB. Nothing's been plugged up wrongly, (i.e. they've not accidentally had my Snaps or Hicap chucking 24v at them) they've been plugged into a Naxo for a number of years (they've not had to power a preamp during my ownership), though the system has been stored unused for 3-4 years prior to the summer...

I'm going to service them all but just wondering if this is just something that eventually gives up due to age or if there's another reason for it? The electrolytics on the preamp PSU boards look a bit suspect, so can their degradation lead to this?

 

[martin clark]

Old, very old, Tantalum caps can do exactly what you describe. It's a known component-failure issue in other realms too (e.g old Tektronix scope PSUs)

It's not your fault; and at such age, the fact it all happened in the same powered channel, is not nec. anything systemic at all.

Replace the lot with modern equivalents, being careful about polarity markings - and forget about it.

 

[Arkless Electronics]

... yep. They always fail dead short so if there's heft behind the PSU they blow up sometimes quite spectacularly or crack in two.... if the PSU has little heft other bits tend to die trying to drive the short... Hopefully only a series resistor as in Tek scopes

 

[mark121211]

I've seen a few Farlowe era exposure pre-amp pcbs with burnt out PCB traces due to shorted Tants.

 

[MJS]

I've had my few share of these. It's normal for tantalum capacitors.

 

[chartz]

Don’t forget the nice and smelly Rifas

 

[Arkless Electronics]

Don’t forget the nice and smelly Rifas

Oh I never forget those!! ... Ah you mean the capacitors!?

 

[cobbers]

Don’t forget the nice and smelly Rifas

Some of the spelling around here is apalling

 

[smee]

Thanks for the replies folks, that's (sort of!) reassuring to know that the tants can just do that eventually. Progress so far:

Damage appears restricted to the Preamp PSU boards, so youngest NAP has been cleaned of exploded tant and had its Preamp PSU disconnected. Having dug the briks' passive crossovers out from storage (and made up some XLR-banana adaptors) it's been happily running with no further noises or smells, so at least I've got music back...
First delivery of parts has arrived, so I can begin the work on the oldest of the trio and rotate them into service as I get them finished.

​

Next question - when buying replacement tants, how important is the tolerance value? Seems to be tricky to find the right values at the same tolerance at the moment... I've got a decent DMM so AFAIK can check the true values before installing.

 

[a.palfreyman]

Ball-park is good enough in most cases TBH. Years ago electrolytic caps used to have horrendous tolerances (-50% / +100% not untypical). They are much better nowadays, and usually within the centre of the tolerance band anyway.

 

[MJS]

I wouldn't worry about the tantalum tolerance in these amps. Any new one is going to be better than what was in there before. Good choice of reservoir capacitor, those Kemets have a healthy voltage margin and should see the amp good for a couple of decades more.

 

[Gervais Cote]

A good amplifier designer will use a 10uf capacitor when he needs 8 so tolerance is not that critical, especially on British stuff where they rarely downsize components.

 

[smee]

Required components are all either here or on the way, so let battle commence...

​

State of the Preamp PSU:

​

Not sure whether to repair these at the moment, as I'm not planning on re-installing them - I don't need the Preamp PSU in my setup, so they'll likely get stashed away for posterity...

 

[chartz]

Just replace the tantalum caps with regular electrolytics. A 2 minute job really.

 

[Gervais Cote]

I would replace the tantals as well while you’re in and this will help keeping its resell value.
Even by today’s standards, this amp would be top of my wish list !

 

[sktn77a]

NAIM says to recap those amps every 8-9years. I guess this is why. Interesting that 50 year old Japanese amps don't need this...........................

 

[martin clark]

NAIM says to recap those amps every 8-9years. I guess this is why. Interesting that 50 year old Japanese amps don't need this...........................

Not quite - Naim always (used to ) recommend leaving their amps on 24/7, and so ... here's an explication I've posted previously:

https://pinkfishmedia.net/forum/threads/how-long-can-an-amp-last.135940/page-2#post-1968157

 

[wd40addict]

Maybe because the owner of the Japanese amp isn't persuaded to leave it on 24/7...

 

[Gervais Cote]

Recapping a Naim like this one is a 1 hour job for me while recapping a Japanese equivalent integrated is a 1 1/2 day job for so leaving a Naim amp turned on at all time doesn’t bother me too much.

 

[smee]

Tbf, as (to my knowledge) none of my amps have been serviced, all their components are about 40 years old, so one exploded tant isn't too bad to my mind - I'm only a couple of years older and feel like I could do with a few components replacing...
Finished stripping out the sled today, replaced the speaker terminals (the plastic on the original ones is going brittle) and power switch neon, and test fitted a set of the new reservoir caps to check the clamp fitment. Was surprised by how dry and un-messy the heatsink paste was when I removed the amp modules though... good job I ordered fresh stuff!