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Transistor Life

E

EdoE

pfm Member
Dear all,

Naim have warned me about servicing my 25 year old 250: the transistors might need to be replaced. I'm wondering what the typical life expectance for a transistor is: would I need to worry about having to replace them within, say, five years if I upgrade to a 1987 or so model? Thanks.

Cheers,

Edo
 
If it ain't broken don't fix it!

What kill transistors if they work within their limits is:

Temperature cycling

Moisture

Air pollutions

Lifetime is long is the working conditions are good, 25 years is no age. I tihink a bigger problem is cracking solder joints and dry electrolytic caps and this may cause chain reactions (broken transistors).
 
I'm afraid that Si based chips do have a general lifetime & don't last forever.

Generally Si is only life tested up to about 10years using Accelerated ageing tests. These tests come in 2 sorts, Package tests & Fab Si tests.

I'm afraid that after a simulated ageing, chips DO deteriorate & lose performance over time. This is obviously sooooo gradual in normal use that unless Si EOL is reached you won't notice unless you replace.
The normal fail mode for chips is the package failure which, as it generally happens first, is taken as the chip having failed. The Si inside however may have already stepped out of it's performance criterea when made.

You replace capacitors regularly so I'd recommend that the transistors are also replaced when they reach such an extreme lifetime.

Hope this makes sense,

J
 
... so if you can keep the part cool, and in a rather clean place the lifetime will definitely be longer.

My oldest amp is from 1975/77 and has been used many, many hours and works still so 30 years is no age in home environment.
 
Yeah but think how much better it would sound with new Si in it :D

Seriously though, there are LOTS of mechanisms acting in a chip to degrade performance that's why so much time goes into Reliability testing & quality control.

Older chips were also not made to such high standards as today (interconnect purity, passivation layers, transistor design) so maybe they were over-engineered (Forth Rail Bridge?) to last.

Still not sure it's worth sticking your head in the sand & waiting for them to fail when people go to so much trouble with caps etc.

J

P.S. You'd have to keep it dry too as moisture will penetrate the plastic package & start to eat away at yer exposed metal......
 
Hope this makes sense
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Some of the myths doing the rounds don't make sense actually.
I have an amateur radio rig well over 30 years old and if the solid state devices were prone to degeneration, solely as a consequence of age, this would now be measurable. As it is, the performance of this little marvel measures (at 430MHz), just as well as the day it was made.

It's worth picking up on another point though, that of moisture ingress - where the case seal is degraded in some way. Heat and vibration are the main causes of sealing loss between case and terminations. In some items of kit I make, we use the tab of the device as the centre pin and this has to be cropped away from the package whilst protecting the leg from vibration otherwise the seal can be destroyed leading to dendritic effects within the plastic case. Moisture can then enter the package and affect the semiconductor die.
 
It's worth mentioning that the TO3 devices fitted to output stages seem to be almost immune to these effects. As the man said, "if it ain't broke........".
 
It's the truth that all silicone products are much better manufactured now than 20 years back but if you have an old amp, the design is also old and everything is old including pots, switches etc. so the odds to make things really better just by changing semiconductors are small unless you have eagle control (örnkoll in Swedish) over the whole product. I would let the amp work as long as it will and mend it if it got broken but not do a total makeover.
 
Are the transistors used in a 25 year old NAP250 the same as those in the end-of-generation units?

Maybe this is why Naim would say they need changing.

You cannot also overlook that production techniques and improvements that have been learnt over this product's considerable life may also dictate the output devices are changed, for optimum performance.

Andy.
 
So do I understand from this discussion that it would be a better idea to upgrade to the newer but still old model at a little cost than to keep the older one and have it serviced for a possibly huge amount of money?

Cheers,

Edo
 
Just wondering, do you have any functional problems now? Do you have any possiblities to compare your amp against a newer one?
 
Just wondering, do you have any functional problems now? Do you have any possiblities to compare your amp against a newer one?
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No real problems, the amp just sounds bad because the caps are 25 years old and about to cream. If I ask Naim about a service, they quote the replacement of the transistors as a likely option, and I'm simply wondering if there are better places to put my money.

Cheers,

Edo
 
It's worth picking up on another point though, that of moisture ingress - where the case seal is degraded in some way. Heat and vibration are the main causes of sealing loss between case and terminations.

You're assuming that chips are hermetically sealed, unfortunately, except for a select few (mostly military) they're not. The mould compound commonly used, be it Hitachi or Sumitomo types, is effectively just a form of Bakelite resin. Moisture can and will penetrate these packages, typically coming to a point of balance in a week.
It is this moisture, along with the temp & voltage accelerants, which can degrade the Aluminium lines in the chip leading to failure. (Please refer to the 'Peck' equation for the model of this).

Also, the basis for an Si chip is the Gate Oxide layer of the transistor which has an inherant wear out period depending on charge, thickness, quality etc...... a delicate beast at the best of times but in time will lose switching speed & suffer leakage problems.

Still reckon for people (such as myself) who change caps, resistors & cables to attain the best sound (Naim obviously :rolleyes: ) it is a bit blinkered to ignore yer wee black bits of mumbo jumbo!!
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