Silicon transistor failure
 

Has anyone noticed transistor failure becoming more common? It may be coincidence of course, but I have had quite a lot of repairs lately, dating mainly from the late 70's through to the late 80's, where multiple transistors had failed in the unit. Of note is the failure mode; they appear to all have gone low gain.

Testing such transistors with a multimeter is useless, as they test perfectly fine. However, a proper tester shows low Hfe and in circuit they just don't work. This is primarily in RF and IF positions on course, so junction capacitance may be playing a part as well, but I have also found problems in audio sections where they are still functioning, but the amp is low gain until the offending component is replaced.

Has anyone else noticed this? I do think it is the age that some of these are now, and suspect it may become more common. Oh, and I am referring to TO92 silicon transistors, not the dreaded lockfit types!

Re: Silicon transistor failure
 

Hi
Yes I have experienced this.
A good example is the Sony icf 6800 where the rf transistors go low gain so the set stops working on SW .I’ve repaired two with the same issue.
A big job to sort but can be done with modern silicon transistors ( does not need to be Sony types )
Cheers
Pete

Re: Silicon transistor failure
 

And eighty year old valves are still working perfectly!

Re: Silicon transistor failure
 

Some of them are, but I expect many more failed due to low emission.

Re: Silicon transistor failure
 

What types/brands are failing?

Re: Silicon transistor failure
 

To be honest I haven't paid much attention, but certainly various types. All have been TO92 packages, not that that means anything. Basically signal transistors.

Re: Silicon transistor failure
 

I suspect the failure is somewhat similar to that of mlockfits; creepage of contaminants along the failing bond between the leads and the epoxy encapsulation.

Personally I never really liked TO92; I always specified TO18 in things I designed, though sometimes had to use the little TO92-alike Ferranti transistors [the ZTX series] for commercial reasons. ZTX ones always seemed reliable.

Re: Silicon transistor failure
 

It's not only gain, but noise. I've had quite a few noisy ones, e.g. the Texas ones used
in Quad 33's. Certainly the early TO92s could have corroded leads.

Re: Silicon transistor failure
 

I've had lots of TO92 types with various faults such as noise, low gain and an annoying one recently where it looked more like a pair of resistors than a transistor. I have noticed for some reason that PNP types seem to be more prone to failure than NPN but why that should be I have no idea.

Re: Silicon transistor failure
 

I went for TO18, TO5 (=TO39) too, but TO92 seemed to be the package of choice for manufacturers!

I also liked the silicone E-line ZTX range, but these went obsolete, which was a great shame.

I'm rather watching this thread - TO92 doesn't have a particularly impressive temperature range, but I didn't know there was a life-limiting mechanism.

One of the longest-life packages was the Mullard/Philips glass encapsulation as used for OC71 etc. The later metal TO7... well we all know what happens to AF117's...

Re: Silicon transistor failure
 

I have had this epoxy rot on a few of the Quad 303 in the past where the HFE drops to 35 on a signal transistor and goes noisy where the HFE normally id around 200-400

Re: Silicon transistor failure
 

Lots of experiences so far, but is it becoming more common?

Re: Silicon transistor failure
 

With the passage of time, slow degradation has had more time to do its thing, so more of the transistors in the historic population have gone bad.

Parts where there is a deliberate void over the active face of the silicon and the bond wires seem to be OK, but the ones where epoxy is forced to coat the whole thing seem most at risk.

There also seems to be two mechanisms at work; contamination, and also mechanical stress on bond wires.

David

Re: Silicon transistor failure
 

I had an old Akai amplifier that developed noise when switching on....like a rushing noise that died away after a few seconds. When I investigated, I found that the small driver and pre-amp transistors were actually sensitive to tapping.....one actually seemed microphonic......you could tap the top or gently scratch the top and hear it in the speakers.....all very odd. I fitted standard BC548's for the pre-amp types and BC637/638's for the drivers. No problems after that.

Re: Silicon transistor failure
 

Yes, I have certainly has noisy ones too. Not sure how old my replacement stocks are, but new old stock transistors have all been fine so far as replacements. Not like the Germanium AF117 old stock issue.

Re: Silicon transistor failure
 

The black death syndrome was/is also a failure mode, see here for details:

https://www.vintage-radio.net/forum/...4&postcount=22

Lawrence.

Re: Silicon transistor failure
 

Perhaps "old" used to mean germanium transistors, now "old" encompasses silicon too.

Re: Silicon transistor failure
 

Hello,

Interesting thread.

As I don’t commercially repair/restore equipment and the only ‘old’ silicon solid state equipment I see is from Swap meets and the Audio Jumble etc., for my own use, so I can only see it from that perspective.

I suppose we’re possibly facing the terminal aging and the eventual demise of a lot of the 30–50-year-old solid state ‘silicon’ audio gear, as these are littered with TO92 parts. 30–50-years ago the TO92 was the go-to transistor in the first and driver stages of the ‘RCA’ type power amplifier circuit.

Transistor faults in front ends, IF stages and preamplifiers will cause ‘localized’ problems, but in DC coupled amplifiers of 30-100Watts (or more) it’s a different proposition as these higher power DC coupled amplifiers are often to be found in 70’s and 80’s audio equipment, and if multiple small signal transistors in the early and driver stages, go way, way, way off specification, the circuit could catastrophically fall over and fail, and with a domino effect take out the output transistors made from unobtanium and not to mention the speakers if they are DC coupled!

In strange way this thread runs alongside the tread about old radios and “AM closures, less impetus to restore sets?” I suppose if this 70-80’s audio gear starts to fail wholesale on a regular basis, because of multiple transistor failure, it will become uneconomic to fix them, [in a commercial environment], when each fault occurs due to a transistor failure, only to have another transistor fail a week, month or year later.

Maybe the time is approaching where a restoration/recommission will have to include the full careful testing and inspection of ALL the transistors to see if they meet the original basic specification?

The availability of the scarcer power devices is also influencing longevity of these [higher] power amplifiers.

Terry

Re: Silicon transistor failure
 

"Old" (eighties) ECL logic chips often showed deterioration in speed after only a few years of operation.

Re: Silicon transistor failure
 

An afterthought, does the failure mechanisms in these transistors effect the breakdown voltage? If so, this could be a problem where a voltage amplifier or [pre] driver has a high collector voltage.

Terry