Corroded transistors?

[Richard]

Having a bit of time these dark days I finally got around to fixing one of my Farnell power meters which conked out last year. These are from the early 80's and very useful working up to 50W with a range of impedance settings and scaled in W and dB. Sad to think we made such lovely stuff in this country just 30 years ago.

Anyway, it lit up from mains and battery and there was 18V on the metering pcb so the psu board was ok. The inputs all checked fine but the meter didn't move with any signal, so it was something on the metering pcb.

Fortunately I have the circuit and another good unit to compare and check against. Working on the theory that if it looks broke it probably is broke I homed in on 2 transistors with corroded green legs. Checked with the meter in situe and found one open B to E.

BC182L are gone now but 2N3904 are close and cheap so changed them both and recalibrated both meters and now have a good pair again

Why it went though is a mystery. I wondered if transistors wore out like valves but found nothing conclusive on the web and they don't run hot to the touch so seem to have an easy life. Then I chanced across a line or two saying corrosion has been noted to break the seal of legs to case and cause failure of transistors.

So have a look here. Only the 2 transistors were like this, everything else, and the other unit, is fine. I can only think looking at the joints that they were hand soldered using a flux which has later caused this corrosion, the rest of the pcb was flow soldered and clean of flux,

[bob orbell]

Well done Richard, that corrosion looks just the same as in the SUT's that I have fixed, green, could be sweat from the fingers on assembly, but why just those two components, stange.

[Richard]

Hi Bob, yes strange indeed. The legs are tinned so the corrosion must be quite deep to go through to the copper to show that green. Perhaps they'd been soldered separately to the rest of the pcb for heat or assembly reasons. They were the only parts showing flux residue, so that, or they'd been changed later and solder with corrosive flux used.

I wondered about something being spilt on them but it's just them and no splashes anywhere else. So, like your step up TX wires I think, bad flux, and something to keep in mind when checking stuff over

It's not conclusive that corrosion was the reason for failure of these transistors though and would be interesting to hear of similar sudden failures for no (other) apparent reason.

I've got the little Hameg scope to have a look at this week, fixed it a few years ago but it's playing up again

[Richard]

...and success with the Hameg 103 too, going well again now just a duff transistor again.

The scope still worked but showed the rise and trigger trace at far left. A look at the circuit shows 2 transistors in the unblanking circuit. They're plug in so perhaps they do fail from time to time. I'd changed them maybe 10 years ago for a similar but worse fault. Checked them and just one, the BF199, was duff this time. A look through the box found I'd bought a few of them as spares back then so the job was done without even switching on the soldering iron!

Got to love this little scope, only 10 Meg and single channel but so handy and smaller than a shoe box, here he is for the re-union photo

[bob orbell]

You'r just showing off now Richard.

[Richard]

ha ha yes you're right Bob, I'm well pleased, me and transistors don't usually get along so well still at least they only have 3 legs ECB or is it BEC and which ones conduct, damn, forgotten all I knew already

[colin.hepburn]

Originally Posted by Richard

ha ha yes you're right Bob, I'm well pleased, me and transistors don't usually get along so well still at least they only have 3 legs ECB or is it BEC and which ones conduct, damn, forgotten all I knew already

CBE. EBC then its NPN .PNP can never remember which is what

[Richard]

Originally Posted by colin.hepburn

CBE. EBC then its NPN .PNP can never remember which is what

Nothing to it Col, just remember PNP transistors are easier to fit as they're Plug N Play. NPN are the tough ones as they're Not Plug Nplay...