common failure mode for digital transisors?

[Maynard A. Philbrook Jr.]

I do some repair work now and then and I got a HF transceiver
that I once replaced a DTC114ES digital (pre biased) transistor.

It didn't appear shorted, it just wouldn't turn on. I just got
a call from the owner and from what he's telling me and from what
I can gather in the schematic, it appears that maybe another
digital transistor of the same type has failed, but only in a different
location?

This unit is rather old, it uses through hole digital transistors. Was
there some fab problems back then that made those fail internally over
time ?

Jamie

 

[Maynard A. Philbrook Jr.]

I dunno about this transistor, but oftwn tranmitter output transistors
fail due to bad SWR, where they get the power right back that should have gone into the antenna.
If it is a driver maybe it could be affected too.
Normally RF transitors (low power) stay OK.

No, these have nothing to do with power RF actually.

These are just switching transistors with pre-biased networks in them.

This unit has a slew of them in various places. It's possible the ones
that are failing could be exerting more current than others in there.

This one that failed this time actives the early stages for the
transmitter, as did the first one that failed but for the VOX.

I used a IR camera on the unit the last time to measure possible
heating after I replaced the first failed digital transistor and I saw
nothing alarming.

I remember years ago when I was a kid doing TV repair work for side
cash, there was a series of TV's that were using this one particular
transistor that always seem to fail, those that were not failed you
could tap on them and they would fail intermittently.. I can't be sure
but these may have been in the audio section as part of a driver. We
stocked
up on lots of subs for that unit

Jamie