partially degraded transistor network degraded by good transistor?

[coppercone2]

I am working on a unit with a few transistors and vacuum tubes, but its a general question

has anyone seen a situation where a unit is working poorly (say low gain) with a bunch of transistors, when one transistor is replaced (in spec), the unit stops working?

like a series of failures where adding a proper part makes it fail (i.e. trim stops working)?

[T3sl4co1l]

Uhh, sure I guess?

Network as in what, some arrangement of transistors that works at DC? Or something at RF?

In either case, it's entirely possible the bias wasn't designed to be stable beyond some range of gain or other characteristics, or the capacitance is wrong, detuning the network.

"Transistor networks" in general are Turing-complete, so, you don't have to do much different to have some potentially very different results from device parameters.

Tim

[coppercone2]

its just weird that there is alot of damaged/degraded stuff that totally crap out with one new part. they don't like new members in the group

[floobydust]

Check your workbench and your technique - last time I had this happen, I traced it down to the ESD mat ground snap had fallen off. Line cords lying on the mat bring it some AC due to capacitive coupling (1MEG=bad idea), as well when you walk over and sit down at your bench, you carry a large static charge. Desk chairs are terrible as well.
I guess the board under repair was getting zapped by the discharge to the soldering iron.

How I proved all this was adding two GaN green LED's in series with the ESD mat. They will flash bright if there is a discharge from myself, and glow steady dim if the mat has anything leaking to it such ac AC line cords lying on it. Very handy.

Check your soldering/desoldering iron is grounded as well as the ESD mat. When you sit down, never touch a board or hand it to/from someone or set it down on a bench without first discharging yourself to GND- then you can put the board down. If someone is handing me a board I tap their other arm first to get to the same potential.

[coppercone2]

nah it was in a chassis

[EggertEnjoyer123]

Is it a differential amplifier or something similar that requires matched transistors?

[floobydust]

nah it was in a chassis

Not when you're in there replacing a transistor- touching nodes I'm saying ESD hits happen from the soldering iron/desoldering, pliers, screwdriver etc.

[coppercone2]

Well I got more transistors coming I will replace two this time with more precautions

i would be suprized if it was ESD on such a old transistor though. 30 years before ESD guidelines?

[floobydust]

30 years before everything was made of plastic - your clothes, flooring, chair fabrics etc. triboelectric central? Or your humidity is low. Here I 've seen it go below 20% beating Death Valley sometimes.
I would check though if something is zapping the gear being repaired. For me it was one open ground wire. I've seen it at work too where they move benches around for cleaning/floor wax and break the ground wires to the ESD mats/wrist strap jacks, and nobody notices.

Rarely I have seen equipment literally have a nervous breakdown on the bench, where 10 things crater at once. Hate when that happens.