partially degraded transistor network degraded by good transistor? - Page 1
Forum section: electronics / repair

author: coppercone2
author_rank: Super Contributor
timestamp: August 26, 2023, 06:23:55 am
content: I am working on a unit with a few transistors and vacuum tubes, but its a general questionhas 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)?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 26, 2023, 07:28:03 am
content: 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

author: coppercone2
author_rank: Super Contributor
timestamp: August 26, 2023, 08:42:46 am
content: 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

author: floobydust
author_rank: Super Contributor
timestamp: August 28, 2023, 07:14:54 pm
content: 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.

author: coppercone2
author_rank: Super Contributor
timestamp: August 28, 2023, 08:20:57 pm
content: nah it was in a chassis

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 28, 2023, 08:25:06 pm
content: Is it a differential amplifier or something similar that requires matched transistors?

author: floobydust
author_rank: Super Contributor
timestamp: August 28, 2023, 08:32:56 pm
content: Quote from: coppercone2 on August 28, 2023, 08:20:57 pmNot when you're in there replacing a transistor- touching nodes I'm saying ESD hits happen from the soldering iron/desoldering, pliers, screwdriver etc.

author: coppercone2
author_rank: Super Contributor
timestamp: August 28, 2023, 08:37:29 pm
content: Well I got more transistors coming I will replace two this time with more precautionsi would be suprized if it was ESD on such a old transistor though. 30 years before ESD guidelines?

author: floobydust
author_rank: Super Contributor
timestamp: August 29, 2023, 01:05:10 am
content: 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.