Maker Pro
Maker Pro

Question about transistor failure modes

W

Wim Ton

Jan 1, 1970
0
Hi,

While restoring an ancient HP5245 counter, I had to replace several
transistors, most of them 2N708 or 2N709 and one 2N3640

Most of them showed perfectly normal DC characteristics on the small 'Atlas'
tester.

How do these transistors fail and is there a way to test this easily (except
for replacing them one by one ;-)

TIA, Wim Ton
 
J

Jamie

Jan 1, 1970
0
Wim said:
Hi,

While restoring an ancient HP5245 counter, I had to replace several
transistors, most of them 2N708 or 2N709 and one 2N3640

Most of them showed perfectly normal DC characteristics on the small 'Atlas'
tester.

How do these transistors fail and is there a way to test this easily (except
for replacing them one by one ;-)

TIA, Wim Ton
You should have tried to wash the board and parts first.
if you ask me, it sounds like you had a substance that may
have collected on the surface over the years.
you would be surprised and how high freq things work after
washing down a board with parts on.
 
E

Eeyore

Jan 1, 1970
0
Wim said:
Hi,

While restoring an ancient HP5245 counter, I had to replace several
transistors, most of them 2N708 or 2N709 and one 2N3640

Most of them showed perfectly normal DC characteristics on the small 'Atlas'
tester.

What do you mean by normal DC characteristics ? Did they have any current gain ?

Graham
 
D

D from BC

Jan 1, 1970
0
Hi,

While restoring an ancient HP5245 counter, I had to replace several
transistors, most of them 2N708 or 2N709 and one 2N3640

Most of them showed perfectly normal DC characteristics on the small 'Atlas'
tester.

How do these transistors fail and is there a way to test this easily (except
for replacing them one by one ;-)

TIA, Wim Ton

Somewhat relevant....
Just yesterday I was just going through the achieves and read posts on
how reverse biasing a BE transistor junction is damaging.
I'm working on a design where, upon unit power down, a BE junction
gets reversed.. So the more often the unit is turned off the more
supposed transistor damage.
D from BC
 
Hi,

While restoring an ancient HP5245 counter, I had to replace several
transistors, most of them 2N708 or 2N709 and one 2N3640

Most of them showed perfectly normal DC characteristics on the small 'Atlas'
tester.

How do these transistors fail and is there a way to test this easily (except
for replacing them one by one ;-)

There was a tale told when I was youngish - around 1978 - about fixing
HP counters by stubbing out a lighted cigarette on the metal can of
one of the transistors.

Apparently the base-emitter junction of the affected part was
regularly reverse biased into avalanche breakdown. The avalanche
current was small, and the avalanche was brief, so the transistor
wasn't destroyed, but the forward current gain was progressively
degraded.

The cigarette was supposed to get the transistor junction hot enough
to anneal out the damage.

Measuring forward current gain in-circuit could be a bit difficult.
 
C

colin

Jan 1, 1970
0
D from BC said:
Somewhat relevant....
Just yesterday I was just going through the achieves and read posts on
how reverse biasing a BE transistor junction is damaging.
I'm working on a design where, upon unit power down, a BE junction
gets reversed.. So the more often the unit is turned off the more
supposed transistor damage.
D from BC

Yes ive had a few designs suffer from unforseen reverse vbe.
some can be rather sensitive, especialy RF sections that burst into
catastrophic oscillation.

IME 8/10 transistor failures are short circuit, shortly folowed by open
circuit if the current is not limited, or transistor ejecting itself off the
board.

Colin =^.^=
 
J

Jamie

Jan 1, 1970
0
D said:
Somewhat relevant....
Just yesterday I was just going through the achieves and read posts on
how reverse biasing a BE transistor junction is damaging.
I'm working on a design where, upon unit power down, a BE junction
gets reversed.. So the more often the unit is turned off the more
supposed transistor damage.
D from BC
Be junctions have zener effects.
commonly around the 6 volt region.
the Vbe in reverse that is.
 
D

D from BC

Jan 1, 1970
0
Be junctions have zener effects.
commonly around the 6 volt region.
the Vbe in reverse that is.

I should add...
The achieved discussions I read was about the damage in the BE reverse
breakdown (zener ..also called avalanche IIRC) region.
I'm guessing the transistor is ok with a BE junction reversed biased
before that point. Any more and zener current starts kicking in and I
suppose heat is the killer.
IIRC somebody posted that the damage is accumulative and leads to
eventual transistor failure.

I might have an example of this:
http://focus.ti.com/lit/ds/symlink/tl431.pdf
Figure 24.
Change the R's for say 20V output.
Put a cap on the output..Charge up the cct..
Then ground out Vin to imitate other loads present when the power is
turned off.
Doesn't it look like the transistor BE junction gets a reverse hit?
If so, this could be a design flaw that can sneak through product
testing..
Might be part of mysterious transistor failure..
D from BC
 
D

D from BC

Jan 1, 1970
0
I should add...
The achieved discussions I read was about the damage in the BE reverse
breakdown (zener ..also called avalanche IIRC) region.
I'm guessing the transistor is ok with a BE junction reversed biased
before that point. Any more and zener current starts kicking in and I
suppose heat is the killer.
IIRC somebody posted that the damage is accumulative and leads to
eventual transistor failure.

I might have an example of this:
http://focus.ti.com/lit/ds/symlink/tl431.pdf
Figure 24.
Change the R's for say 20V output.
Put a cap on the output..Charge up the cct..
Then ground out Vin to imitate other loads present when the power is
turned off.
Doesn't it look like the transistor BE junction gets a reverse hit?
If so, this could be a design flaw that can sneak through product
testing..
Might be part of mysterious transistor failure..
D from BC

ooops spellcheck goof..."archived discussions" not achieved.
D from BC
 
G

Gibbo

Jan 1, 1970
0
D said:
I should add...
The achieved discussions I read was about the damage in the BE reverse
breakdown (zener ..also called avalanche IIRC) region.
I'm guessing the transistor is ok with a BE junction reversed biased
before that point. Any more and zener current starts kicking in and I
suppose heat is the killer.
IIRC somebody posted that the damage is accumulative and leads to
eventual transistor failure.

I might have an example of this:
http://focus.ti.com/lit/ds/symlink/tl431.pdf
Figure 24.
Change the R's for say 20V output.
Put a cap on the output..Charge up the cct..
Then ground out Vin to imitate other loads present when the power is
turned off.
Doesn't it look like the transistor BE junction gets a reverse hit?
If so, this could be a design flaw that can sneak through product
testing..
Might be part of mysterious transistor failure..
D from BC

It's a pretty common input PSU circuit and a pretty common solution is
to put a diode between be, reverse connected of course.
 
F

Fred Bartoli

Jan 1, 1970
0
[email protected] a écrit :
There was a tale told when I was youngish - around 1978 - about fixing
HP counters by stubbing out a lighted cigarette on the metal can of
one of the transistors.

Letting the magic smoke getting back in?
 
Top