Odd Transistor Readings

-------------------

** See, all you need was a simple ohm meter test.

Checking C-E is a standard practice, cos it's a common failure when a transistor is over voltaged or over heated under load.

Commonly known as " punch through" or "second breakdown".

...... Phil

I've never known a BJT fail in *this* way. Has anyone

CD-

That sounds like leakage to me. Did you measure C, B and E voltages in-circuit?

I wonder if a coupling capacitor from the previous stage is leaking, and it might have damaged that transistor?

Fred

---------------------

** The OP is working on a scope horizontal sweep amplifier PCB.

It's direct coupled throughout.

Despite my advice, he failed to carry out basic ohm meter tests with the devices in circuit.

Folk pick their favourite replies here and pay the price for that.

.... Phil

Yes, I did. They were all over place and not just for the transistor in question:

Q10: Vc +1.1v, Vb +0.5v, Ve +1.2v Q9: Vc -3.2v, Vb +1.1v, Ve -1.34v(this is the faulty one) Q8: Vc -3.8v, Vb -3.2v, Ve -3.9v

Q15: Vc -2v, Vb +0.11v, Ve +0.78v Q14: Vc -7.74v, Vb -2v, Ve -1.34v Q13: Vc +3.5v, Vb -7.74v, Ve -7v

I'm not sure if just Q9 alone being faulty could account for three other transistors showing 'impossible' bias voltages: Q8 fully saturated; Q10 inverse Vbc; Q13 inverse Vbe - but with them all being inter-dependent as a consequence of direct coupling, who knows?

It's all *direct* coupled, which is 9/10 of the problem in trying to pin down the fault. Here's the schematic:

--
This message may be freely reproduced without limit or charge only via  
the Usenet protocol. Reproduction in whole or part through other  
protocols, whether for profit or not, is conditional upon a charge of  
GBP10.00 per reproduction. Publication in this manner via non-Usenet  
protocols constitutes acceptance of this condition.

Let's try and make that clearer:

Q10: Vc +1.1v, Vb +0.5v, Ve +1.2v

Q9: Vc -3.2v, Vb +1.1v, Ve -1.34v (this is the faulty one)

Q8: Vc -3.8v, Vb -3.2v, Ve -3.9v

Q15: Vc -2v, Vb +0.11v, Ve +0.78v

Q14: Vc -7.74v, Vb -2v, Ve -1.34v

Q13: Vc +3.5v, Vb -7.74v, Ve -7v

Having checked for replies in google groups...

I did actually carry out in-circuit resistance checks with the board unpowered using a meter capable of exceeding the 0.7v before removing anything. It indicated discrepancies compared to an identical board in the area around Q10, but not a specific enough area nor specific enough discrepancies to be of use.

--
This message may be freely reproduced without limit or charge only via  
the Usenet protocol. Reproduction in whole or part through other  
protocols, whether for profit or not, is conditional upon a charge of  
GBP10.00 per reproduction. Publication in this manner via non-Usenet  
protocols constitutes acceptance of this condition.

-------------------------

** You are being very coy about what you REALLY did.

My post said to use an analogue ohm meter on the X1 range - nothing else works near as well, especially any DMM I know of.

The ohms reading found corresponds to millivolts and volts across the load - easily calibrated with a DVM and few resistors. I inked voltage numbers on the ohms scale of my meter 45 years ago.

Most importantly, such meters deliver up to 50mA into the load, making redundant any resistors that might be in parallel with device junctions.

Betcha you did nothing like that or else the near shorted C-E would have jumped up and bit you.

.... Phil

CD-

Diagram appears to have a mix of PNP and NPN transistors.

Fred

---------------------

** Hey - don't confuse the OP with facts !!

..... Phil

Well I don't do repair work for a living so defer to your wider knowledge of the subject. I just find it baffling how such a transistor can give readings like those I described, with a Rc-e lower than each of its Rc-b and Rb-e readings!

It wouldn't be worth if for the number of times a 'mass extraction' like this has been necessary, which before this happened was zero.

It would be nice to have *all* the meters in the Peake range, but on an occasional need basis I can't justify it.

--
This message may be freely reproduced without limit or charge only via  
the Usenet protocol. Reproduction in whole or part through other  
protocols, whether for profit or not, is conditional upon a charge of  
GBP10.00 per reproduction. Publication in this manner via non-Usenet  
protocols constitutes acceptance of this condition.

-------------------

Trevor Wils>

** Never seen a shorted power transistor ?

The two junctions test OK, but there is a near dead short from C to E.

If you open one, the chip usually has a black spot on it where the silicon has melted.

Your example involves a low resistance, likely the result of an over voltage or over current spike.

Mistreat a BJT and a short from C to E is the MOST likely outcome.

Same goes for mosfets, the short is then from D to S.

Seen many hundreds of both.

Diodes fail short too, plus zeners, triacs and SCRs.

Silicon dies are fragile things.

.... Phil

PITA having to cut and paste from Google Groups but anyway...

"** You are being very coy about what you REALLY did.

My post said to use an analogue ohm meter on the X1 range - nothing else works near as well, especially any DMM I know of.

The ohms reading found corresponds to millivolts and volts across the load - easily calibrated with a DVM and few resistors. I inked voltage numbers on the ohms scale of my meter 45 years ago.

Most importantly, such meters deliver up to 50mA into the load, making redundant any resistors that might be in parallel with device junctions.

Betcha you did nothing like that or else the near shorted C-E would have jumped up and bit you. "

Nope. I actually used one of these:

Not the same exact model as couldn't find one on Ebay to show, but same make made about 60 years ago. I also keep an AVO 8 but the batteries are dead in that one. And yes, I *did* check it on the 1X range. YOU could probably have made sense of the readings I was getting; they were definitely lower on the damaged board in that region and *now* I know why, but I'm no repair technician and never pretended to be so the significance on me was a bit lost.

Are we clear now?

--
This message may be freely reproduced without limit or charge only via  
the Usenet protocol. Reproduction in whole or part through other  
protocols, whether for profit or not, is conditional upon a charge of  
GBP10.00 per reproduction. Publication in this manner via non-Usenet  
protocols constitutes acceptance of this condition.

Which is a lot more 'modern' than the one in the Ebay advert, plus mine goes up to 2,500VDC.

-- This message may be freely reproduced without limit or charge only via the Usenet protocol. Reproduction in whole or part through other protocols, whether for profit or not, is conditional upon a charge of GBP10.00 per reproduction. Publication in this manner via non-Usenet protocols constitutes acceptance of this condition.

Found it! Four hours of searching didn't go to waste after all.

Not as old as I'd thought; 1972 vintage according to the advert.

--
This message may be freely reproduced without limit or charge only via  
the Usenet protocol. Reproduction in whole or part through other  
protocols, whether for profit or not, is conditional upon a charge of  
GBP10.00 per reproduction. Publication in this manner via non-Usenet  
protocols constitutes acceptance of this condition.

---------------------------

** So what happened to your ohm readings ??

You did not post any.

Each junction should read about 10 to 20 ohms, on that meter.

B-C a bit lower than B-E, all the same for good devices that are the same.

FFS fess up and shame the Devil.

..... Phil

** So what happened to your ohm readings ??

You did not post any.

Each junction should read about 10 to 20 ohms, on that meter.

B-C a bit lower than B-E, all the same for good devices that are the same.

FFS fess up and shame the Devil.

----------------------------------------------

There's no point now. I have checked all 6 of those transistors and found one faulty one as described. The question in my mind now becomes, "can that one transistor (Q9) being partly shorted cause the voltage readings on the other transistors to be so far out of whack?" Now most people would probably say "just stick a new transistor in there and see if it works" but I'm going to Spice the voltage-to-current section just out of curiosity. Since I'm not a repair tech and time is not a factor I can do this. I'm lucky to have an identical board, the Y-amplifier, which is identical in every way except that it works so I've been able to get some useful values from measuring it under power and getting meaningful and sane voltage readings from it to compare to the faulty one's measurements which are all over the place. The simulation is almost ready to run but I have other things to do right now so it'll have to wait until tomorrow.

--
This message may be freely reproduced without limit or charge only via  
the Usenet protocol. Reproduction in whole or part through other  
protocols, whether for profit or not, is conditional upon a charge of  
GBP10.00 per reproduction. Publication in this manner via non-Usenet  
protocols constitutes acceptance of this condition.

That sounds to me as if the fault isn't in the B/E or B/C junction(s). Rather, it's acting as if something ohmic is _bypassing_ the junctions.

I wonder whether something inside the case (e.g. some form of contamination, debris, etc.) has fallen onto the surface of the die and is bridging the C and E wells (or the contacts or the bond wires).

------------------

** Crikey - another one who has no clue about how transistors typically fail with second breakdown or gross leakage.

BJTs that merely hot for a long time often develop gross leakage and reduced C-E breakdown voltage.

Take any TO3 pak silicon power tranny, put it in an oven at 250C for ten minutes.

Then test it.

..... Phil

Phil Alison:

"** Crikey - another one who has no clue about how transistors typically fail with second breakdown or gross leakage.

BJTs that merely hot for a long time often develop gross leakage and reduced C-E breakdown voltage.

Take any TO3 pak silicon power tranny, put it in an oven at 250C for ten minutes.

Then test it. "

Yes, but we're talking about a small signal transistor here and your example of a test is a little on the extreme side to say the least. Whilst secondary breakdown *can* happen with small signal devices, it's much less common. I suspect your outlook is coloured by years of seeing burned out TO-3s in audio amps, but it's nothing like as prevalent with SSTs so let's give the guy a break, eh?

--
This message may be freely reproduced without limit or charge only via  
the Usenet protocol. Reproduction in whole or part through other  
protocols, whether for profit or not, is conditional upon a charge of  
GBP10.00 per reproduction. Publication in this manner via non-Usenet  
protocols constitutes acceptance of this condition.