[morzh]

1. Are you sure? Can you post how you checked them? How did they fail? (in what mode - open/short/leak, if open-short - which junction , BE-BC, or is it EC breakdown?).
2. Q2,3 and 10 are unrelated. If they failed this may be potentially a symptom of something systemic.
Go and check 9V and 10V points and see if those are OK.

[BrendaAnnD]

Did you check them in or out of circuit? In-circuit checks are unreliable at best (unless you are using something like a Cricket to test actual function.)

[morzh]

A valid question; what I described is "out of circuit" test.

In-circuit it is possible also but requires lots of experience so you can discern those subtle differences between leakage and external elements conducting, or...or. And even that sometimes not really possible so you have to desolder it.

[Eric Adams]

I checked them in the circuit. Here's what I found:
Q2 - EC shorted
Q3 - shorted with leads in any position
Q10 - EC shorted

Voltages:
Q2 - -0- volts all pins
Q3 - B - 3.7 volts, C,E - -0- volts

[morzh]

Eric

Try to do what we are asking you to do.

1. Desolder the transistors you want to check. Attention: semiconductors are VERY sensitive to the long heating so do NOT apply solder heat for long time. Best, grab a lead you are desoldering with needlenose pliers or a large tweezer, so to sink the heat from the iron tip. Note the way the leads go into the board.
If the board is a copper laminate, do not overheat the laminate or it will delaminate.

2. Do measure the power supply 10V and 9V voltages as I asked you before.

3. If indeed the power is 9V and you do see those zero volts relative to GND it is possible the transistors are shorted.
What I am trying to figure out is what caused several transistors to go short.

4. Measure from Q3.E to GND, see if it is shorted.


5. When you say "short", do you mean 0 Ohm or close, or just conductivity of a few hundred Ohm?

[Eric Adams]

OK, Now you tell me.
Q3 - BE is open. All others check OK. I pulled them out before I saw your last post so I didn't check the power supply voltages.

[morzh]

Well that makes more sense.
What abougt the CE of the Q3? (not that it is too important but I want to see why you had 0V at its collector).

But if BE is open E will indeed be at 0V and not at 1.3V igt is supposed to be if there is 2V at the base.

PS. You still can check the power voltages even with the Qs out.

[Eric Adams]

CE of Q3 wasn't shorted. I'll check the voltages tomorrow and let you know.
Thanks for the help. I think we're on the right track.

[morzh]

Well there are two ways for BE to be open.

The most popular is, the junction is shot, so no current flows, so the CE has to be open. But then the C would be at full Vcc (9V) and not at zero.

Another way is, the bonding wire got detached but the junction is intact.
Then the junction is autobiased and the transistor gets to close, at which point the C goes to 0V or so.

Anyways that's not terribly important. It does look a bit like the Q3 has bought the farm and is actively pushing the daisies now.

[Eric Adams]

I'm with you. I'll keep you posted.

[Eric Adams]

Power supply voltages -9v and 10 volt check OK. Transistor on order. I'll report back after it is installed.

[morzh]

Have you ordered just one? They should be under 50c a piece.

[Eric Adams]

No, I went all out and ordered 2!

[morzh]

So you felt like splurging!

Some people are living large!

[Eric Adams]

When you got it, flaunt it!