Which transistor is bad

I am having problems with the 7.5 Ohm resistor burning up when no output is connected. I am thinking that one of the output transistors is bad, but do you guys see any other one that it could be?

Also, what are some good substitutes for the 2n4036, and 2n2102?




Thanks,
Andrew

> the 7.5 Ohm resistor burning up when no output is connected. I am thinking that one of the output transistors is bad

Direct-coupled tranny amps, the failure is rarely where the smoke comes out.

Remove the parts which "can" smoke. Lift two leads of Q9 and Q10 so they are inert. Short CR6 CR7 for clarity. Short CR8 for clarity. Q7 Q8 can not drive a heavy load, but they can drive a 10K:220R NFB network. With the other input grounded, see if the output sits very near ground; it should. If it is stuck at the positive rail, could be Q7 or Q2 is shorted C-E.

In general: if the base-emitter voltage is less than 0.6V, but the collector appears to be sucking current, that transistor is bad. Maybe.

If it set-up happy without the output stage, un-short CR6 CR7 and check for 1.2V end-to-end. 1.3V is not wrong, 3V or 50V is wrong... a diode has gone open. 1N400x is a fine emergency replacement.

In fact. hmmmm... one of CR6 CR7 CR8 Q10 -must- be bad, because if they were all good there could never be more than 0.6V+0.6V+0.6V-0.6V= 1.2V across 7.5R, which won't smoke a 1/4W resistor.

Thanks for the reply. Looks like the 2n4036 is toast, and digikey is out. Is there a suitable replacement that you know of in the bd1xx series?

Thanks again,
Andrew

> replacement that you know of in the bd1xx series?

I don't know the BD series. The essential specs for full load are 60V, >200mA, 600mW. If you never drive less than 600 ohms, 300mW is enough.

It is still un-likely Q10 could have blown unless there is a diode failure also. Unless it was driven into a dead-short... then it could take a 2W rating to survive, but a big power device might not be fast enough for a 45MHz BW amplifier.

Well it's possible that the whole "dead short" scenario might have happened...

What about the MJE1xx series?

Thanks again,
Andrew

MPSW56 might not be too bad (in the tall TO-92 package, about 1W free-air).

> it's possible that the whole "dead short" scenario might have happened...

This amp (assuming "small" transistors) is protected against high momentary current but not against sustained shorts. It will over-heat. Probably the assumption was that a well-wired console should not have internal shorts, and external shorts will be mediated through transformers which will reduce the abuse.

OK, if you plan to do that again (as in: it is removed from the original console and exposed to Strange Loads), lift the collector leads of Q9 Q10 out of the PCB, tack 150 ohms 0.5W resistors in there.

Sustained DC into dead-short will still smoke a resistor or transistor, but big audio will just burn the paint, it probably won't just quit.

This will reduce power into 150 ohms, but that is no longer a common load. It will reduce power in 600 ohms, but still +24dBm, and few true 600 ohm loads are still around. Or need >+24dBm.

What about the MJE172? I have one of those kicking around right now.

Thanks,
Andrew

[quote author="Andrew"]What about the MJE172? I have one of those kicking around right now.

Thanks,
Andrew[/quote]

That should work too. Careful with the pinout.

Turns out it was a diode. It's fixed and fully working.

Thanks again for the help, I really appreciate it.
Andrew