transistor testing.

[52 Bill]

The other thing, when i pulled the first one,a 2SC 5200, it tested shorted. I tested the other output trans. in circuit and the short was gone. Is that what Enzo and 52 Bill is talking about ? I sided with caution and pulled all of them for test. The question i have, if they don't show a short in circuit, but show a short outside of the circuit, am i missing something here?

If you look at the schematic for this amp, or almost any power amp, the output transistors are paralleled to get more power from the output stage. Look at page 39 of the manual.

Output transistors Q203, Q207, Q209 and Q211 are wired together. All of the Collectors are wired to each other, all of the Bases are wired together, and the Emitters are connected to each other through 0.22R resistors. If one of the transistors develops a short between any two of its' terminals, then the other transistors will also show as shorted across those same two terminals. If you remove the transistor with the short, the other transistors will now test as okay unless they re shorted as well.

If the other transistors tested as okay in circuit, but then test as shorted when removed from circuit, then either you misread the test in circuit or you damaged them when removing them or I suppose that you could also be misreading the out of circuit test. For me it would help to know what terminals on the transistors are showing as shorted, what meter setting you are using and what readings you are getting.

[catstrat]

Hi Bill. The criteria i'm using is whether it show's a short in the circuit or not, if i remove the transistor and it beeps with my meter on diode test, then it's shorted. It is a real possibility i ruined them with too much heat. But i'm aware of too much heat destroying a transistor and i try to be very cautious. I usually heatsink the legs before any soldering but i didn't do it this time. I'm using a 25 watt iron so it's not a 40 watt iron. I try not to keep heat applied for more than 10 seconds. Ten seconds max is what i was told when i worked at Raytheon. Is this another piece of mis information i should dis regard? Thanks.

[g1]

It will be a constant beep if shorted. Some meters will give a quick beep to tell a junction is forward biased.

[Enzo]

This^^^^

If the meter makes a steady beeeeeeeeeeep, then we are probably shorted. If it just goes beep, that usually means it sees a junction. Look at the meter reading. If it beeps and says zero, then we have a short, but if it beeps and says 0.45v or something, that is OK.

[52 Bill]

If the meter makes a steady beeeeeeeeeeep, then we are probably shorted. If it just goes beep, that usually means it sees a junction. Look at the meter reading. If it beeps and says zero, then we have a short, but if it beeps and says 0.45v or something, that is OK.

Until i got my newest Fluke, I never had a meter that beeped when testing semiconductor junctions. It does make it easy to not have to look at the meter when doing tests, but i still like to know the voltage numbers.

As for soldering heat damage to transistors, I don't see it happen often. There are other ways to damage solid state devices when installing or removing them.

[Enzo]

I used to use a little probe style meter a lot, it was autoranging. And it made a quick beep each time it shifted range. So when I touched it to say a 5v node, it would go "bededeep" as it transitioned from 200v to 20v to 10v or whatever its ranges were. I loved that little meter in field service.

I'd wager the innards were the same as this one:
DM73C Pen Probe Style Digital Multimeter | Amprobe

[catstrat]

Thanks everyone. I always wondered about how much heat is too much. I probably will continue to heatsink the pins when i can just for piece of mind. My meter gives a short quick beep and when i look at the readout it show the transistor is ok. I'll have to check out the meter you use Enzo. I bought one similar but it was huge and bulky to use. I don't know how many times i've been checking a voltage and when i turned my head to read the meter, the probes slipped and,pow. That's not good.

[Enzo]

Then use clips instead of probes.


I haven't used that pen style meter in years, it was just another example of how meters might use the beep tome. But I ought to dig it out and play with it a while.

[mhuss]

That would be a great little product - a pen-sized diode tester/continuity tester with the short and long beep functionality.