Uh oh... sigma 22 mistake

[oneplustwo]

Ok, now I get 30V+ but I get 3V+ on the negative rail. Why would I get +? D3 and D4 bad? Replace Q2? I'm getting nearly 50V across that one like I did with Q1.

EDIT: Replaced Q2 and got the same result. Checked Q2 again and it's blown again (I heard a small "snap" at power on). What could be causing Q2 to blow? *sigh* Someday I'll contribute to this forum as much as I receive because I will have made all the mistakes!

 

[amb]

Sounds like something else is damaged downstream from Q2, causing Q2 to blow. I mentioned elsewhere in the β22 thread about using the DMM's diode check mode to test the transistors. You should do that for all the BC546B and BC556B transistors on the negative rail.

 

[oneplustwo]

Checked and two of them were definitely bad, one of them I think only looked bad because one of the legs was shorted to the other on the PCB (Q10).

Replaced and powered up and it worked. Thanks everyone for the help!

Now I just need someone else to have the same exact problem so I can impart my new found wisdom!

Any takers on problem sigma #2? Seems like it has to be a problem with a resistor or two in the wrong position.

 

[oneplustwo]

Just out of curiosity, anyone have a theory as to why one or more of the Q's was bad? Is it most likely that I applied too much heat to one of them during soldering? Or did I just happen to get a bad one? I didn't make a mistake in terms of orientation or wrong part/wrong place. Just trying to learn what the potential issue was so I can try to avoid the issue again later.

Also, still wondering if anyone has any theories about the sigma giving me the 15 volts instead of the 30. I'll just try testing voltages if not and hopefully that will shed some light on the problem.

 

[amb]

Quote:

Originally Posted by oneplustwo /img/forum/go_quote.gif Just out of curiosity, anyone have a theory as to why one or more of the Q's was bad? Is it most likely that I applied too much heat to one of them during soldering? Or did I just happen to get a bad one? I didn't make a mistake in terms of orientation or wrong part/wrong place. Just trying to learn what the potential issue was so I can try to avoid the issue again later.

Too much heat may have done it, but more likely it was due to an accidental short circuit (meter probes or other objects touching certain points while you're measuring, either when the power is still on, or when the power is off but there is still a large charge in the bulk caps.

Quote:

Also, still wondering if anyone has any theories about the sigma giving me the 15 volts instead of the 30. I'll just try testing voltages if not and hopefully that will shed some light on the problem.

Did you measure the D5 zener diode voltage while powered up? Also measure the resistance of R8, R9, R10 and R11 to make sure that they are what you expect, and check for bad solder joints.

 

[oneplustwo]

Hmm... something weird is going on. All the resistors check out fine except two. R10 is supposed to be 6.81kOhm but I measure around 5.56kOhm. I checked it and it was the right one but I replaced it anyway (and double checked on the DMM before I installed.) Same result. R8 is supposed to be 10kOhm but I measure around 7.8kOhm. Checked fine, replaced it anyway just like R10 and still the same result. Also, I checked the D5 zener while powered up and get just under 12V so that seems to be fine.

Anyone have any ideas for what to check next?

 

[amb]

Disconnect any load at V+ and V- before you measure any resistors. The load might throw off the measurements because it goes in parallel with R8 + R10.

Other than that, time to do some more exhaustive voltage measurements and compare against those listed in the operating points PDF (found at the σ22 website initial check section).

 

[oneplustwo]

Hmm... ok. There was no load on the board if that makes any difference. I'll go through the operating points I guess then. Thanks for the direction!