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Old 2nd August 2007, 01:46 PM   #1
adi81bv is offline adi81bv  Romania
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Unhappy P3A problem

hi
i have constructed a P3A amp and when i powerd it up the replace transistors C546 burned. (i understand that that was the problem with the transistors)

after that i have build the amp again using new part (others) and I get 33V on output.

I am using BD249C & BD250C with BD139/140 and BC546A.

I measured the voltage and all the results are presented in the picture:
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File Type: jpg elliot_3a-masurat.jpg (84.9 KB, 939 views)
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Old 2nd August 2007, 02:04 PM   #2
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You have a fault with your input stage most likely, the voltage at the emitters of the LTP should be about 0.65 below ground, not several volts above ground.
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Old 2nd August 2007, 02:35 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
the reverse voltage on BE of Q7 and the 3.7Vbe of Q8 probably mean both these are damaged. Replace carefully. Do you sink the small device legs while you solder? I don't and I get away with it. I know I should!!
That 1.1V across the multiplier may indicate that the output stage is undamaged.
Why is there only 6.9V across the DC blocking NFB cap? Is it leaking badly or damaged or is that normal for a high output offset? C4 is back to front. It is normally biased the other way when the circuit is operating correctly. That probably explains the ONLY 6.9V across it.
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Old 2nd August 2007, 11:19 PM   #4
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check Q 6, 7, 8 for shorts or incorrect leg positions. Also check LED1, R5 and R6 for incorrect values or open circuits. Your bias servo seems to be working right and the output stage also so check the front end of the amp.
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Old 3rd August 2007, 04:40 AM   #5
anatech is offline anatech  Canada
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Hi adi81bv,
The voltages around your diff pair are normal for this fault condition. They have been damaged. They may still work but the hFE may have shifted and they will be noisier than they were. Replace for that reason and match for beta. Andrew is correct in that your emitter voltages should be approximately -0.65 VDC from a ground reference.

Q3 appears to be biased on but should be off (ie 0.00 VDC from emitter to base). You should check it to make sure it isn't shorted, also check that R4 is soldered properly and measures correctly. If Q1 was shorted or your output, you may get these readings also. However, Q3 should be off and it could be Q7 in reverse breakdown.

Thermally couple Q7 and Q8 when you replace them with a matched pair.

-Chris
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Old 3rd August 2007, 01:54 PM   #6
adi81bv is offline adi81bv  Romania
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i verified every transistor. hFE, and resistance - they all seem to work fine. I verified 4 times the tracks and the solder. I verified the correct soldering of the transistors on the pcb.

anyone have another ideea? i dont know to much electronics.... i was told that this is a easy amp to build, but i build 2 of them and both are dead. dont know what to do....
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Old 3rd August 2007, 06:05 PM   #7
anatech is offline anatech  Canada
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Hi adi81bv,
Quote:
i dont know to much electronics.... i was told that this is a easy amp to build, but i build 2 of them and both are dead. dont know what to do....
At least you are consistent. There is something that you think is right, that is not. At this point you need someone more familiar with electronics to look over what you have done. Perhaps if you could take a picture of your work and post it?

-Chris
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Old 3rd August 2007, 11:13 PM   #8
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Default Hi,

it looks like R13 is disconnected (open cicuit),
voltage over it don't make sense.
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Old 3rd August 2007, 11:26 PM   #9
adi81bv is offline adi81bv  Romania
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i checked R13. it is correctly soldered. may be the transistors? there can not be the resistors (it's the second amp that i am building; the first didn't work either and i changed the pcb and all parts) i measured all the resistors prior to soldering and checked 3 times the circuit for correct alignment and short-circuit.
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Old 4th August 2007, 01:02 AM   #10
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Default ok

as is at list Q8 doesn't do its job and may be damaged as
mentioned before.
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