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troubleshooting a P3A
troubleshooting a P3A
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Old 19th December 2002, 04:52 AM   #11
Patrat is offline Patrat
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Join Date: Dec 2002
Location: San Antonio
Default progress

OK then, removed the "safety resistors", now get (weak) light from led, also now Q4 gets warm and Q9 gets HOT! Thats progress!
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Old 19th December 2002, 04:10 PM   #12
EnvisionAudio is offline EnvisionAudio  United States
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Location: Dayton, OH
Default Just a thought....

I know this could be a possibility that I have seen other constructors miss. Are resistors R9 and R10 2.7k ohms? I have seen a few people read 2k7 as 2.7 ohms!! (Should be red, purple, red, gold (or whatever tolerance you chose). Heck, I used 2.2k and have no problems, though that's only a 500 ohms difference. This would certainly make Q9 get hot (or explode!), Q4 to get warm and pull down the entire bottom half of the VA stage, causing a large DC offset at the output.

I guess it's possible that since the LED is driven from the negative 12 volt (at this time) supply, a high current draw through Q9 would pull the supply down considerably causing the LED to appear dim. Check all the color codes for all the resistors. I'm not saying you're stupid, I think we've all seen brown bands that look red on resistors, especially five banders. If in doubt, pull one resistor lead out of the board and remeasure with your meter.

Give it a shot - something's gotta give!
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Old 19th December 2002, 10:57 PM   #13
Patrat is offline Patrat
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Join Date: Dec 2002
Location: San Antonio
Default well I'll be...

Envision, you hit the nail on the head, I misread the number and did it more than once. Boy, do I feel stupid. The voltages are now correct, led lights, about 300mv at output. The closest I thing had on hand was 2.2K, I will change them out when the 2.7k get here.

Hey! UPS just brought my toroidal transformer!

I cant tell you how much I appreciate everyones' help and for sticking with me on this. Happy Holidays,

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