P3a problem

[Morph]

OK I need some help from you lot!

I have just built my first amp. It is the p3a. I built one channel perfectly and everything worked first time!

However I have now built the second channel, and when I first powered it up by mystake I only had the positive supply. This resulted in what seems a well known situation with the p3 amp where the most of the bnoard was at the same voltage as the positive rail (+38v) including the output.

So I sorted out the power supply now with both rails on and 2 safety resistors in place of the fuses. But now there is a large drop in voltage accross the safety resistors and the rails are at about +12v and -12v. Obviously lots of current is being drawn. The output is almost at 0 volts so thats ok.

It seems to me that my iniatail mistake with only the positve rail may have damaged something resulting in my current problem.

Any suggestion of how to fix this would be appreciated. oh and ive already checked and double checked that everything is in the right place and it is all exactly the same as the working channel.

thanks for any help.

[Morph]

Anybody??

[paulb]

How much current is being drawn (what value are the safety resistors)?
If the output is at 0, I'd look for a biasing problem (the trimpot and associated transistor). If the resistors aren't getting too hot, try putting them on both channels and compare voltages between channels. I have a record of voltage measurements on the P3A somewhere I could dig up if needed, but you have a working channel so use that as your reference.
Are you sure the trimpot is set to maximum resistance?

[Morph]

The trimpot is definatly set to maximum resistance and im pretty sure the associated transistor is fine becuase if it had gone I wouldnt be able to get a valid resistance readfing for the trimpot while it was still in circuit.

I ran the working channel with the same safety resistors (22 ohm/ 7w) and there was only a very slight drop in voltage across them, less than 1v. Which correlates with rod's test plan from the web site.

Where as this second channel has a huge drop across the safety resisters from 37v to 12v for both the + and - rails. And the resistors get VERY hot.

This says to me that the amp is drawing way too much power.

I think i'm gonna just have to change all the BC546's.

[tiroth]

The bias servo transistor is Q9, I believe.

Do you have bypass electrolytics on board? They may have been damaged, resulting in high leakage current.

Check for shorts in all the transistors. Generally it seems like the outputs or the drivers are damaged by things like this, although I am sure all sorts of things could potentially die.

If this is unsuccessful, you might use cold spray to determine which parts are sucking up a lot of current. I would guess that a transistor was damaged if not shorted, and needs to be replaced.

[pedro]

Morph
I'm new in this forum and I'm an unqualified diyer but I when I first built the P3a I did something wrong very similar to what you have done.
1. the best way is to ask Rod. He's a very kind and helpful man who knows more than anybody on P3a.
2. from my experience I'm nearly sure you has burned-out some transistors. So, I'm afraid you have to desolder one by one and to check which ones have been damaged.
3. In my first experience with P3a I had to replace most of the transitors twice.
Now my P3a runs wonderful, very good sounding amp.

Pedro.

[Morph]

Its ok I found the problem. I checked all the transitors and they were fine but i still had the problem. Then it clicked I had got the output transistors mixed up! I had PNP in place NPN and vice versa.


Well now I feel stupid.

anyway both channels are now working perfectly. i think Ive been lucky as niether of my 2 major mistakes have damaged the transistors.

Well it must say i was very skeptical about how much difference an amp could make, but this p3a just blow my technics AV out of the water!

[paulb]

Originally posted by Morph
Well it must say i was very skeptical about how much difference an amp could make, but this p3a just blow my technics AV out of the water!

I agree. Even with crappy speakers I could hear a marked difference from my old chip amp (LM1875s).

[saurus]

Wow, don't feel stupid - I couldn't count the number of times I slapped my forehead and began desoldering. I think I spent more time and money fixing my self-inflicted destruction than I did building it in the first place. But once I got it right, it was all worth it!

[pedro]

Morph,
You've been very lucky. As I said in my first post, I blow-out some transistors twice.
Have to tried the P88 hi-fi preamp. I have built it (I first changed the suggested opa2134 to 2 paired opa637's, now I'm using ad825's). P88 combined with P3a sounds magnificent.
Pedro