need some trouble shooting help

Hello !

I need some help in figuring out why my amplifier is blowing fuses.

I fired up one channel of P3a today for the first time. At first everything seemed ok, led lit up. I measured DC output and its about 65mvolts. There was no DC output on the input. Then I connected old 8 ohm speaker to the output and all was ok. It didn't suck in or out, I could hear normal humm when I touched the input wire. I measured voltage between two 5 watt resistors and it was under 100 mv DC.

So then I went ahead and connected signal to the input and all I could hear was crackling like its out of power. I went ahead and then adjusted vr1 and thats when negative rail fuse blew and speaker sucked in. I went ahead and disconnected the AC
power to the power supply. I checked all the components for visual damage and didn't find anything, only one of the 5watt resistors got warm and one blown fuse.

I pulled both fuses and checked the power supply again and it was ok. I am using 25-0-25
500va transformer, bridge and two 10000 uF caps. I put the fuses back in and turned it on
again. Led lit up for a second and then it blew the other fuse (positive rail).

What should I check next ?

Tia
 
darn rail fuses

Happened to me a couple of time
the rails fuses just gave away leaving me with crossover cooked to perfection :bawling: I now believe that the problem was the speaker cable I was using, there was too much capacitance thru it.

Now when you turned power down before connecting audio source? what was your source?

You should not check the drivers BD139 BD140
and the OP tansistors
those were the ones ceasing to function in my amp.

Hope you find the problem
 
KenP, I bought the boards from Rod Elliot online. The layout is pretty straight forward.

I used 1% metal resistors, including the 5 watters on the output transistors and panasonic caps. I measured all resistors one at a time before soldering them to the board. I have not measured the caps or transistors though. I am not still sure why it started blowing fuses. It still does on one channel, I have to take the caps out now because I have already replaced all transistors
twice and its driving me nuts.

The board layout was made to be cut in a half if needed, and thats what I did for ease of installation.

My power supply is pretty simple. I am using 500va 25-0-25 avel lindberg tranny, two 35a bridge rectifiers and 4 10000uF panasonic caps (two per rail). If I remember correctly that gets me about +/- 36volts.

Once I get the second channel working I'll start putting it all together, its not in a chassis just yet and I have not done any listening on my speakers (only a raw 8 ohm driver for testing).

I'll keep you posted, also if you need a part # for the trim pots that fit perfect let me know.
 
sam9

I've had a couple mysteries like that. It had nothing to do with board either time. Once it was an error with installing the output connectors - something I thought was insulated wasn't. On another occassion it only happened when the enclosure was closed - turned out something was contacting the end of the bolt that attached the trasformer creating a short coil - very major short.

While your situation doesn't sound quite like these, consider looking beyond the PCB just in case.
 
Re: sam9

sam9 said:
I've had a couple mysteries like that. It had nothing to do with board either time. Once it was an error with installing the output connectors - something I thought was insulated wasn't. On another occassion it only happened when the enclosure was closed - turned out something was contacting the end of the bolt that attached the trasformer creating a short coil - very major short.

While your situation doesn't sound quite like these, consider looking beyond the PCB just in case.

The board and the power supply is not installed in the case yet, I can't possibly think of anything that could be shorting anything out. I'll start removing and testing caps and then transistors again tonight.
 
tips from an unintentional arsenist

Having to replace transistors each time while you are trying to track down the problem is a pain in the neck. What I do is to put a 10-ohm high-power WW resistors in the collectors of the output transistors (or in the power line feeding the output transistors). This will not be good for normal operation but will enable the amp to function adequately to debug it. If there is a short or instability the resistors will get really hot but the transistors will be saved.

Another tip is to power up the amp without the o/p transistors connected at all (no drivers or power transistors). Move the feedback point to the voltage gain stage output (the point before the output drivers) and check all the voltages and currents look sensible. Then add the output stage (with the 10-ohm safety resistors) and test again.

I'm not familiar with the circuit you are using, but generally it is possible to do this.