p3a not working, help!

[benmanf]

i did some fixes in the soldering of my P3A and it stoped working!

i get 35 VDC on the output

when the speakers are dissconnected everything looks OK

and the green led lights ok

but when i connect the speaker a hummm noise comes from it

and the led lights lower .

when i dissconect the amp from the 220V

the supply caps discharge immidiatly

but when the speaker is not connected the caps discharge slowly....

(the speaker is OK!)


can you help me please?!?

 

[Mattyo5]

Sounds like the output transistors are blown.... at least on the + side. Or, you made a soldering mistake and bridged the + rail to the speaker output somehow. Are you using rods boards?

-Matthew K. Olson

 

[benmanf]

the transistors are ok

and i dont think i connected the + to the output...

maybe something else?

 

[still4given]

I built a three channel P101 and after testing the boards and everything was working properly I installed it into the chassis. Somewhere along the way of installing everything a drop of solder fell between two of the diodes where I couldn't see it and created a short. That blew Q7 which caused me to have rail voltage at the output. I cleared the solder bridge and replaced Q7 and all was well.

Hope this helps.

Blessings, Terry

 

[Mattyo5]

well, somehow rail voltage is getting to the output.... i doubt its oscillation...its probably a solder bridge or a bad transistor... double check the transistor for shorts

-Matthew K. Olson

 

[benmanf]

i fixed the problem!

it took me a lot of time!

now everything is working but the output transistors are hot
(60 deg. C)!

even when im not listening to any music or something...

what can it be?

(its the same with those transistors on the second channel)

maybe the output trans are damaged...what do you think?
(but the amp is working...)


tnx!!

 

[edl]

the idle current is too much.
let's reduce it with the trimmpot. 20-25mA is enough, 50mA is perfect. let's measure the voltage on the emmiter resistors. on the 0,33ohm rated resistors you need to adjust about 15mV.

(or maybe your heatsink is too small?)

 

[benmanf]

but, the bias trimpot is even not connected

and the heatsink is ok

at the begining it was'nt like that...the transistors were cold

 

[Greg Erskine]

but, the bias trimpot is even not connected

and the heatsink is ok

at the begining it was'nt like that...the transistors were cold

Sounds like you have modified the P3A design. You can't ask questions without telling us what you have done. Why don't you post the schematic.

A bias trimpot is pretty important in most designs. So if you had it working somehow without it and you were happy then put the amp back to the way it was when it worked.

A P3A constructed as designed by Rod works perfectly. Your not using Rod's PCBs or following Rod's construction instructions are you?

Greg

 

[benmanf]

my p3a is not modified...

and im not using rod's pcb...