Sony TA-AX380 Stopped Working

[rudypoochris]

I am relatively new to the forums and apologize if this is not the right place to ask for help - I figured a DIY audio forum would be the best place to look for amplifier repair.

Anyway, I bought this amplifier two days ago - it worked perfectly when I first received it. After leaving the unit on over night with no music playing, I found the power indicator light to be "off". The receiver does not play audio now. If I turn the volume up all the way I do not hear any hum at all. I opened the case and cannot see anything burnt, fuse tested to have ~0 ohms resistance. When I power on the unit the relays fire, the transformer hums slightly (is energized). I really have no clue at all what it could be and it is driving me nuts.

This unit was actually purchased to replace another Sony receiver I had that "melted down" due to my own abuse. That unit similarly did not show any signs of burnt components or failed fuses. Maybe if I am lucky both of these can be fixed. What is peculiar to me is that this receiver failed with no load on it.

Could the transformers be going bad somehow?

Thank you!

[rudypoochris]

This is an old 1980's amp by the way, just to be clear.

[Mooly]

One thing to check first is that both units are rated for your mains supply voltage... if they are adjustable are they set correctly ?

You really need a circuit...

however faultfinding at a very basic level means measuring the main rail voltages (across the big caps in the PSU) and seeing if the output devices are supplied with voltage (discrete or power IC ?)

A good clue has to be the power light... trace it back... what feeds it. Look for low voltage regulators... if the amp has remote control there will be a 5 volt rail... is there 5 volts on the remote receiver etc.

Transformers fail through overheating... very unlikely, and usually there is a thermal fuse buried in the primary winding. If it buzzes and fire the relay it's not that.

Pictures ?????

[rudypoochris]

Thanks for the response! The amplifier is only rated for 120V which is the correct voltage, it is not adjustable.

Where would these main rail/big caps in the PSU be... here is the main board below as well as the auxiliary board.





I will check what powers the light tomorrow morning. I notice that on the header to the front panel there is a +5v, +9v, and -9v marked. Maybe I should start there. I believe it is a discrete amplifier, how do I check?

Thank you!

Here are the direct picture links so you can read the circuit board...

http://img.photobucket.com/albums/v4...s/IMG_1571.jpg
http://img.photobucket.com/albums/v4...s/IMG_1566.jpg

[jaycee]

This is a discrete amp as you can see 4 large power transistors (they probably have Sanken or SK written on em, right?).

The main filter capacitors are those two black cylinders at the top of the first picture. They could probably do with replacement, although that's a later job after you've found the first fault.

Checking if those 5v and 9v voltages are present is a good start. I'd lay good money that one of them isnt.

[rudypoochris]

Originally Posted by jaycee

This is a discrete amp as you can see 4 large power transistors (they probably have Sanken or SK written on em, right?).

The main filter capacitors are those two black cylinders at the top of the first picture. They could probably do with replacement, although that's a later job after you've found the first fault.

Checking if those 5v and 9v voltages are present is a good start. I'd lay good money that one of them isnt.

Will check. Yes Sanken.

I have noticed now that the power indicator light glows ever so slightly - so it must be receiving some current. When the muting button is pressed, the light vanishes entirely. Just food for thought... probably very low voltage.

[sakis]

the specific amp features an stand by circuit ( see the small trafo next to the big one ) you need to carefully check how voltages a runing arround various circuits ( caution there is needed since there is dangeroius voltages related ) to find out if both of traffos are suplied

the click you may hear on start up might be the stand by relay but after that you need to listen to the click that comes from the protection relay

sorry to say that this cannot be trouble shooted by any one without any expirience ....

[rudypoochris]

There are two clicks, first the power switch - second the switch on the main board pictured above. I will see if I can check for power to the smaller transformer. What does the stand by circuit do? Thanks!

[sakis]

hmmm that is good then you should focus on the secondary power supply that give the 5 and 9 volt

a comon failure for this type of amps is the input selector IC

[rudypoochris]

Originally Posted by sakis

hmmm that is good then you should focus on the secondary power supply that give the 5 and 9 volt

a comon failure for this type of amps is the input selector IC

You are referring to the smaller transformer? I will trace back the 9v and 5v source tomorrow. Is that 5/9v source signal or is it something else that merely lets you know if power is running through specific stages?