Recap problems...I need help!

[rob29]

Alright, well I am fairly new to the audio scene as I only started this hobby within the past year. I just graduated from an electronics technician course this past week and am off within the month to join the Navy as an electronics tech (cant wait!!). I am very comfortable working around electronics so I decided to try out some things with my first reciever, which is currently being replaced by a Marantz sr100 which I got for $30(Sounds beautiful).

The reciever that I am currently working on is an AKAI AA-1020 that I picked up for free and I thought sounded amazing until I started listening to the little things. I was getting what sounded like AC distortion in the speakers when the volume was turned up with no signal so I figured it was the capacitors in the rectifier/ power amp stages. I just completed recapping them, double checked everything, and turned it on to test.

Well... the noise is still there. I poked and proded a little bit more and found that when i touched some capacitors the noise slightly went away. I also tested the DC offset and got a extremely bad 600mV from both channels :thumbsdn:.

So this has led me to think that there may be a bad ground somewhere and decided it clip from chassis to grounding prong on a power bar and the noise dropped quite dramatically. So next I am going to put a new power cord with a grounding prong on it just to be safe. But there is still some noise and I really want this amp back in order.

Any help would be greatly appreciated for a yound guy who is looking to waste some time.

 

[rob29]

On further inspection I have found that one main driver transistor has been replaced and the others seem to be bulging slightly. I am now wondering if the original capacitors were also replaced as they do not look too bad and if someone could have possibly put in wrong values.

The problem with this is that I do not have any scematics for the reciever and have been unable to find any online. This is going to be fun lol.

 

[EchoWars]

Adding a ground wire to the power cable is almost certain to introduce a permanent hum through a ground loop with external devices (if you intend to NEVER connect an external device, such as a CD player or turntable, the grounded power cable isn't a problem).

You more than likely have a broken ground connection somewhere in the amp, in the form of a broken wire or PC board trace, or a burnt resistor.

Find the main ground for the amp (generally the connection between the two large power supply caps). Use this point for your reference, and check the voltage at all the PC board grounds you can find.