Onkyo power amp board

Karl vd Berg

Member
2012-07-16 12:49 am
Hi all,

Tried in other places, but no help, so I hope you give me a hand here...

I have an Onkyo TX-84 to service. The 2 main problem was a permanent and annoying "buzz" and a relay (12V) going under protection. I've heard that this 1985 receiver has a common issue with its relays so I decided to change the 2 relays it has in the power amp board....

Ahead:

The amp had no ground connected, so I added a power cord with ground connection at the chassis and stopped to feel that current when I touched the case. The audio "buzz" dropped a lot, but still I could hear it when I switched to another input line in silent moments.

I decided to replace the two main power caps (10,000uF 50V Nichicon VX(M) series) as they were leaking a lot - as you can see from the picture, with two new BC Vishay M 10,000uF 63V.

When I switched it on to test the amp, the 5A fuse blows all the time when I just turn the Volume pot a bit louder...

Checked polarity of power caps, everything seems perfect.

Can it be one of the relays (they have same 12V and 24V as the originals had).

Or maybe one of the caps is in short, but I don't know where...

Listening to very low volume I notice a huge improvement on the bass and dinamics, but once I put the volume a bit higher, the fuse blows up! :(

Any idea?

[IMGDEAD]http://img832.imageshack.us/img832/7849/p2200604.jpg[/IMGDEAD]

[IMGDEAD]http://img585.imageshack.us/img585/8947/p2200593i.jpg[/IMGDEAD]
 
Last edited:

unclejed613

Member
2006-12-28 12:19 am
two things to check. dried out electrolytics are common in Onkyo receivers, may cause oscillation or other weird failures. they run the amps so hot at idle, it's a given you should check all of the small electrolytics on the amp board. second, you may have something wrong with the amplifier bias circuits. Onkyo uses a strange 2-transistor bias circuit, with one transistor for the outputs and one for the drivers, but the two bias transistors are in a strange series-parallel arrangement in the base circuits of the drivers. check all of the small flameproof resistors, as it;s common for one to be open, and that's even true in the bias circuit. check all of the base resistors for the outputs and drivers, and all the way back to the voltage amplifier stage, which is often a symmetrical VAS. also check all of the soldering. as i said Onkyo amps run hot, and ring joints in the solder are very common (it gets worse with their multichannel amps, because the component density on the amp board is much higher)