Help repairing old Akai AA-V1500 Amplifier

[galapogos01]

After a little help with a repair to my trusty old AA-V1500.

I have had this amplifier for roughly 15 years and it has done me well until this week, when the wife turned it on one day only to find it going into Protect mode. Just the night before it had been working just fine.

Upon disassembly, even with the pre-amp and inputs boards removed, it still goes into Protect mode. No obvious signs of damage, a little heat discolouration around some heatsinks but nothing else I can see. The power rails I can find (+/- 15V and +/- 50V) both measure OK.

If I bypass the protection circuit and engage the output relays I get +20V at the positive speaker terminals, both left and right channel. This suggests a common fault to all channels yet from what I can tell the power supplies are fine?

 

I have noticed the power ground is separate from the signal ground, but I can't see where they are coupled.

If anyone can give some tips or tell me where to find a service manual it would be appreciated, I'd love to breathe some life back into this trusty old amp.

Some pics:






Cheers,
Jason

[Equaliser]

Lot's of discolouration going on there in the output stages for each channel. Sounds like a resistor has gone open.

[galapogos01]

Yeah, they're underneath the main heatsink tube (which is internally fan cooled).

 

Agree regarding a resistor.  Just can't find which one!  I have measured the high wattage ones and haven't found an open one yet.

 

Without a schematic it's hard to find where the two grounds are coupled.  I have been over the board for hours and can't seem to find it. 

Edited May 16, 2015 by galapogos01

[galapogos01]

So.

 

If I bodge the 2 earths together, it clears up the voltage offset on the output terminals, and will play audio.  Unfortunately I still have to override Protect mode - seems wherever the protection circuit picks up it's earth from is also damaged.

 

I really need a schematic!

[Zaphod Beeblebrox]

I can't locate a manual for that model. I can, however, tell you that unless ALL daughter boards are installed AND screwed to the rear panel, that the amp is unlikely to work at all. This is normal with modern AV amps, as earthing systems are complex and the rear panel is normally used for earthing. 

 

Without a manual, it will be a difficult job (if I was presented with it, I'd simply hand it back to the owner), as there is usually a bypass procedure which is accessible via front panel keystrokes. No way of knowing what this is. 

 

Good luck. You'll need it. 

[galapogos01]

Anybody got any other advice or perhaps a source for rare service manuals?