AR A-07 amp with ticking phono stage

68Spider

Member
2011-11-15 9:28 am
UK
I'm trying to fix an old Acoustic Research A-07 integrated amp that has developed a ticking on the right hand channel, phono input. The ticking is at about 16Hz and after some time prodding about with a test meter it disappears. Switching off and on again immediately and it stays silent. Switching off and leaving it for a couple of minutes makes it reappear.

It is a pretty simple circuit - an NE5532 opamp and 4 transistors. When checking voltages I see that the side of the NE5532 with the problem has 9V on both inputs (like the working side) but -17.8V (same as the negative supply) on the opamp output :eek: That doesn't seem right. Can an opamp fail like this - putting the negative rail onto the output? Would such a voltage issue cause the ticking problem or do I have two problems?
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
What is the reference point for 9V measurement on the inputs? If the supply rails are ~+/-18V, inputs should be close to 0V. If it is a signal opamp that appears to be shot, it likely is something to do with the 16Hz time constant, since an electrolytic would have to be in play to generate that. A scope would show it as appearing on an electrolytic such as an input cap, and one of those could well be the source of your problem, allowing DC on the opamp input and so forth.
 

68Spider

Member
2011-11-15 9:28 am
UK
The reference point for the 9V on inputs is signal ground. I have found a schematic for the A-06 which looks to have the same phono stage circuit as the A-07 but a different board layout. It wasn't scanned very well but I can just about make out a 10V designation at each of the opamp inputs so I guess 9-10V is by design.

I can't measure anything lower than the -17.8 on supply rail and RHS opamp output so it looks like an internal short. One thing it has done is dump reverse voltage to a 10uF cap at the output of this stage so that will be broken, perhaps causing the ticking.

I'm planning on replacing the opamp and the cap and see if that fixes it - but stop me if you have a better idea :). Running an NE5532 at +/-18V is over the max rating of +/-15V, I wonder if that has fried it and I should try dropping that supply voltage down.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Good work. I'm sure you'll work out what happened to simultaneously cut the tape loop channel. BTW, 5532 is normally rated to
+/-18V but some versions go to +/-22V without problems. just Google the full part number for a datasheet and check it for free.
There are many sources and almost as many quality levels of these mega-popular parts, though.