Modified aaa5

firstly this is not my machine and I've not done the mods [except for fitting opamp sockets, removing the muting transistors and fitting hard wired rca] The symtptoms where present before the current mods.

The problem is that there a low level hum [reading 6.1mv] on the output between sig and ground .....with the machine powered up only

using my scope its clearly a sine wave and on the longest time setting looks lens so theres a large sine over the top of the smaller.

this is audible as a hum via head phones with the wick turned up ...press play then pause its cear another elements been added as it introduces a beat to the hum.

Placing the probe on either of the opamps power rails its clear that this is the source as overlays the dc voltage ...

Suggestions to remove would be helpful !

 

I don't know much about this kind of thing, but the 1st thing that springs to mind is any capacitors around the regulators.

 

As far as I can tell nothing has been changes ...the mods previously carried out seem to be whole sale replacment of caps with blackgates.........but the schematics show nothing out the ordinary but I'll do a detailed check to see whats been added.

Dik did you get my pm ?

 

Not yet, Pete.
Sorry I cant help more, but my knowlege of electrickery is limited to soldering a few kits together, and trying to wade through a few textbooks without my head exploding.

 

Pete, what's the frequency of teh main component(s)? If it's 50, 100Hz (or multiples thereof) its PSU related, and I'd have a look at the supply pins of the output stage opamps; if you see the ripple there on your scope, work back through the power supply looking for a dud 'rail'. In particular, check voltages before and after all/any regulators. 3-pin regs generally need 3v or more overhead to work properly, so if a main PSU rail has dropped - for example, due to a dying reservoir cap or a blown diode in a bridge rectifier - then the reg drops-out and ripple from the raw supply will pass right through.

If the ripple is at some none-mains related frequency you've got something oscillating somewhere...

 

Cheer .......will take a look !

 

Problem solved ......[to a great extent] the four 1000uf caps c404 c406 c407 c408 were down by 100uf so swappedthem out for 1200uf [nearest I'd got to hand] reassembled switched on spun a disc set to pause turned the volume up full ....no hum [well just the tiniest bit]

so either it was badly rebuilt after the last mods [several loose screws], or the new caps have sorted it ........many thanks to everyone

the next mod will be to swap the diodes for schottky's and loose the snubbers ....any thoughts ?