AU888 restore discussion

I put it more or less together (front plate and knobs).
Several observations:

1. There is a need to clean the input switch. Because it is also in the feedback loop of the input amp, moving the switch selector from Tuner towards Phono and in between Phono and Mike selections, gives a rather loud switching effect. I guess I also need a high ohms resistor to ground at any place where a capacitor is switched (to have 0V DC level at it beforehand) Those places are the F1228 input amp output cap and the RIAA filter.

2. The amp is sensitive to hum from the outside, when getting close to the input amp with my hand I hear hum getting more. So it really NEEDS the metal shielding plate to get rid of the hum, as it has a wooden case.
The hum is gone having the metal shield attached.

3. The main output amps slowly hunts a bit within a millivolt. Also, from cold to warm, there is close to a millivolt difference. So, offset definately stays within 2 millivolts from zero, whether warm or cold. The decision to be a bit paranoid for component selection and modifying the circuit pays off for DC offset measurement.

4. Still, a "baxandall diode"is to be considered for the main amps.
 
2. The amp is sensitive to hum from the outside, when getting close to the input amp with my hand I hear hum getting more. So it really NEEDS the metal shielding plate to get rid of the hum, as it has a wooden case.
The hum is gone having the metal shield attached.

I made recordings of the noise floor on the 888 with and without the metal shield and even without a hand nearby the difference is striking.
 
Yes, although we should not be surprised, having the most sensitive circuits sticking out in the air, if having the plating is removed.;)
 
To add more to the au-888 info, mine seems to be a rare beast considered other internet pictures. It is one of those having extra fuses (which were relocated). Although one of the schematics I have lists those fuses in a drawing, this actually seems to be some modification, as it did add a lot to the wiring mess. The fuses were located where I did put the realys and the relay circuit. Some pictures of the schematic and original wiring. It seems the original transformer wires were too short. The place where those were extended was hidden below the plating covering the transformer.


two fuses back left

au888fuses1.jpg


Four fuses back right

au888fuses2.jpg

The plating the fuse holders did reside on.

au888fuses3.jpg

The part of the schematic apparently in use for my amp. Those fuses are NOT to be found on most internet pictures. However, as this mod is found on some schematic, it is "normal"

au888fuses4.jpg


Noteworthy is the fuse in the center winding offtake of the preamp 30V windings...

I do not know what they were thinking or whether some "problem"was supposed to be addressed.

The fuses are together with the square rectifier, as their original location is occupied now....
I did not put back the lamp fuse, as the thing is resistor current limited LED now

au888bottom.jpg
 
Last edited:
Yes, although we should not be surprised, having the most sensitive circuits sticking out in the air, if having the plating is removed.;)

have you considered lining the interior of the wooden cabinet with some copper foil tape to add some EMI shielding?
 
...also the amps that came with a metal case ( vs wood ) I presume, would not suffer from the shielding issue?
 
The AU-888 has a shield over preamp. Discussion above about noise was discussing when this sheild is off. There is no need for additional shield once factory preamp shield is reinstalled .

The models with metal cover also mostly have preamp shields as most of the noise comes from power supply, not from outside of amp.
 
Last edited:
As the old big caps had signs of heating by the R003 68 ohm resistor, I took a little bigger one and mounted it a few millimeters above the card te enable a little more airflow.
I'm going to switch to a smaller diameter wire wound resistor for R003. Might help out. Mouser ID 756-W21-68RJI
 
Back
Top Bottom