Refurbishing and upgrading the AU-D11 II

Here is the completed unit back in my equipment rack, complete with brand new front panel courtesy of Nico at Sansui Parts Shop.

Where is this Sansui parts shop?

Did you notice anything strange about the power transformer? It looks like it should be toroidal from the round case, but I thought it looked like a regular square frame transformer encased in some sort of resin.

Nice work! It's making me not want to sell the two I have.

Lee.
 
Hello Lee,
The transformer looked like a toroidal to me. It is encased in resin as you describe, and the only things that make the underside resin uneven are where the primary and secondary wires feed round.
 
Hello Lee,
The transformer looked like a toroidal to me. It is encased in resin as you describe, and the only things that make the underside resin uneven are where the primary and secondary wires feed round.

Hmmm... I wonder what this big square thing is then inside the resin?

Sansui AU-D11 II transformer.jpg
 
Rumour has it that its actually an 'EI' core transformer potted in a toroid can, but I disagree. ;)
 
Hi phonomac,

While your at it, if I recall correctly, there was a bulletin on modifications to the headphhone circuit because, in some of the D11 IIs, the headphone transformer would blow and there are no replacements for this. The bulletin was available on the obsinc site which no longer exists.

Marc
 
The bulletin was available on the obsinc site which no longer exists.
I have a copy of the bulletins, they have been posted on AK at least once, I am pretty sure phonomac is aware of them and has applied them to his example.
 
Yes, I received a copy of the bulletins, along with some supporting information on the AU-D11 II from riverrat who used to post here. It is a pity about the Obsinc site, Isao had a very useful information resource. I believe it stopped because eBay discontinued a small market service that he had used..
 
Is the calibration (offsets, bias) on this amp inter-linked between the different boards? For example, if you change the calibration on the Phono amp or control amp, will it also impact the readings on the power amp (and vice-versa)?
The reason I ask is that, as some of you know mine is getting worked over by the tech, but the repairs are running over budget, so I might have to skip on a mod or two. If the amp was expertly calibrated and later on I decided to so something local to one section (like the headphone service bulletin), will the recalibration be limited to just the pre-amp or will the changes cascade into the other sections as well?
 
The hot-ground trim works off an unregulated supply and is entirely dependent on line voltage. There's no way to trim it closely for other than a stabilized line voltage. Is that normal and expected?
 
The hot-ground trim works off an unregulated supply and is entirely dependent on line voltage. There's no way to trim it closely for other than a stabilized line voltage. Is that normal and expected?
Exactly Conrad, that is correct - it is an unattainable setting, you can get it roughly right, but in a matter of seconds it will fly around all over the place. To expect your local mains voltage to be mV perfect is asking too much. ;)

I actually wonder whether it would be better to attempt the adjustment and get the relevant trimmers roughly right - then use a multimeter and set them so they are the same resistance - but as close to the first setting as possible. You could even replace the trimmers with fixed resistors after doing this.
 
While the hot-ground offset works from an unregulated supply, I think the real problem is that the 'ground' in question is artificial. The ground reference is established by two 6.8k resistors working as a potential divider across the + and - power rails. So line voltage swings in theory don't matter, the reference point remains at half the total voltage. What upsets that theory is differential currents flowing in the positive and negative portions of the circuit, which have to flow through one or other resistor so shifting the balance point.

As noted above I achieved a better situation using 1k resistors in place of the 6.8k originals. The effect of this change is that unequal currents don't create such an offset voltage unbalance so the range that it drifts over is much diminished.

But the fact remains that it does drift!
 
That makes sense, as the supplies are balanced, so a good design would allow for them to track up and down with line voltage. When I get other problems sorted out, I'll try changing the resistors. I'm afraid I had a problem with the connectors on top of the driver boards getting old and oxidized. That made them intermittent- tapping the boards would change the offset adjustments by a huge amount. A bit of cleaning and Deoxit took care of it, but in the process I managed to lose one channels hot-ground adjustment range entirely. :no: Probably popped a FET or something. Need to look at that today. I'm also hoping the oxidized connectors would explain excessive distortion seen on both channels.
 
On the square thing under the transformer, the one I'm working on has it a little more visible. I think it's some sort of shielding or cover plate, not the transformer itself. Thus, the toroid question remains a mystery.

Back to the offsets- I found the connectors on top of the driver boards hugely affect the hot-cold offset. I tried Deoxit and even disassembled the contacts, but they still change the offset if you tap them. I'm about ready to ditch them entirely and solder the input wires directly to the board. Thoughts?
 
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