AU-999 Restoration & Modification

The phono board has two types of transistors, one set compared to the other is in some weird mirror wise config that makes it easy to pin out wrong - if the replacement tranny's pin out are different too.....so double and triple check those.

Test again with covers on and after thorough inspection of phono board pinouts.

I checked the transistors and they are correct. Then I placed the cover to those boards on and the hum went away! Yay!

The foam that was under that cover was crumbling. Replace? If so, with what?

What about the click/pop/thump sound when switching inputs? Is it worth trying deoxit on that switch? Anything to worry about?
 
Scrape off the old flaking foam.
I put some foam I had on hand to apply medium pressure to the top of the PCBs to keep them seated.
 
Scrape off the old flaking foam.
I put some foam I had on hand to apply medium pressure to the top of the PCBs to keep them seated.

I just shop-vac'd the foam right off. So its only purpose is to secure seating the boards? I may have some I can use. If not, I'll get some.
 
Wow, glad that solved the loud hum on the phono inputs, don't recall having that, just a faint hum when covers off. A band of weather strip might do the trick in lieu of the foam.

If using the AUX, make sure you have cleaned the aux trim pot in the back. I avoid AUX all together and use tuner for my digital source, since it has a dedicated quality switch with no trimpot and it skips the rotary selector.
 
proper pops when flipping switches sometimes indicates DC leaking through a signal path cap. The noise due to dirt is more just a scratchy sounds, or fingernails down a chalkboard if you go very slowly, whereas DC issues are more like a pop/thump.... I hope that makes sense.

Nice work on the bias servo transistor. FWIW, I just opened up my AU-X111MOS last night and check what they use in that; a pair of C1845.
 
proper pops when flipping switches sometimes indicates DC leaking through a signal path cap...
Would that be one or more of the green mylars?

It's not a concern since stereofun pointed out the advantages of using the Tuner input. And since I only use two inputs (digital and vinyl) I will just have to flick a switch to change between the two - win/win.

I couldn't get to it yesterday, but I plan on replacing the foam and putting it into my main system to finally give it a proper test.

Although I still need to refinish the wood case...
 
I've been listening to it through my KEF 104/2 and all I can say is wow! Depth, detail and oh so clean. It lacked presence and ooomph (for lack of a better word) before but it's all there now with plenty to spare. I don't have time tonight but I'll be pulling out all my favorites to hear them again through this.

Thank you all for all the help, guidance, and most of all, patience.

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Nice work, now kick back and enjoy..
Make sure to share your experience once you've become more familiar with her character.
 
Still enjoying the amp but have a question...Why is the DC offset not adjusted to zero, or near zero on this amp?

Also realized I never posted the obligatory old parts pic...

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What is your offset ? - I remember at some point during the thread there was talk of using different transistors on the driver board, which is ok as long as they don't work in pairs - please make sure that TR801/803 ARE the same type, and preferably matched in hfe. I think you already verified that both VR801 pots are 5Kohms (you can do a quick measure on the two outer legs to confirm - I had a pot in my 777a completely out of spec in one side, preventing proper adjustment.

Here is a great thread from Echowars on the subject.
 
I'm very familiar with Echowars thread and I don't have any issues adjusting offset. But I set it somewhere between 12 and 15 mV for each channel (don't remember exact measurements, I don't have my notes with me), which is what I thought was recommended. Looking at the service manual, it states 0+/-50mV. I'm not experiencing any issues, but I'm thinking I should get it as close to 0 as possible, right?
 
Yes, technically ideally as close to zero, but I don't think anybody can hear 12-15mV - and with a manual stating +/- 50mV I wouldn't worry about, but I get the curiosity of why ? Try to turn on your amp and let it sit for a while just idle with no signal, then in 20 minutes time probe at the speakers (not amp terminal) - this will give you a realistic picture of what the speakers actually see after amp has settled down to a stable temperature - just like bias you never want to measure on an amp that is just turned on.
 
I'm very familiar with Echowars thread and I don't have any issues adjusting offset. But I set it somewhere between 12 and 15 mV for each channel (don't remember exact measurements, I don't have my notes with me), which is what I thought was recommended. Looking at the service manual, it states 0+/-50mV. I'm not experiencing any issues, but I'm thinking I should get it as close to 0 as possible, right?

Those instructions are saying to set it to zero, they're just saying if it's within 50mV of zero, in either the positive or negative direction it's good enough, and there's no sense trying to get it closer, because it won't affect anything. If you think about it, 50mV into 8 ohms is 0.0003 W of DC offset, which will do no harm to your speakers, and won't put the voice coils off center enough to cause distortion. You got it even closer, being only 12-15mV, so even less to worry about.
 
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