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AU-999 initial powerup, after restoration

Will do, thx!

Headphone jack L/R are both isolated from ground, and there is continuity from it's sleeve to chassis ground, as well as the knobs. So it appears to be wired ok.
 
If the build is same as AU888, probably the hum is because it picks up environment hum because the shielding is off.

Just try with shielding mounted.
 
Good theory! I tried it with the shield plate installed though. Thanks!

It doesn't have the obnoxious ground hum ( when I touch the amp ) once a turntable is connected & I'm just running the speakers. Like I said, I'd never tried headphones before.

The good news is the loud rice krispies crackling is gone now that I replaced all the transistors on board 1228.

But there is still some low level noise/buzz present if phono 1/2/mic is selected. It increases as the volume is turned up. I noticed the power transformer is audibly humming a bit, but I assume that's normal.

The amp is very quiet once a line level source is selected. I'll try a different TT to rule that out. But I suspect there is some other noisy component on 1228 or crosstalk going on. I examined the board closely & checked for continuity along all the traces with my MM, it looks ok.
 
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TO-3 said:
Check any loose wiring, broken pads, the ''balance/test'' lever. report.
Click to expand...

Examined all the wiring & touched up a pad under TR701 on board 1208, but I don't think it affected anything. I did learn you need to switch to mono before you use the balance check! I was just doing it in stereo. I hope that wouldn't mess anything up? That's when the problems started appearing though.

I haven't worked up the nerve to plug headphones in again yet. Listening to Miles Davis on CD, sounds great.

Ps the bias takes a few minutes to go from 6mV to 13 mV, once powered up. Then it overshoots to 15mV and slowly drifts back down to 13 mV. This is normal?

Thanks again, everyone
 
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Re Balance check
In our off thread conversation I mentioned balance check gives out a mono signal to both channels (L-R). If I remember correctly. I hope you did not misinterpret me. Does it say in the users manual that you should switch the mode selector to mono when in BC mode? Wasn't it used to cancel volume differences in the input signal, not so much to aid volume balance with respect to speaker placement? It has probably been 30 years since I used it on the 777a so I could be mistaken...

Also, since you're familiar with it, how does the AU-999 heatsink itself? Isn't it a set of output transistors behind a metal mesh cage? Very little contact induction?
 
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Thanks! It's in the manual: tells you to switch it to any of the mono modes first

Output transistors mounted on big heatsinks inside the amp. Running cool.

The noise only happens with phono selected. I guess it could be related to the volume trimmers at the back, but I've worked them all & they seem to be functioning ok. I'll attach a sound clip, it's not regular TT background noise & wasn't doing it before. At least, I don't think it was...
 
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Here's a clip. Rename the file extension to .mp3 instead of .txt

Wouldn't upload otherwise

I also tried flipping around the power plugs on the TT ( which is plugged into the 999 power sockets ) and the 999.
 

Attachments

That's the universal sound of the 60Hz power signature. That's the most commonly heard buzz in the buzziness.

sounds more saw-toothy than sine wave. It is either at 60 hz or a harmonic. The phono circuit which is where you are hearing it? Is more Amped than the line in circuits, but it is probably there as well. Maybe at 20 or 30 db less. Perhaps the old salts on here could tell you that can be a PIA to find.
 
Yes, only happening on phono.

A clue could be the headphone noise from before. I'll check that out again & start measuring resistors etc on 1228 to make sure they're all good. And hit the switches with contact cleaner.I still think it's a ground leak/wiring issue.

Probably the new transistors on 1228 are higher gain than what I pulled out.
 
Yeah I don't think that's your regular 60Hz hum, there's something getting injected into the signal. I hear sputtering in there, and if I crank the vol I hear faint radio chatter, even though the tuner isn't powered on.

The headphones don't induce any hum, if I plug them in while the TT & speakers are hooked up. I'll try again with the 999 on the " bench."
 
Does anyone know what this thing is?

Attached to the phono inputs with 4 wires. Looks like a cap, it's called "NF-C" in the schems & SM, but I don't see it in the parts list anywhere. I guess it's some kind of filter?

NF-C.jpg
 
Great info in those threads!

I'll try a .047uf poly cap to ground on one phono input instead of that, and see if it does anything
 
Ha!

You know, for the purists, it wouldn't take much to completely bypass those rear volume pots for

Phono 1
Phono 2
Aux

and just reroute the wiring.

I'm also thinking of converting the mic input into another phono, as they share the phono preamp. RIAA is bypassed in mic though, stock. ( Credit to Kevzepp for that note! )
 
I wish an old salt would help you. I didn't see any NF-C in the pictures in the SM. But it is alluded to on page 38. It looks like a post design fix, or even a latter day repair. But you did well to find what you could about it in previous AK threads. You were not the first to be wondering about it.

For those of us frustrated Sui owners who are watching their old equipment limp towards the final stretch of their repatched lifetimes.

upload_2022-12-8_20-4-11.png

It's your thread and I'm thinking you're a good sport.
 
Thanks! I think I need to figure this out based on what's in front of me. Pretty hard for someone else to guess what's going on?

I'm just documenting it in case it's useful for someone down the road.

I don't think NF-C is the problem though: the mic inputs bypass it & the noise is present with mic source selected too. And the chances all 4x of those .05uF caps have gone bad must be pretty slim?

So I'm going to test 1228 components instead. The wiring under 1228 where it slots in all look good.
 
Unless I'm reading the schematic wrong, it looks like the phono inputs go:

1. from the rear of the amp chassis, up to board 1228 at the front, and through it
2. then all the way back to those variable resistors at the rear ( VR601-606 )
3. then back up to the front & into board 1208

So cutting all that out & jumpering the connections from board 1228 directly to board 1208 would sure simplify things, and cut down on stray noise/interference as well?
 
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Ah, not so easy. I guess I'd have to do it on the main selector switch, which means figuring out how it's 3 wafers/toggles are routed to everything.
 
Allright, I traced out where phono 1 sends & returns it's LR from the Variable Resistors to the main switch. So theoretically I could disconnect these wires & bridge them like so: ( the ground wiring should be unaffected, I think? )

999_phono_mod_switch.jpg


What I can't figure out is why at the back on the Variable Resistors, phono 1 & phono 2 have a signal bridged between them, off the 3rd outer lug of the pots. This is in addition to the ground lugs on the other side.

So say the ground is on lug#1, signal going in is lug#2, and signal going back out is lug#3. Pic below: the circled bits are common between phono 1 & 2 VR's, but it's not ground.

999_phono_mod_VR.jpg

* edit: I may have it backwards & lug#2 is the return. But either way I'd think phono 1 & phono 2 signals would be totally separate?
 
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