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Yet another Au-999 rebuild

Efver

Efver

New Member
Hello there. Been lurking around here for a while, finally pulled the trigger regards a rebuild and modding of my Au-999. I´ve had it for about five years; these units are somewhat rarer here in Sweden I believe.
I have never done a rebuild of this magnitude, and it all feels a bit overwhelming to tell you the truth!
I am a novice, so please be forgiving with me!

To make it a bit more comprehensible I´m planning to divide the rebuild into sections. Also, this is my only amp at the moment, so the time it will be up on the work bench could perhaps be kept at shorter streaks.

I have already begun with the removal of the subsonic filter. The unit is otherwise stock, apart from four new bindingposts.

My plan for the first section of this rebuild is to start with the the mods (ordering the correct parts for these mods) + ordering and changing the filter caps and bridge rectifier.

Thoughts?
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For filter caps Nichicon LKG 10000uf 63V

https://www.mouser.se/ProductDetail/Nichicon/LKG1J103MKN?qs=Ysb/BSeDKl3ywbTlpPp5NQ==

I´ve read recommendations for 12000uf 100V filter caps as well. Would the amp benefit from an "infinite" increase in filter cap capacitance and voltage?

___________

Bridge rectifier upgrade. Would this one be "appropriate"?:

https://www.mouser.se/ProductDetail...V2i03Pvz7eu30Uz9yLVjBMQ7vWr1tpa3oSGHnEttezA==


Greatful for any kind of help here! Thanks
 
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Efver said:
For filter caps Nichicon LKG 10000uf 63V
Click to expand...
Good choice.
Efver said:
I´ve read recommendations for 12000uf 100V filter caps as well. Would the amp benefit from an "infinite" increase in filter cap capacitance and voltage?
Click to expand...
Nope, specially voltages 100v cap on a 44v rail would not add anything, regarding the amount of capacitance there is a point of diminishing return, plus this little amps have a very simple power supply with no soft start circuit, so keep adding capacitance could be detrimental at some point due the inrush current. Homework---> Look on the internet how dual linear power supply works

Efver said:
https://www.mouser.se/ProductDetail...V2i03Pvz7eu30Uz9yLVjBMQ7vWr1tpa3oSGHnEttezA==
Click to expand...
Is overkill and it will work very good. I think I used one of those on one of mine.
.
 
Thanks for you reply, Hippocrates.

Great! I will go ahead and order those filter caps and the overkill bridge rectifier then. I will add resistors in the order from mouser, for those mods:

Preamp mod
2 x 3.3KΩ 1/4watt metal film resistors
2 x 1megΩ 1/4watt metal film
2 x 1kΩ 1/4watt metal film
2 x 2.2kΩ 1/4watt metal film
2 x 100Ω 1/4 watt metal film

Bass Eq mod
2 x 1meg (1/4watt metal films)
2 x 2.2kΩ (1/4watt metal films)

Any recommendations regarding resistor brands, series etc? Any gains to be made, sonically or other?
 
Great! I will order some of those then.

About the bridge rectifier; how did you mount it? It looks rather different from the stock one I believe. And it´s only one bridge rectifier on the au-999 right?
 
Efver said:
Great! I will order some of those then.

About the bridge rectifier; how did you mount it? It looks rather different from the stock one I believe. And it´s only one bridge rectifier on the au-999 right?
Click to expand...

The old 5B2 bridge rectifier used 2 screw to mount. just reuse 1 original screw and 1 old mounting hole for the 5B2 of your choosing. did that on my au888 and 777D.
20210227_102822.jpg

Peter
 
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Update: Bridge rectifier and filter caps done.
IMG_20210420_164451.jpg IMG-20210424-WA0005.jpeg IMG_20210424_195841.jpg

I'm doing the pre amp mod at the moment. However, I am a bit unsure about the trace side placement of the 1M resistor, does this look alright?

IMG_20210424_225352.jpg IMG_20210424_225326.jpg
 
OK, completed both mods today. Did some test listening in headphones to check that everything worked and noticed that there were some problemos with the bass eq in the left channel. It´s in constant -8 level(I presume), only to start working at +6. To be honest I´m not 100% sure this problem occured when fiddeling with the bass eq mod. Although I haven´t listened this critical before, there have always been some issues with the bass eq, as if the signal became weaker when engaging the "defeat 200hz 400hz" pot. It could very well be the same issue, or it occured today...

Any ideas? :(
 
Clean those switches (deOxit, 99% alcohol, CRC)... no change?
Check work done.
Change F-1209 sides to see if the problem follow
 
Clarification: Midrange and treble works in both channels.

Bass works right channel.

On the left channel bass eq kicks in only when turned up to +6, where it gets super bassy, probably equivalent to +12(or more).

I have cleaned switches and pots. I will try swapping those f-1209 boards and see what happens!
 
Switched 1209 boards with each other, issues with bass eq still in left channel.

Here is the bass pot.
IMG_20210427_182718.jpg IMG_20210427_182924.jpg
Are the two "wheels" liable for one channel each?
 
Efver said:
Are the two "wheels" liable for one channel each?
Click to expand...
Yupe, I would re-check the work done and compare the two sides looking for differences or a loose wire.
tracing the signal would be good, at least with the multimeter
 
The problem here is we don't know if this problem existed before the work was done.

Logic would have us assume there is a problem with the work you've done.
You are going to have to compare the side which is working correctly, against the side which is not working correctly.

I looked at the pictures, the work could be tidied up, there is melted insulation and so forth, you never know, something could be shorting out......

I can't see a magic bullet solution here, unfortunately its going to be the slow process of stepping through until you find the problem.
 
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