AU-X1 Troubles

FireGuruMN

Active Member
I fired up (literally) my AU-X1 yesterday because I was feeling nostalgic. One channel was silent. So I went around to the back and found that on of the RCA connectors was loose. When I plugged it in I heard a distressing "sizzle" from deep inside the amplifier. The power/protection light started flashing. I turned it off. When I opened it up to investigate I found that two resistors on one of the driver boards were destroyed. See pictures. So I unsoldered the leads going to that board and pulled out the heat sink. I found R41 (47ohm, 1 watt) blown in half and R39 (560 ohm 1/2 watt) melted. I also noticed that R36 was modified into two resistors at some point in the amplifier's life. But that does not seem to be a problem. It would also seem that someone upgraded all the R41's to 1 watt from the original 1/2 watt values.

My question is: Do I just replace the two resistors and put it all back together or are the resistors indicative of a larger problem like a failed transistor or such? It would appear that transistors on this driver board have been replace previously in this unit since the colors do not match. I did not check part numbers on them. Is there an easy test for a transistor? Forgive my ignorance.

Also, can I "just" Deoxit (F5 for faders) the volume knob and two level knobs to get rid of scratchy noise or will that destroy what appears to be a very sophisticated potentiometer?

Anything else I should do while it is on the operating table?

Serial number 239050093 if that is relevant.20180126_194745.jpg20180126_194850.jpg
 

Attachments

  • 20180126_194745.jpg
    20180126_194745.jpg
    120.8 KB · Views: 6
  • 20180126_194850.jpg
    20180126_194850.jpg
    95.3 KB · Views: 4
My question is: Do I just replace the two resistors and put it all back together or are the resistors indicative of a larger problem like a failed transistor or such? It would appear that transistors on this driver board have been replace previously in this unit since the colors do not match. I did not check part numbers on them. Is there an easy test for a transistor? Forgive my ignorance.

Oh my goodness, where to start...

Firstly, with those resistors looking like that, I would be very surprised if you don't have at least blown driver transistors, and maybe some blown output transistors too. There is evidence of some makeshift (poor) repairs on that driver board other than the destroyed resistor. (i.e. 2 pairs of resistors 'standing up' next to the burnt resistors).

You appear to have a completely unrestored AU-X1 - with the infamous 'Black Flag' capacitors still in evidence. Anyone still using an AU-X1 in this condition can expect failures, particularly when changing / plugging in, input connections.

Your AU-X1 needs to be restored, and by someone who knows what they are doing, otherwise you will continue to have failures like this - each time destroying very hard to find output transistors upon which, the special sound quality of this amplifier is partly thought to depend. Lastly, this is definitely not a job for any kind of electronics novice.
 
Last edited:
Mate - don't even me started on how difficult this thing is to work on.

Like Hyperion says - get it to someone who definitely knows how to work on one (and can cite past experience with having done so).

At the very least, you need all those little blag vertical capacitors replaced (black flags) and you need to test all your outputs plus the pre-driver transistors in your output boards.

The outputs are now obsolete as metal TO3 types, and the pre-drivers on your driver boards are starting to go the same way.

Tricky tricky tricky.....
 
I feel for you. I don't even switch sources with power on. Recap, black flag removal, check all transistors and put resistors back to original.
 
Thanks all. Your comments were pretty much what I was expecting after reading what the internet had to say about servicing this amplifier. Do you have any recommendations for a qualified technician in the greater Minneapolis area? I suspect that even the diagnostics will be more than I want to fork out. Sadly I will probably have to just button this one up and put it on the "eye candy" shelf. Fortunately I have some others I can use.
upload_2018-2-2_7-17-8.png
When does it change from collecting to hoarding?
 
It's alive!
I read through all of the other AU-X1 restoration projects and got some great assistance from Bigdrive. And some NOS parts from Kale. Thank you both very much.

I don't think there is anything to be gained by writing up the whole story and filling AK with pictures but here is a bit of what I did:
Replace all black flags with C0G capacitors. Many black flags were out of spec or completely dead.
Installed the grounding mod on the control amp board.
Replaced R36,37,39,40,40,41 on both driver boards with 1% metal film resistors
Replaced TR15,16,10,11 on the driver board that blew up.
Installed MJ transistors on both heat sinks to get everything tweaked in.
Sprayed the amplifier pots and main volume pots with deoxit F5 a couple of times.
Cleaned the terminals on all of the plug connectors that I worked on.
Replaced ZD01 on the damaged driver board (someone had two 13volt diodes in series so I put in a 27v diode)
Replaced ZD01 on the undamaged board also. It was reading 25volts.
Replaced SC01. The previous device appeared to have legs crossed and was not testing out correctly.
Moved the white bias wire to the correct terminal on the transistor socket on both heat sinks.
Replaced 3 of the 4.7 ohm resistors on the transistor sockets. They were reading open after the transistors blew.
I pulled several capacitors on several boards and everything read in spec. I don't think this unit has seen much action in its life.
Adjusted the bias and DC offset to spec after things warned up.
Blew the power fuse once by not waiting for the caps to discharge. arg.
I pulled the phono boards because the fewer things to go wrong the better.
I played it for several days with FLAC files through a DAC. Sounded very good.
Today I swapped out the MJ's and installed the original Sankens on the good heat sink and the NOS replacements from Kale.
It sounds VERY good.
I am currently using a pair of Boston Acoustic A400's in my workshop. Once I have some run time on the Sankens I will button it up and bring it into the main room and see how the Polk RTI-A9's like it.

Of course it was not as simple as this repair list makes it sound. I had 9 pages of conversations with Bigdrive on the PM's. And I think I pulled the heat sinks out 5 times. The last time I hooked it up with alligator clips and everything worked perfectly. Murphy rules.

Mandatory picture of everything I have replaced:
upload_2018-3-28_14-35-56.png
I did hookup a turntable to see how that sounded. Not good. I have ordered replacement relays and trim pots all around. We will see if that helps. The bias on those boards is all over the place as they heat up. And boy, they do get hot.

With out a doubt my AU-X1 sounds very good. I have an AU-9500 that I also love. At some point it becomes a matter of personal taste but the AU-9500 has a richer sound to it.

I'm going do all the oscilloscope tests and make sure the oscillations are gone. And make sure the output matches the input with some various wave forms.

For my amusement and yours I came up with a play list to follow my AU-X1 saga. From my Pink Floyd folder:
--Sorrow
--On the Turning Away
--Learning to Fly
--Time
--Money
--High Hopes
--Round and Around
--Signs of Life
--A New Machine, Part 1
--Lost for Words
--A New Machine, Part 2
--Shine On You Crazy Diamond

Thanks to everyone who took the time to document their restorations on AK. Priceless. :bowdown:

I have lots of parts lists and pictures if anyone sees value in me posting them.

Time to damage my hearing a bit. . .:music:
 
someone had two 13volt diodes in series so I put in a 27v diode

Interesting - would that just mean that diode would only pass 13v?

You did some great work on this!

Theres one hell of a guitar sound on this track aye. Apparently Gilmour and co hired out an entire football stadium, rigged it up with tons of speakers, and hit "record." Magic.

I will need to rebuild my MC phono boards soon - what work did you do on those?
 
Diodes when placed in series are additive in nature, for example two 4148's in series will equate a vd1212 ( 0.56v + 0.56v), very handy with some of the oddball voltages the transistor era amp regulated power supplies use.
Nice rebuild Fire.
-Lee
 
Attached are the orders that I could find. There are many choices for soucing most of these components and some of my choices were based on availability. I did not include quantities as that seems to vary on different models. But here are my black flag counts:
Equalizer Boards F2770 and F2771
30pf times 2 boards
30pf times 2 boards
6pf times 2 boards
6pf times 2 boards
20pf times 2 boards
Flat Amp Board F2819
2pf on bottom of board
2pf on bottom of board
30pf
4pf schematic says 3pf
10pf
30pf
10pf
4pf schematic says 3pf
Driver Board F2275
22pf times 2 boards
3pf times 2 boards
1pf times 2 boards
1pf times 2 boards
33pf used 47pf as advised
33pf used 47pf as advised
3pf times 2 boards

I have my AU-X1 back on the bench to see if I can get the phono section working by replacing the relays. If not the relays then I am sure there will be other cans of worms to open.

I have a pair of W5M's filling in nicely but not as loudly.

I am occasionally hearing "crispies" in the undamaged channel until the AU-X1 warms up. I really have to try to hear them though. I'm thinking about replacing the predriver and driver transistors on that driver board to see if it goes away. But I so hate pulling those board out. . .
 

Attachments

  • Orders.pdf
    19.7 KB · Views: 32
We finally had a rainy weekend here in Minnesota so I was able to install the relays on the phono motherboard. I used Digikey G5V-2-DC24 (Z769-ND). As you can read in other posts the pins are not the same configuration so they have to be raised off the motherboard and a couple of then get swapped around. It is a royal pain to get all 8 pins back through the holes but if I can do it anyone can. I cut some leads off of resistors to extend the relay pins. I made tiny hooks in the pins and leads to get a good solder joint. I can send pictures if anyone thinks that is valuable.

Here are some other part numbers of multi-turn potentiometers to replace the original single turn ones.
3296P-1-101LF 100 ohm qty 6
3296P-1-501LF 500 ohm qty 2
3296P-1-102LF 1000 ohm qty 2
3296P-1-502LF 5000 ohm qty 2
3296P-1-202LF 2000 ohm qty 2
I might have ordered some extras so you can count on your own for your specific boards.
I have not installed these pots yet since things are working quite well right now.

I plugged in a Beogram 9000 for a test and both phono sections sound wonderful now. I don't have a MC turntable so I can't test that section properly.

When I put all the sheet metal back together one of the copper coils on the amplifier boards was sticking out too far and coming in contact with the bottom plate so I had to solder that back in to the amp better.

And for some reason my cover plate for the speaker relay and binding post section has vaporized. Or I never had it. It does not show up in any of my pictures. If anyone has one to spare please PM me. I really do not want to put the outside cover on the amp but it is rather dangerous leaving that section exposed.

I recently acquired a pair of Polk Audio SDA 1A's and they make nice sound together. Even before I recap the Polks.
 
Wow, that is quite a journey! Congrats on getting it going again and finding original outputs. After working on my AU-919 I can somewhat appreciate what you went through, though I gather the X1 is quite a bit harder.
 
Nice work man!!
Doing your homework, research, and careful methodical work approach wins the day!!

They are not the most forgiving amplifier to work on when things start going wrong thats for sure....
 
Thanks all. Your comments were pretty much what I was expecting after reading what the internet had to say about servicing this amplifier. Do you have any recommendations for a qualified technician in the greater Minneapolis area? I suspect that even the diagnostics will be more than I want to fork out. Sadly I will probably have to just button this one up and put it on the "eye candy" shelf. Fortunately I have some others I can use.
View attachment 1104093
When does it change from collecting to hoarding?
Now I know who's been buying all the gear in the twin cities!! Nice collection.
 
Back
Top Bottom