Early AU-20000 restoration project - oh boy!

w1jim

I can fix it but good...
Subscriber
Not that I need more gear but I just put together a deal with s fellow AKer for an AU-20000 that needs repair.

First off, this is the earliest AU-20000 I've ever seen referenced - it could be the 4th one ever made with S/N 215100004 - Oct 1975.

He brought it to a local tech who said everything looked good except for the power supply PCB (F2508).

I don't have complete confidence in that conclusion since he was unable to power it up but so it goes.
The guy does have a huge amount of experience but is also a bit quirky - locals likely can guess who I'm referring to.

But the PS board certainly does need attention as the power resistors have melted their solder joints.

I plan on completely redoing that board and would like to install larger wattage power resistors off the board (those gold chassis mount resistors) to prevent heat damage.

Looking at the service manual R11 & R14 are listed as 150Ω/5watt but the actual board (and other pictures I've seen) have a 120Ω and 150Ω in series (for 270Ω total). Mine also has a 1200Ω/5watt Carbon resistor on the back side which results in 220Ω total value.

The next curiosity is that there are 3 diodes mounted on the back of the board - not sure what's going on there.

Due to my work schedule this project may be a bit drawn out (plus I first have to finish up a friends Marantz 2270).
I have fixed a AU-20000 before so I'm comfortable working on it and the modular design with boards that unplug will make this less of a challenge.

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That Sansui is certainly on my most wanted list and hard as hen's teeth to find. Should make for an interesting thread.
-Lee
 
Lol now you gonna make me go download the service manual and drag out the calculator for those 5watt dropping resistors The off board mount of the aluminum cased versions should do nicely, good thinking sir.
 
Thanks Giang!
Giang was the source of this beauty - now he'll have to go through the agony of watching me restore what was once his baby.
I'm sure she's better off with you making her sing agian! Good luck
 
He brought it to a local tech who said everything looked good except for the power supply PCB (F2508).

I don't have complete confidence in that conclusion since he was unable to power it up but so it goes.

Hi, I would caution you that perhaps the power supply problem is the xfmr. "since he was unable to power it up..."

I've heard of several of these having a bad xfmr. It just might be a factory problem. I have one in my shop that we spent months getting a new xfmr built for it. The new one is installed, we just have to bring it up, now.
 
Hi, I would caution you that perhaps the power supply problem is the xfmr. "since he was unable to power it up..."
Well, I hope not!
My remark about his being unable to power it up is in reference to the obvious defects on the PS regulator board.

We shall see - I'm ordering up the parts I don't have on hand - specifically the power resistors,some higher voltage caps and the 2 relays and I hope to begin this in about 2 weeks after an overseas business trip.
 
well, if you've got power, that is good. The xfmr we had that was bad had the two 120v primaries blown.
 
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While I'm waiting for the parts I removed the large power resistors and mounted with epoxy a connector for the remote mounted resistors.
Fortunately I have the proper tooling to terminate the matching connector.
 
Progress being made on the PS PCB between other projects and my work travel commitments.

There was heat damage on the PCB and when I was cleaning it it developed a crack which I had to epoxy back together. Traces were also damaged which I replicated with some 22ga Belden buss wire covered with teflon tubing.
I replaced all of the resistors and electrolytic capacitors - some of which had serious heat damage.
I replaced the VD-1212's with a pair of 1N4148 in series.
The transistors all tested good so hopefully I'll be good there.

I built a harness with 220 ohm/10 watt resistors. The resistors will mount somewhere nearby on the chassis.

Some additional guidance will be from this thread by Trnsfrmr:

http://audiokarma.org/forums/index....blems-and-must-fix-for-safe-operation.733713/

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I had problems with the power supply board also. It seemed to incredibly finicky, I would replace some part but a new cascading series of faults would appear. I finally checked and rechecked every TR and got it back together with no bad ones. Then... it was completely solid. I was worried about the zeners, but they never went bad.
 
After over half a year of my trying to get to my AU-20000 the day is finally coming for it to rise to the top.
Before I get into it I have seen reference to these oscillating and in fact I have a local friend who says his does just that after a few minutes of operation.
Any thoughts as to what might cause this and a possible fix?
 
Any thoughts as to what might cause this and a possible fix?

Check the Boucherot cells // Zobel networks on the output, I recommend changing the capacitors for better quality ones, check the resistors are ok.

I don't know this to be a definitive fix but it's a start. ;)
 
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Getting back into it!
Recapped and replaced the low power signal transistors in the F2507 driver boards.
These are tricky since not only do they plug in but they have 5 short wires each that go through a small rectangular hole to the back of the main PCB.
Once back in position I reinstalled the power supply regulator PCB and remote mounted the power resistors on the side panel. These were previously mounted on the PCB and are a major design flaw.


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Next are some of the smaller boards at the front of the amp. These required very little disassembly - basically plugging in.

On the F2501 EQ PCB C25 & C26 are bipolar 3.3uf capacitors that look like they were bypassed on the back of the board. Not sure what to make of that - for now they're installed and not bypassed.

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F2495 Tone Amp-

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F2505 Flat Amp-

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F2511 -Protection PCB.
Recapped and replaced the two relays.
The power (vs speaker) relay required that the PCB be modified slightly for the pins to fit.

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Starting to really have seller's remorse now! Nice work Jim! I knew you would take good care of her.
 
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