AU-777A fix-up

c.coyle

Fighting the Dunning-Kruger effect.
Here is 1970 Sansui AU-777A. It's in beautiful shape outside, but has mild-to-moderate smoking residue inside. Looks like the original owner was a smoker who took care of his equipment. Pots and switches are scratchy, but all the ins, outs, and controls work, and it sounds OK. A quick visual inspection reveals no obviously damaged components or bad solder joints. No glue on the boards.

This will be a winter project. First step is a thorough interior cleaning to get rid of the smoke residue, followed by cleaning the pots, switches, and connectors.

It has some cool bells and whistles: separate stepped tone controls for each channel (with defeat), balance check switch, stereo reverse, three mono options, selectable pickup load for phono 2, and mic input.

A little quirk: The loudness lever switch doesn’t detent. No clicks or stops, it just moves smoothly from one end to the other. Loudness comes on OK, but it doesn’t sound like variable loudness. The schematic just shows an off-on switch. I don’t know if the switch is defective or if it’s built that way.

And the pièce de résistance: An original fold-out schematic in an envelope on the bottom! The schematics on Hi-Fi Engine the AK database are pretty fuzzy, so I'll try to do a high-res scan and upload it.

These triple-digit black beauties are gorgeous.
 

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A few more pix
 

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Hi Chris,

Looks like it is in pretty good condition, nice pick up.

Re the loudness switch, it should feel the same as the other lever switches. From memory these switches have a spring - and that may have become dislodged or broken on the loudness switch.

Cheers

John

(PS- I posted this answer on the other forum as well)
 
Great score there. This amp is littered with 2sc458 signal trannies, 16 of them. Can be subbed with Ksc1845fta's for a lower noisefloor and a slightly cleaner more detailed sound.

While recapping, the main supply cap can be upgraded to a 10K which helps the amp breathe better as does upgrading the 2 sets of output caps. When time comes I can share my restoration notes for caps that perfectly fits the existing brackets.

It is an amazing amp, once restored. The best in my collection (havent restored the 999 yet)

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Nice Tom,you interior looks very clean compare to mine. I love the AU 777,very nice amplifier. Mine is with my cousin now. Good luck Coyle, it's a keeper.
 
Great score there. This amp is littered with 2sc458 signal trannies, 16 of them. Can be subbed with Ksc1845fta's for a lower noisefloor and a slightly cleaner more detailed sound.

While recapping, the main supply cap can be upgraded to a 10K which helps the amp breathe better as does upgrading the 2 sets of output caps. When time comes I can share my restoration notes for caps that perfectly fits the existing brackets.

It is an amazing amp, once restored. The best in my collection (havent restored the 999 yet)

Thanks, that's the plan. I'm thinking basically double all the big chassis caps. I've already started scoping out the big caps. I found a 10,000uf/100v/105c Chemicon KMH that's a perfect 40mm fit. Haven't gone beyond that. I bought two dozen KSC1845FTAs a few months ago and thought "these will last me a long time." :no:

With the bigger PS caps, I'll put a triac on the power switch and possibly go to a heftier rectifier.

It also looks like there are lots of 1.0uf and 3.3uf electrolytics which will probably be replaced by films.

But first, five or six boring hours with citrus degreaser, dish soap, DeOxit, and toothbrushes.
 
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Great score there. This amp is littered with 2sc458 signal trannies, 16 of them. Can be subbed with Ksc1845fta's for a lower noisefloor and a slightly cleaner more detailed sound.



X2 on this. I have used this sub in all my Sansui rebuilds including my CA-3k.

On the bigger power filter caps, some say it's best to upgrade the rectifier diodes. The larger caps take more "fill time" which can place a larger load on the rectifiers. I had zero issues in my 5000x when I went from 1500µf to 6800µf on the filter/ps caps.
 
Smoke removal underway. Right side is clean, left side still sludgy. Q-tips and Simple Green are doing the trick on the boards. Finish Line Citrus Degreaser on the chassis.

Just put in a Mouser order for the big caps, and the PS board (F-1153) caps, rectifiers, and diodes.
 

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Crowbar

While there are several threads on removing the AU-999A crowbar circuit, which is on its own board, I haven't found much at all on the AU-777A crowbar, a simpler circuit that is incorporated into the F-1153 power supply board. Surprisingly, the service manual has nothing on setting the SCR trigger with VR001.

So, while I plan on rebuilding the power supply, down to resistors, I'm inclined to leave the crowbar circuit alone.

Anybody have experience fooling around with the AU-777A crowbar circuit? Any thoughts?
 
Inside cleanup is finished. Clamp power supply electrolytics have been replaced with 105 degrees Chemicons. Main filter went from 4000/80v to 10,000/100v. Kept the others at same value, but gunched up the voltage.

Underneath, I put in a heftier bridge rectifier to handle the bigger inrush current.

The 5W resistors on the F-1153 power supply board, underneath, get too hot to touch. You can see heat discoloration on the underside of the chassis, right below C820, one of the pre-output caps. That board gets rebuilt next.
 

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Looks really nice. Would you happen to have come up with a suitable reccomendation for a rectifier upgrade on my 5000x? I rebuilt it all and like yourself I added capacity in the Power supply caps and fear the original rectifier might be holding back some performance? Thanks in advance, again really nice work. :)
 
I used a BR-108, Mouser 583-BR-108. 10A 800V. $1.40 U.S. I am not familiar with your unit, so I'm not going to recommend anything.

For my AU-777A:

56VAC out of the transformer secondary (with load) * 2.5 is the rule of thumb I was taught. So, at least a 140 volt bridge. I'm measuring 73 VDC into the filter cap. Schematic says it should be 70.

I also do at least 2.5 times the expected load current. Really, I was primarily looking for one that was going to fit and let me use the original hole. For the price, why not go big?

Question for you: Do the two 5 watt cement resistors on your F-1153 power supply board, R604 and R605, get too hot to touch? I'm thinking of moving them to the foil side of the board to get them a little further away from the big caps up top.
 
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So, while I plan on rebuilding the power supply, down to resistors, I'm inclined to leave the crowbar circuit alone.

Anybody have experience fooling around with the AU-777A crowbar circuit? Any thoughts?

Hi,

The crowbar circuit in the 777A cuts the power supply voltage to the input driver stage of the power amp in the case of an over current being detected in the output transistors. It is not the same as the crowbar circuit in the 999 where the speaker outputs are shorted if there is a high DC detected on either speaker output in a effort to blow the protection fuses before either the speakers or the output transistors are damaged.

I would leave it alone unless you experience nuisance tripping.

Cheers

John
 
Hi,

The crowbar circuit in the 777A cuts the power supply voltage to the input driver stage of the power amp in the case of an over current being detected in the output transistors. It is not the same as the crowbar circuit in the 999 where the speaker outputs are shorted if there is a high DC detected on either speaker output in a effort to blow the protection fuses before either the speakers or the output transistors are damaged.

I would leave it alone unless you experience nuisance tripping.

Cheers

John

Right. Although I think the crowbar is intended to detect overvoltage, not excessive current. If the voltage at the series regulator exceeds the set point (usually a few volts above rail voltage, set by the pot in this case), the SCR fires and the power fuse blows. At least that's how I see it.

But you are correct, and I will leave it alone.

I still don't know why the service manual says zero about it.
 
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Hi,

The crowbar circuit definitely monitors the current in the output stage via the voltage developed across the lower emitter resistor in each channel.

If it triggers, the SCR shorts the input power supply voltage going to the first stage in the main output drivers to ground via the protection lamp ( the lamp acts like a dim bulb tester) - it does not blow any power supply fuse.

Cheers

John
 
Been wondering about that protection, if it should go like on the 999, and what it does. Thanks for the explanation John.

P.s Resistors in mine gets really hot too.
 
Hi,

The crowbar circuit definitely monitors the current in the output stage via the voltage developed across the lower emitter resistor in each channel.

If it triggers, the SCR shorts the input power supply voltage going to the first stage in the main output drivers to ground via the protection lamp ( the lamp acts like a dim bulb tester) - it does not blow any power supply fuse.

Cheers

John

Yup. I should have read the schematic more carefully. Interesting circuit. I might have gone ahead and put a LED in there without thinking. Goes to show that a little bit of knowledge can be dangerous.
 
. . . P.s Resistors in mine gets really hot too.

I was thinking of moving them to the underside of the board, with long leads. That will get them a little farther away from the caps. Conrad H. just started a thread on the effect of heat on capacitors in tube equipment, and he suggests putting hot running resistors on remote terminal strips.
 
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Aside from the hot resistors, I find that the 777a suffers from inadequate heat sinking. I think this is one of the reasons the plastic plate housing the input jacks are cracked in mine. That whole section beneath the output grills gets quite warm. There is a decent amount of space internally - have wondered if there is a way to mount small heat sinks, either vertically on the plate where the pre-drivers are located or on the vertical plate of the outputs ? My bias is stable at 30mA

Good idea with the resistors - that heat sure doesn't help so close to the chassis.

By all means, very curious to hear your take on this amp - It has really surprised me how good it is after restoration. The kind of amp, I'll never sell and keep on tweaking for the better ;-)
 
Preliminary listening test

Aside from a clean up, all I've done so far is rebuild and beef up the power supply and check voltages and currents. I haven't touched anything else.

So this morning I hooked up some speakers and played some flacs for half an hour before work. All I can say is Holy Moly, this thing sounds like a million bucks! Big, open, 3D soundstage, individual instruments well-defined, slightly warmish, and plenty of slightly liquid, but not flabby or boomy, bass. I wasn't expecting this.

So I think I may be at the first critical decision point in my still newbie audio career: Do I recap or just stick with the 45 year old grey Elnas?

I'm going to listen to it for a week or two and try to compare it to my AU-555A and my direct coupled Kenwood.

I'm amazed.
 
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