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AU-719 on the bench

Hyperion

Hyperion

Roobarb & Custard
Subscriber
I have an AU-719 from an AK member for attention to various small problems - going in to protection now and again, noisy controls, the usual in other words. This will be my third one of these, so I don't expect any surprises. ;)

Check out the shrunken sleeve on the capacitor in the middle. ;)
IMG_3354a.jpg

Bias was set at 25mV (left) and 35mV (right). ;)
 
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People blame the shrinkage on heat but I've never been convinced. I've seen it on parts that are perfectly fine and have never been subjected to unusual heat, unless it was during shipping. The stuff is shrink tubing after all, so if it was a tad short to begin with it could easily move down over the years. It's even possible that, in a large batch of caps, you just get a few like that right from the factory. Or, some terrible thing could have happened that I've not thought of! Bottom line is, AFAIK, I've never tested a cap with short shrink and found a problem. That's not to say the unit isn't ready for a complete rebuild. :D
 
Zekeman said:
How does the sound of the 719 compare to that of the 717?
Click to expand...
They are very different, the richer sound of the AU-717, contrasting with the more clinical delivery of the AU-719.
ConradH said:
People blame the shrinkage on heat but I've never been convinced.
Click to expand...
I am coming round to that idea too, especially with this example, where no other signs of excess heat were seen. And as is my habit now, I test all the capacitors I remove, (ESR and value only), and the one with the shrunken sleeve was no different to its twin on the same board. ;)

And in other news:-
I have almost completed one driver board, and started on the second, my Mouser order seems stuck 'In Warehouse' - has been for the last 3 days. :(
 
Very nice! I like the 719 a lot. I thought it had a bit more power than the 717. As you mentioned it is a bit more clinical sounded, lacking that special magic trick the 717 does with that warm low end. I was very impressed when I finished a 719 restore, unfortunately it was not my unit and I had to give it back to the original owner. I told him, "you're going to hard to please after this one".

I remember when I first saw the 719 outputs, I thought someone had been in there before me and retrofitted them in there. They are tiny! From what I have seen it's pretty rare to see that case style in this era of gear. So much so that when I bought my 919 I figured the outputs would be the same case style and was very surprised when I looked in the grill vents to see they were not!

That solder error sticking out could be very bad, I know I've accidentally crossed traces with filter caps when the end of an RCA cable hit just the coated traces, similar to your photo. It when BOOM and I saw a huge blue flash. Luckily no major damage was done.

Excited to see this one progress, don't skimp us out on the photos!
 
Thanks John, - C615 - I know it well ;)

I have completed both Driver Boards, and also the Control Amp - that is, aside from the components I ordered from Mouser which unusually took 4 days to get moving in my direction - normally it is same day or the day after. I am stopped now, I really need those components to proceed - predicted to arrive on Thursday. ;)
 
I just received an order from Mouser and it took two days to get to shipping. Must be having covid issues... All in all, I can't say that I mind the short delay.
 
Here is the 'before' picture of the Left Channel driver assembly
IMG_3359a.jpg

And here is the 'After' picture.
IMG_3362a.jpg

Here is the 'before' picture of the Right Channel driver assembly
IMG_3357a.jpg

And here is the 'After' picture.
IMG_3363a.jpg

Component changes are as follows
C05, C08 - 4.7µF, 25V electrolytic, replaced with 4.7µF, 63V WIMA Film. (these were a very tight fit).
C09 - 33µF, 25V, replaced with 33µF, 35V Panasonic FC.
C15, C24 - 1µF 50V electrolytic, replaced with 1µF 63V WIMA Film.
C16, C17 - 470µF, 80V, replaced with 680µF 80V Nichicon PM.
VR01 - 100Ω (offset), replaced with 100Ω 25 turn Bourns.
VR02 - 470Ω (bias), replaced with 500Ω 25 turn Bourns.
ZD01 - 27V 1W zener, replaced with 27V 1W zener.
ZD02 - 13V 400mW zener, replaced with 13V 500mW zener.
C03, C10, C13, C18, C21 low value ceramic disk capacitors, replaced with same value Murata C0G/NPO capacitors.
R45, R46, 4.7Ω ½ Watt MOX resistors, replaced with 4.7Ω ½ Watt metal film resistors.

Notes:
All PCB joints 100% inspected with an eyeglass, individual suspect joints were remade. All thermal paste replaced with new. The square brass nuts on the driver transistor mounting screws have had one flat filed clean and were then soldered to the PCB.

Found one bias test point connection (on one driver assembly) was solder bridged to the middle (unconnected) pin, making it easier to short the + & - bias test points - the bridge has been removed. (with sharp eyes you can see this bridge in the picture in post #1) ;)
Found one of the screws that secures the driver & bias transistors to the main heatsink had stripped the thread in the aluminium bracket. Changed this to a machine screw, and added a 'Nyloc nut' to properly secure this thermal path.
 
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Here is a picture of the finished and reinstalled Control Amp. (sorry - no 'before' picture).

IMG_3364a.jpg

Component changes are as follows:-
C25, C26, C27, C28 - 10µF 50V, replaced with 10µF 63V Nichicon PW.
C10, C11 - 10µF 25V, replaced with 10µF, 25V Nichicon KL.
C17, C19 & C18, C20 each combination replaced with 4.7µF 63V WIMA film, with vacant position jumpered.
TR01, TR03, & TR02, TR04 - 2SC1313 each combination replaced with a matched pair of KSC1845.
 
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