Replacement filter caps for AU-919

[lbcgav]

On the connector on the MM board I get ground on 110 and 109.

I tested R107 and R108...got 100 Ohms on both. TR107 and TR108 tested OK in my parts tester.



Here are the voltage readings on the drains of the FETs on the MC head amp:
FET01 10V
FET02 10V
FET13 -5.75V MAX. I can lower it, but can't increase with VR05
FET14 .03V No change by turning VR06

I pulled VR05 and VR06 and made sure they would change resistance when adjusting them. They both worked fine.

I did some C-E voltage measurements:
TR106 11.3V
TR108 -43.65V
TR105 10.18V
TR107 -15.39V

TR101 13.96V
TR102 14.77V
TR103 -19.14V
TR104 -42.10V

TR104 tested as a resistor in my parts tester. I replaced TR103 and TR104 with KSA992's. I then proceeded to test FET14 and started out with something around 14V and then it started to slowly go down back to the .03 I got before changing out TR103 and 104. I pulled TR104, which was just replaced with a KSA992 and it read damaged on my parts tester. Some good news, I can now get a solid 10V from FET13.

What could be damaging TR104? I was going to install KSA1220's and KSC2690's in place of the SB527's and SD357's, but I don't want to damage those, as I only have two of each.

 

[Hyperion]

Remove and test TR04 (F-2833) - replace if faulty.
Check the orientation of C104, C108 (F-2835) & C14 (F-2833) - if you can't see anything wrong - change TR104 and C104 together and try it again.

 

[lbcgav]

Remove and test TR04 (F-2833) - replace if faulty.
Check the orientation of C104, C108 (F-2835) & C14 (F-2833) - if you can't see anything wrong - change TR104 and C104 together and try it again.

I think I've got C12 in backwards. Would this cause TR104 to get damaged? Wow, what a rookie move!

 

[Hyperion]

Would this cause TR104 to get damaged?

Yes it would !

(sorry, through this latest hitch you've had, I have been referring sometimes to the circuit references for the wrong channel, it seems you were translating my advice for the affected channel very well. )

 

[lbcgav]

Yes it would !

(sorry, through this latest hitch you've had, I have been referring sometimes to the circuit references for the wrong channel, it seems you were translating my advice for the affected channel very well. )

That did it! Thank you John!
After replacing C12 (and C13 for symmetry's sake) with Panasonic FCs I had on hand, I pulled TR04 and TR03 on F-2833 just to be sure. Both of the transistors tested fine, so I put them back in. I fired it up and tested the voltage on FET14 and got -14V! I dialed in VR06 until I got -10V and we're off and running!

I've done about 12 to 14 amps/receivers so far, and this is the first time I've put in a cap backwards. I was sure it was a faulty component somewhere. I guess as the old saying goes, "Pride cometh before the Fall."

I just wanted to give a huge thanks to all who have made it possible for an electronic idiot such as myself to repair complex amplifiers like the venerable AU-919...in this instance, Leestereo and Hyperion especially.

 

[lbcgav]

My celebration may have been premature. I'm now starting to realize the DC offset on the MM board is swinging quite a bit. I can get it to around zero, and then it fluctuates by about 100mV in either direction. What is the most likely culprit for this fluctuation? I've replaced all the components Leestereo mentions in his wonderful guide.

 

[lbcgav]

After more testing, the 0V offset on the MM head amp is really bad. It starts out at around -500mV and then keeps creeping up past 100mV. There is no way to keep it near zero. This is the measurement off of R69 for the left channel. The right channel is a bit better.

 

[Hyperion]

My celebration may have been premature. I'm now starting to realize the DC offset on the MM board is swinging quite a bit. I can get it to around zero, and then it fluctuates by about 100mV in either direction. What is the most likely culprit for this fluctuation? I've replaced all the components Leestereo mentions in his wonderful guide.

Unfortunately this is normal - even EW himself could not find an answer. However, with the benefit of time I have unearthed a possible fix for this which I haven't had a chance to try on my AU-919, I'll give it a try as soon as I can, and will post up the 'how to' if it works.

With patience, and after a warm up, you can sometimes find a spot where it 'only' swings by about 150mV, sometimes a little better. Some AU-919 examples may not respond to this 'patience' treatment. It is apparently caused by input currents adversely affecting the differential pair.

 

[lbcgav]

Unfortunately this is normal - even EW himself could not find an answer. However, with the benefit of time I have unearthed a possible fix for this which I haven't had a chance to try on my AU-919, I'll give it a try as soon as I can, and will post up the 'how to' if it works.
With patience, and after a warm up, you can sometimes find a spot where it 'only' swings by about 150mV, sometimes a little better. Some AU-919 examples may not respond to this 'patience' treatment. It is apparently caused by input currents adversely affecting the differential pair.

I guess that's both good and bad news. I was hoping dial it in a bit tighter, but if that's the nature of the beast, then so be it. Everything else is within spec, so I guess it's now time to give her a test spin. Thanks again for all of your help!

 

[Leestereo]

The DC offset for the phono board is very sensitive to temperature; what may help is the use of a temporary cardboard cover (with cut-outs for the trimmer access and the DMM probes) during the adjustment procedure; let the temperature stabilize for ~10 minutes with cardboard cover in place prior to adjustment.

 

[lbcgav]

A couple more final questions (hopefully). I replaced R25-R28 on F-2835 with KSC2690AYS and KSA1220AYS. Should I put the originals back in? I didn't see any problems when they were tested, and I did not see any improvement after the new ones were put in. Also, would there be any benefit to replacing all the 2SC1845's and 2SA992's with KSA equivalents?

 

[Hyperion]

That's up to you on both questions, AK member scottrt did report quieter output noise when he changed all the driver board transistors of those type numbers with KSA replacements, but I didn't feel I needed to do it with my own, or the 2 other AU-919's that I reconditioned. YMMV

 

[ghamilton]

 

[lbcgav]

I've finally got it all buttoned up and running with my KEF Q100's. This thing sounds incredible. I have to say this sounds better than my accuphase E-202. It could just be expectation bias, but it sounds clearer with a better soundstage.

Here are some pics:

The face is in great shape except for a small nick on top:

After a little touch up with Birchwood Casey Super Black

 

[lbcgav]

The DC offset for the phono board is very sensitive to temperature; what may help is the use of a temporary cardboard cover (with cut-outs for the trimmer access and the DMM probes) during the adjustment procedure; let the temperature stabilize for ~10 minutes with cardboard cover in place prior to adjustment.

I used this little trick. Thanks for the suggestion.



The DC offset was still drifting, but not as badly as before.

 

[lbcgav]

Unfortunately this is normal - even EW himself could not find an answer. However, with the benefit of time I have unearthed a possible fix for this which I haven't had a chance to try on my AU-919, I'll give it a try as soon as I can, and will post up the 'how to' if it works.

With patience, and after a warm up, you can sometimes find a spot where it 'only' swings by about 150mV, sometimes a little better. Some AU-919 examples may not respond to this 'patience' treatment. It is apparently caused by input currents adversely affecting the differential pair.

I'd like to know what your fix is. I haven't used the phono amp yet, but the drifting DC offset is a concern for me, as unfounded as it may be.

 

[lbcgav]

Ok, I set up my turntable and have been using the MC input with my Denon DL-301 II. It sounds wonderful! I think the 330 ohm resistors in place of the original 33 ohm ones on the head amp board were a good call. I have a nice SUT, but I don't think I'll need it anymore.

 

[lbcgav]

I apologize for all the questions, but what type of speaker wire termination works best with the binding posts on this amp? I assume banana plugs don't work with them. I tried to insert some BFA style banana plugs but they wouldn't fit.

 

[Hyperion]

but what type of speaker wire termination works best with the binding posts on this amp?

I assume you have the stock OEM binding posts, and no, banana plugs don't work with them. I have found that stout single strand copper wire, or 'speaker pins' work best. I use QED multi stranded copper speaker wire - so I have soldered short (1") lengths of thick copper wire to them, seems to work well enough. I might upgrade the binding posts on mine, but there is other stuff to do first.

BTW: The MM Phono fix is the proposed addition of one capacitor and one resistor to each channel, just as was done to the AU-X1 MM Phono stage for the same reason, with excellent results.

 

[lbcgav]

I assume you have the stock OEM binding posts, and no, banana plugs don't work with them. I have found that stout single strand copper wire, or 'speaker pins' work best. I use QED multi stranded copper speaker wire - so I have soldered short (1") lengths of thick copper wire to them, seems to work well enough. I might upgrade the binding posts on mine, but there is other stuff to do first.

BTW: The MM Phono fix is the proposed addition of one capacitor and one resistor to each channel, just as was done to the AU-X1 MM Phono stage for the same reason, with excellent results.

Ah yes. I've got some thick wire at work I can use. Thanks again!

I'm looking forward to hearing more about your MM phono fix.