AU 717 Phono Hum

LBPete

Rolling Along
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I have an AU 717 that was re capped a while back. It's developed a pronounce 60 cycle hum at moderate to high volume when in phono mode with nothing plugged into the phono inputs. If you feed a signal into either aux or tuner, it bleeds over when in phono mode. Putting shorting plugs into the phono inputs, eliminates the hum and bleedover. The hum is not present at minimum volume and gets more pronounced as you increase volume,

It works normally and sounds great otherwise. I've gone over the input and EQ boards with a fine tooth comb checking grounds and don't see cold solder joints or cracked traces.

Anyone run into this?

- Pete
 
Could be a broken ribbon cable joint(s) to the rear panel phono connectors?
 
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I looked at the solder joints with a magnifying lamp and didn't see anything unusual. I'm going to start resoldering shielded cable grounds.

- Pete
 
Pete,
When I worked on my 919, I distinctly remember the RCA sockets (aux) seemed to short out the input signal when no plug was in the socket. I wonder if one of these types may be giving you a problem on the phono section input.
Mano
 
Played around with it a little more this morning. It's much more pronounced in the left channel. I disconnected the ribbon cable from the inputs on the back panel and the hum continued. So that seems to indicate the issue is on the EQ board.

Reconnected everything and ran a jumper from the power supply ground for the phono section right to the ground traces for the phono section on the EQ board. I jumped it at both ends of the phono section of the EQ board. Pin 25, the "I" connection from the power supply ground and at pin 5 of the ribbon cable from the back panel. No change.

Putting shorting plugs in just the left channel phono inputs quiets it down but if I bring my hand near the solder joints for the function switch on the EQ board increases the hum. You can play it like a Theremin by waving your hand over that section of the EQ board.

I have to think about this for awhile.

- Pete
 

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  • AU-717 EQ Board.pdf
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Mano,
If I feed a signal from a CD player into the AUX input, it bleeds over into the phono section when the mode selector is in the phono position with noting plugged into the phono inputs. I didn't check the aux input when I had the ribbon cable disconnected.

- Pete
 
This is still giving me fits. I've resoldered grounds and have it to the point that the hum is the same for both phono 1 and phono 2. If I put shorting plugs in either input, that input quiet but the input without the shorting plugs hums.

I don't have a turntable handy in the shop so I use a phono attenuator to feed a CD player into the phono inputs. I do not hear the hum when it's playing this way.

Now I'm wondering if the phono stage is picking up the 60 cycle hum from a bad chassis ground or rectifier diode. I see a sine wave on the scope if probe the chassis but I'm not sure that's significant.

- Pete
 
It's all sorted out and it was somewhat self inflicted. The AU-717 has a lot of internal covers that provide shielding. If you don't have them in place, you will get a hum in the phono section at higher volume. Also, you need to have all the connectors between boards connected. I missed one.

Does anyone have a source for those plastic rivets that hold the phenolic boards with the RCA jacks to the back panel?

- Pete
 
You're looking for a plastic push rivet, but that type with the head that matches the diameter of the inserted part appears to be harder to come by.

Here's a crapload of similar parts, but I didn't see any that match that original style plus not sure what diameter you need:

http://www.digikey.com/product-sear...ee=0&rohs=0&quantity=&ptm=0&fid=0&pageSize=25

Edit: here's some with caps in a range of sizes:

http://www.skiffy.com/docs/menufr.c...ina=inhoudfr.cfm&pag=skifprod.cfm&serie=094-1

I'll keep looking. I am pretty sure I've stashed some originals too, I never throw that kind of thing away if I part a component.


John
 
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Pete....for those rivets.....just search 'nylon rivet' on Ebay.........that is how I found the ones that I needed for the exact same thing as you.
 
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.135 dia. x .220 len. - http://www.mouser.com/ProductDetail...2pyFadug%2bkWUOonVuxijCjfs7URzGAt0iCr/%2b2Ao=

.135 dia. x .280 len. - http://www.mouser.com/ProductDetail...2pyFadug%2bkWUOonVuxiMb%2bxkIYqfzk8UzPV3QDA8=

Forget which one is proper for the 717 back panel. Ought to be easy to determine.

Edit: All Products > Eagle Plastic Devices > Electromechanical > Hardware > Screws & Fasteners - Search for 'Snap Rivet' and 'Snap Rivets' and you'll see a large selection.

For what it's worth, Mouser only stocks the longer ones.

- Pete
 
The 717 uses the shorter ones. I've been scrounging plastic washers to shim the backs of the long rivets.

- Pete
 
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