Sansui au 7700 rca plugs shorted on the left side only??? Help!!

MrMonster

Totally Lost In The 70's
Hello Lads,
Hope you can help me with a problem
I have a Sansui AU 7700 integrated amp I picked up at a yard sale two years ago, Gave it the typical cleaning of an interior dusting and all switches cleaned and lubed, and ran great until a month ago the left channel started going out, checked everything when I found that a little wiggle to the RCa cable brought it back in. I replaced the cable and used it as a tuner and CD player in my office. I brought it down stairs to use in the kitchen for the holidays and the left channel RCA problem started up again, Change cables no good, wiggle cable it worked.
So I tested all the RCA inputs, and by God they are all short on the left channel. Again, if I hold the cable jack to one side it works and of course let go of cable and no left side.
I have no loose wires to the board that I can find, so I am assuming this is on the board itself? and it looks like a bugger to get to.

Any ideas or suggestions? is there a history of this problem with these rear panel that slope to a horizontal input?

Thanks,

Donald Bowman
 
RCA Problem

Hello,
I guess I should not have said "test" it was basically hooking one channel of my tuner to every left RCA input, and getting nothing till I applied some sideways pressure to the jacks. From Phono to Tape inputs, all left channels don't respond without some tweaking of the cable jack.

Donald Bowman
 
By the way, I love the R. Crumb avatar. I am an old, old, fan of his work.
All the way back to Zap comics when they were new.

Donald bowman
 
How corroded are they? If they are corroded or oxidized, they won't make good contact. Try cleaning them.

R. Crumb is alive and well. He has just published an illustrated book of Genesis.

- Pete
 
I cleaned all the exterior contacts with emery paper and contact cleaner in the holes with a pipe brush.
All right inputs worked fine even before cleaning, just that the left inputs are in trouble all of them.. Like I said I think this is something on the board if none of the Left inputs are making contact without adding a sideways pressure to the jack.
Thanks

Donald Bowman
 
They are not "shorted," they are open. A big difference electrically. You probably have a solder joint or cracked board. Can say exactly without looking. Have you taken the cover off and looked at the back side of the plate the RCA jacks are on?

- Pete
 
Thanks,
Open is the term I was looking for,
I am in the process of getthing the board exposed, going is slowww, on this as I have to take so much apart to get a look at that board.
No cracks that I can see so far, and it looks like only two wires and a ground that actually go to the RCA portion of that board.

Donald Bowman
 
Electricity 101. A short means there is stuff touching that shouldn't be, short circuit. Open means there is stuff that should be touching but isn't, open circuit.

- Pete
 
I don't really understand what you mean by "open" on the left channel. Try this: Some input jacks have a feature in which the input is shorted to ground until a cable is inserted. When the unit is off, insert an RCA cable into the jack. At the cable's other, unplugged end test the resistance between the center pin and the ground "tube" surrounding it. It should read very high, like 100K. Is that the same on both channels, even when wiggled? Test it on the source inputs and on the power amp input, if the amp has one. It seems odd that it would afflict several inputs, which is what it sounded like.

It could also be a switch such as the "connected/separated" switch at the rear of the unit.

It does sound as though you have a case of bad jack, or bad solder joint on the board. I think there is a guy on this board or another called "wirewiggler." I often feel this way when troubleshooting problems like this. :)
 
I'm taking "open" to mean the jack is not connected electrically to the corresponding board. He plugs in a source and the signal doesn't get past the jack. Wiggling it sometimes causes it to make contact.

- Pete
 
I see that the connection you mean is the signal part of the jack's joint to the corresponding trace on the board. Sometimes I test my solder joints by testing the resistance from various leads that have been soldered into a given board trace-not touching the solder itself but the leads. It would seem worth removing the existing solder and resoldering those joints (then testing them) while you have the board extracted. Hopefully the jack isn't bad.
 
"Open" Iput Jacks

Here again is the problem,
what would cause every input jack on the left side only to be open? All of them on left side only, is this a circuit board fault, wiring fault, or are the negative sides of the jacks, series wired to a single ground?
like Xmas tree lights, where one goes bad they all go out?

Because of the design of this amp, I can not get to the inputs without taking apart half the amp? Any tricks to getting this board out to test, or is there an in cabinet way to test the RCA line on that board?

Thanks,
Donald
 
If all of them are bad on the left side, it may not be the jacks. Rather, there may be a problem with the wire or wires that take the signal from the jacks' board to the various amp sections. This is complicated if there is a phono section, since it would be different from the path an AUX or TUNER signal would take.

There could also be a problem with the power amp input, like corrosion somewhere. Lots of opportunities for problems, such as connected/separated switch, board interconnections inside the amp (with plastic-bodied connectors), the input jacks to the power amp, if it uses jumpers.

Thinking about it differently, if you're testing it with the same source on all the various jacks, wiggling the interconnect RCA could reveal that you have a problem on the output jack of the source component.

In sum you need to find out if the preamp or power amp is the problem. If you can separate the two, and test using another amp's preamp with this amp's power amp (for example) that might help isolate the issue so you're not struggling to get a look at things that might not even be possible issues.
 
Hello Lads,
If anyone is still watching, I got the input board out, no cracks, no hot spots and no loose wires.
One thing though, and it's odd I never noticed it before, but the channels are reversed on this amp. If you plug a component into the right jack, the signal comes out on the left speaker output and vice versa, I never noticed as I never needed to adjust the balance before, as I used the amp in my studio for FM radio and cassettes.
My guess there is some prior work done here that less than ideal.

Still, after checking the RCA input board I can not see what would cause one input side to be open until some side pressure is applied to the RCA plug/jack.

I have a feeling this crossed over input output situation is to blame, but I will need a schematic to see the path of the inputs to wherever they lead.

Any suggestions would be appreciated, as I would like to keep this amp working.

Thanks,
Donald bowman
 
Hello Lads,
Forget the Left Right channel business, I found that the last person in here swapped the out put connections, so that is now corrected, but no effect on my problem.
I tried other components just to be sure, and oddly came up with the following,

Bad = Left side open Good= Both channels good

Phono 1 bad
Phono 2 good
Tuner bad
Aux 1 bad
Aux 2 good
Tape 1 bad
Tape 2 both channels bad

In going down this list, with Phono 2 and Aux 2 both good,I am beginning to think that the positive connection on these plugs have bad joints from their use over time and someone really doing a Gorilla on inserting and removin cables.

I guess I will try hitting each input with an iron and a touch of fresh solder and see what happens.

I paid a whole $10.00 for this two years ago, so you'd think for that kind of money it would last a little longer than 40 years. HA!

Donald bowman
 
Hello again for the last time.
I am probably talking to myself at this point, but I finally solved my problem.

I finally wound up having to damn near dismantling the entire amp to get the input board exposed to work on it. Due to to the design of placing the inputs on a horizontal plane the board is tied to the rear of the case above the heat sinks for the transistors, and again by three brackets two on the corners and one inside in the middle of the case, not to mention the long extension rods from selector knobs to the actual switches.

Carefully looking over the board with my visors and strong lamp and checking the connection of the wires from the input board to the amp with my DMM, there was nothing wrong! continuity across the RCA plugs were all good, so I was completely baffled.



I figure the original owner must have taken care of the amp just by the fact he spent so much money on it in the day, and made the choice he did in a time when there was a lot of great audio gear to choose from.
Subsequent owners probably had less affection and care for this amp as the dollar amount paid for it decreased over time.

The final owners before me, more than likely yanked out the RCA cables by pulling on the cords instead of at the jack. If it was in a cabinet without much access it would mean the left jack would have been pulled at severe angle to remove it, as the Left input is tucked in close to the back of the amp where more pressure would be applied to it when removing or installing components.

I installed RCA cables into the inputs and watched them go in from the back of the board. Sure enough, as the positive pin goes in, I can see that the solder joint has separated from the board. It read good on my DMM as there was no pressure on it from the plug so it was making contact with the board, which is also why when I wiggled the plug it would make the circuit.
I have a lot of soldering ahead of me, and then I will be back to listening to music.
Oddly, even with the hard handling, the face is near pristine, and only a few marks to the cabinet.

That is it then.
Donald Bowman
 
Whoa great to see this thread alive again. The boards' traces may have separated from the board itself, too. I couldn't tell from what you said. Now that you've got it out you have the opportunity to really fix it well. Some of those fancy RCA cables have really too much spring pressure on the jacks. Could've contributed at some point down the line. Anyway I hope you are able to get those joints reliable again. Glad to hear that you're most of the way there.

P.S. I don't know if this practice is approved by many around here, but when I have a trace coming off a board, I bypass it with a custom-bent piece of bare solid wire (old clipped leads are great) directly over the old trace. I loop it around where the component leads come through on either end of the old trace. I had some really bad ones that required desperation. I didn't have much success re-glueing the trace back down. I then solder components directly to the wire, which holds itself in place by bracing the old trace on either end. Just my 2 cents there, surely there are better ways to do this. You may not even require it.
 
Mister Monster, glad to hear you found it. Based on your description, it had to be something like that.

- Pete
 
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