Sansui 5000A hum

[seljohs]

Hello all. First post.
This seems so similar to another old thread that I almost piggybacked on it.

http://www.diyaudio.com/forums/solid-state/239475-sansui-5000a-hum-help.html

My receiver has new caps and transistors (except tuner boards). Faint hum unrelated to volume in all inputs. To isolate I disconnected the signal wire from preamp to amp. If I disconnect at the amp it is silent. If I disconnect at the preamp the hum remains. But if I touch the chassis (while disconnected from preamp) the hum stops.

Looks like the signal wire is acting like an antenna for noise? Shouldn't the shielding prevent that?
Should I troubleshoot the signal wire or the noise coming from the chassis?
Maybe find a signal wire with better shielding and/or reroute the wire?
Something else?

John

 

[Welcome]

Is the cabinet completely closed when it hums? Some old receivers will hum when the top is off.

 

[AndrewT]

in an unbalanced amplifier input the screen is the signal return. It is not a true shield/screen.

If you break the signal input circuit by breaking either the flow route, or breaking the return route you stop the flow of signal around the circuit.

You are then left with an input with aerial wires connected. The amplifier will do it's best to amplify anything that gets onto your aerial wires.

 

[Ian Finch]

...Faint hum unrelated to volume....

You have two preamps here, the receiver and preamp, so that's two volume controls. What are the relative volume settings of each and which control are you using to say that hum is unrelated to setting?

 

[seljohs]

Thanks for the replies, guys. This is bringing me down!
It does hum with all covers on.
Ian, - the drawing shows the preamp board and amp board inside the receiver
Andrew the shield stops just before the wire reaches the preamp board - that's how its designed.

First pic shows the signal wires where they leave the preamp (roughly either side of the orange cap). Wires are blue and are bare for 1/2" before the shielding starts (blue and gray).

Second pic shows where the signal enters the amp driver board. In this one I left the shielding attached but have unsoldered the signal wire. No hum.

 

[AndrewT]

it must have a complete circuit for the input to work at all.
If the screen connection is broken, it should not work.
The amplifier must be finding an alternative route for the return to follow. The alternative route could be anywhere. And that is probably why the amplifier is amplifying the interference.

Find that alternative route and break it.
Connect the screen/return at both ends . One end takes both the hot and cold at the RCA/phono. The other end connects to the two adjacent pads/pins for the input signal.

 

[seljohs]

I've rebuilt a few stereos (Pioneer SX434, SX750, SX780, Sansui1000X) so not a total newbie but I'm still climbing the learning curve. That said, here is my understanding. This is a chassis grounded unit that employs braided cable from chassis to the main boards (like the one on the amp board picture). The audio signal passes from the preamp to the amp driver board through the single blue wire for each channel. This wire would be susceptible to picking up noise, so it is shielded. The shield should collect any noise and direct it harmlessly to ground to prevent hum. The shield is connected on one end only to prevent possible ground loop and resultant noise. So my understanding tells me that the hum should not occur - obviously I'm missing something.

 

[ririeiman]

I've rebuilt a few stereos (Pioneer SX434, SX750, SX780, Sansui1000X) so not a total newbie but I'm still climbing the learning curve. That said, here is my understanding. This is a chassis grounded unit that employs braided cable from chassis to the main boards (like the one on the amp board picture). The audio signal passes from the preamp to the amp driver board through the single blue wire for each channel. This wire would be susceptible to picking up noise, so it is shielded. The shield should collect any noise and direct it harmlessly to ground to prevent hum. The shield is connected on one end only to prevent possible ground loop and resultant noise. So my understanding tells me that the hum should not occur - obviously I'm missing something.

AndrewT allrady told you for exery audio signal you must have return signal , for unbalanced case , the shielding is the return signal . You must connect the shield too preamp and amp

 

[AndrewT]

The shield should collect any noise and direct it harmlessly to ground to prevent hum.

Complete balderdash !

 

[seljohs]

I swear I'm not making this up. I know wiki is not necessarily the best reference but...

"A shielded cable is an electrical cable of one or more insulated conductors enclosed by a common conductive layer. The shield may be composed of braided strands of copper (or other metal, such as aluminium), a non-braided spiral winding of copper tape, or a layer of conducting polymer. Usually this shield is covered with a jacket. The shield acts as a Faraday cage to reduce electrical noise from affecting the signals"

"The best method to wire shielded cables for screening is to ground the shield at both ends of the cable.[1] Traditionally there existed a rule of thumb to ground the shield at one end only to avoid ground loops."

Seems the Sansui follows the "ground only one end" so I'll try grounding both.

John

 

[AndrewT]

A coaxial connection for a signal is a TWO WIRE connection.
The core allows the signal current to FLOW to the Receiver.
The screen allows the signal current to RETURN to the Source.

You MUST NOT BREAK that flow and return circuit.

 

[seljohs]

You MUST NOT BREAK that flow and return circuit.

Andrew I'm not breaking any return circuits. I haven't changed anything. It came from the factory this way.

 

[seljohs]

Still trying again to solve this problem. Let me restate that the only things I've done is to recap and put in new transistors - no design changes. I think the hum is the fault of Sansui engineers. This receiver hummed before and after the recap. I have another unmolested Sansui 5000A that has the same hum. I've also seen other threads complaining about hum with this model. Hum is unaffected by volume and occurs at all inputs (even when shorted).

According to this writeup (Grounding and Shielding for your DIY Audio Projects) there should be "three separate grounds; the signal return, the power supply return, and the chassis." But here is the way this receiver is built...

• Signal return - through the chassis
• Power supply return - through the chassis
• Chassis - two wire plug with no ground. Mains are referenced to the chassis through a capacitor and 1M resistor.

There are chassis lugs all over the place. The rectifier board, main filter caps, driver boards, tone board, tuner boards all have their own chassis grounding lugs. After reading about proper grounding techniques, I'd suspect that hum is entering the signal path all over the place, and that the problem would be unfixable. But I did some tinkering this morning that gives me hope. This receiver doesn't have a rear-panel jumper between the preamp and amp sections, so I unsoldered the signal wire and made two tests.

1. Connected a source (iphone) to the aux input of the receiver, then ran the preamp output to a T-amp and speaker (see pic). Good sound with no hum.
2. Connected a source (iphone) directly to the amp driver board with speaker connected to the Sansui (see pic). Good sound with no hum.

So, even though the grounding scheme isn't optimal, the preamp and amp sections are individually noise free. The hum only appears when the two are connected. The Sansui design uses one wire (per channel) between the preamp and amp, and uses the chassis as the signal return. I'm wondering if the problem could be solved by somehow removing the chassis from the signal path and using a shielded cable (inner wire for signal, shield for return). I think this would require modification to the amp driver board in order to isolate the signal input from the ground plane (see pics of driver board schematic and layout). Maybe disconnect one leg of R802 from the ground plane and connect it to the signal cable shield?

John