Sansui 1000 massive hum

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samgraysound

Well-known member
Joined
Jun 9, 2014
Messages
284
Location
Olympia, WA
Hi All,

I'm working on Sansui 1000 which has a hum problem. It is quiet but noticeable in the power amp, put bringing up the volume control (before the 1st preamp tube) makes it incredibly loud on both channels.

The whole unit was recapped and more within the last 5 years (not by me). There is very little B+ ripple, and I tried bypassing power filter caps to no effect. It could be grounding, but every ground point I've checked has been solid.

What else should I try?

Schem attached
 

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  • hfe_sansui_1000_service-8-8.pdf
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I looks like there's a stereo volume control VR10 that will ground the grid of the power amp output when fully CCW. Does it hum if this volume is full CCW (off)? Or just less? By how much?
 
samgraysound said:
What else should I try?
In addition to squarewave's tip, the usual troubleshooting procedure. Ground the grids till noise disappears. Process in the order of signal flow. Do not ground the grids of the output tubes nor the phase inverter.
 
squarewave said:
I looks like there's a stereo volume control VR10 that will ground the grid of the power amp output when fully CCW. Does it hum if this volume is full CCW (off)? Or just less? By how much?

That's the balance control.  I know it gets rid of the preamp hum when fully grounded, I will check if it effects the power amp hum as well.

Sam
 
abbey road d enfer said:
In addition to squarewave's tip, the usual troubleshooting procedure. Ground the grids till noise disappears. Process in the order of signal flow. Do not ground the grids of the output tubes nor the phase inverter.

will try today and report back.
 
abbey road d enfer said:
In addition to squarewave's tip, the usual troubleshooting procedure. Ground the grids till noise disappears. Process in the order of signal flow. Do not ground the grids of the output tubes nor the phase inverter.

Grounding the grid of V6a eliminates the loud (VR5 controled) hum for that channel, but so does the grid of V6b and V7a. The same is true on the other channel.
 
squarewave said:
I looks like there's a stereo volume control VR10 that will ground the grid of the power amp output when fully CCW. Does it hum if this volume is full CCW (off)? Or just less? By how much?

The balance control will turn off the preamp hum, but the power amp hum remains in the left channel.
 
samgraysound said:
Grounding the grid of V6a eliminates the loud (VR5 controled) hum for that channel, but so does the grid of V6b and V7a. The same is true on the other channel.
Ok, so then you know the noise is coming from upstream of VR5. So just keep going. If you terminate the input with 50R, does the hum go away?

If "yes", then the noise isn't even coming from the unit itself. It would be a basic ground loop issue in which case you need isolation transformer or possibly just a special cable if the source is balanced.

If "no", they missed a critical filter capacitor somewhere in the input (like the -17.5V on point T) or less possibly a part is broken in the preamp.
 
The power tube heaters seem to be connected directly to the wall-power side of the transformer. Similar schemes have been "acceptable" but are HIGHLY dependent on heater-cathode insulation. A high hum level could be insulation breakdown; and this could be dangerous.

It's a cheap trick to get the amp sold at a good price 50++ years ago. "All" these amps were discarded when transistors came out (this one has four already). That you are *still* using it means it isan't as throw-away as they intended. It is time to buy it a 24VAC heater transformer.
 
squarewave said:
Ok, so then you know the noise is coming from upstream of VR5. So just keep going. If you terminate the input with 50R, does the hum go away?

Nope. The last place that makes a difference is the wiper of VR5

squarewave said:
If "no", they missed a critical filter capacitor somewhere in the input (like the -17.5V on point T) or less possibly a part is broken in the preamp.

-17.5 on point T only is used by the transistors in the phono preamp, I am testing everything with the aux input so that shouldn't be in the mix at all.  If it was power supply ripple why would it be controlled by the volume control? There's no DC upstream of V6 at all that I can see.

I also replaced all the preamp coupling caps, the plate and cathode resistors, and the cathode cap on both preamp tubes. Also put in new preamp tubes.

I thought perhaps it was being induced inside the volume pot itself. It has the power switch on it, so the mains line is right there, but I desoldered the mains wires and pulled them away and it had no effect.

 
PRR said:
The power tube heaters seem to be connected directly to the wall-power side of the transformer. Similar schemes have been "acceptable" but are HIGHLY dependent on heater-cathode insulation. A high hum level could be insulation breakdown; and this could be dangerous.

It's a cheap trick to get the amp sold at a good price 50++ years ago. "All" these amps were discarded when transistors came out (this one has four already). That you are *still* using it means it isan't as throw-away as they intended. It is time to buy it a 24VAC heater transformer.

Would this cause the preamp hum I'm hearing though or just hum in the power amp?
 
samgraysound said:
Nope. The last place that makes a difference is the wiper of VR5

-17.5 on point T only is used by the transistors in the phono preamp, I am testing everything with the aux input so that shouldn't be in the mix at all.
If you terminate the aux input with 50R, does the hum go away?

If "yes", then the noise isn't even coming from the unit itself. It would be a basic ground loop issue in which case you need isolation transformer or possibly just a special cable if the source is balanced.
 
squarewave said:
If you terminate the aux input with 50R, does the hum go away?

If "yes", then the noise isn't even coming from the unit itself. It would be a basic ground loop issue in which case you need isolation transformer or possibly just a special cable if the source is balanced.

No, if I terminate the aux input with 50R the hum does not go away.
 
When I google "Sansui 1000" in Japanese, it finds an FM radio.
What is the exact model of the amp you are working on? Plus pic of front?

Just asking cos there's tons of information on repairing Sansui units (all in Japanese though), including most common problems, reasons for failure and even design flaws. Maybe I'm lucky and find something -- although I assume yours is not a model for the domestic Japanese market but an export model. Still something might show up. Just an offer.
 
samgraysound said:
No, if I terminate the aux input with 50R the hum does not go away.
Then I must have missed something because I thought you said that the hum goes away if VR5 is "off". Since AUX is directly connected to VR5 there is not much difference between VR5 off and AUX terminated.
 
Script said:
When I google "Sansui 1000" in Japanese, it finds an FM radio.
What is the exact model of the amp you are working on? Plus pic of front?

Just asking cos there's tons of information on repairing Sansui units (all in Japanese though), including most common problems, reasons for failure and even design flaws. Maybe I'm lucky and find something -- although I assume yours is not a model for the domestic Japanese market but an export model. Still something might show up. Just an offer.

Thanks for the offer! It’s this one: https://www.hifiengine.com/manual_library/sansui/1000.shtml
 
squarewave said:
Then I must have missed something because I thought you said that the hum goes away if VR5 is "off". Since AUX is directly connected to VR5 there is not much difference between VR5 off and AUX terminated.

Alas you are not missing anything. When the wiper of vr5 is CCW (at ground) hum is off. When the wiper is connected to ground via 50R hum is off. When top of VR5 is connected to ground via 50R, hum remains. When positive of Aux Jack is connected to ground via 50R, hum remains. It doesn’t make sense to me either.
 
samgraysound said:
When top of VR5 is connected to ground via 50R, hum remains.
Well that is pretty strange. That pot is pretty busy in that it's dual, has a tap for what appears to be some kind of filter and switches connected to the input. But if you just terminate the "top" of VR5 (no 50R, just ground it) and try different positions and switches what happens? If it's fully CW or fully CCW there should be no hum. Right? If it's in the center position, then the source impedance is going to be 100k or so in which case that first stage could be picking up heater hum (maybe because of the heater-cathode isolation issue PRR mentioned).

Check basic continuity of the pot with a meter. Maybe you have a bad solder joint. Try each position of the pot and sanity check the values. Maybe the wiper is loosing contact.

I was sort of leaning toward some kind of grounding problem created by a ground loop with another piece of gear. But this is not connected to any other gear at all right? You just have it connected to stand alone speakers? If you have any connection to another piece of gear then it's still possible that there could be a ground loop issue. Does the power chord have an earth pin? Is the chassis connected to it at the mains input?
 

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