Sansui 1000 hum? What does the HUM. Balance 100 ohm do?

[noise415]

Info for Sansui 1000 not the 1000a

Looking for advice on eliminating low level hum/buzz on right channel.

I’ve recapped it. Replaced the 6an8 and PL36 tubes with matched sets. I’m still getting a slight hum on right channel. I’ve swapped tubes but still getting a buzz. If I reverse channels with the mode selector on the faceplate and the hum travels to the other side.

Their is a right and left channel Hub Balance 100 ohm non-adjustable coil like thing. Labeled VR1 and VR2 in the parts list. Should that be replaced? And if so with what?

I replaced some resistors. When I test some of the resistors with my cheap multi meter the levels slowly rise to spec but new resistors are almost instant. Is that an issue?

I appreciate any advice. Thanks!

 

[EL504]

VR1 and VR2 are drawn as adjustable resistors in the schematic. So I advise you to take another look at them to see how you can adjust them (is there no hole in the middle in which a screwdriver would fit to adjust?).

To test if they are still OK, you could measure if the wiper still makes good contact with the windings of the resistor. According to the schematic the wipers of VR1 and VR2 are connected to a positive voltage of 23.5 V. If the wipers still make contact, you should be able to measure (close) to that 23.5 V between ground and either of the two leads going to the heaters of V6 and V7 (for VR2) and the same for V10 and V11 (for VR1).

 

[gadget73]

they're kind of hard to test in circuit honestly. The two ends of those pots go to the heater winding, so even if there is only connection to one side, it will read through the winding in the transformer and tube heaters. Only way to really get an accurate read would be to disconnect one end of the pot and ohm check it from that end to wiper and to the other end.

but they are indeed adjustable. Its a 100 ohm pot, probably a high wattage wirewound job. If they are in fact bad you can likely sub it with a pair of 100 ohm resistors and call it good enough. I have also done similar with pots that were open to one end. Tie the wiper to the open end with a resistor thats roughly half the value of the pot, and use the pot to tweak it in. IME these don't tend to make all that much difference unless bad but I've never worked on one of these either.

 

[noise415]

Thank you for the advice. Here is an image of it. The item in question are the 2 white coil looking things labeled Suzuki. Doesn't look like any kind of adjustable pot I've seen. Am I missing something?

 

[Mr Boochie]

Thank you for the advice. Here is an image of it. The item in question are the 2 white coil looking things labeled Suzuki. Doesn't look like any kind of adjustable pot I've seen. Am I missing something?

They are adjustable. You need to loosen the set screw holding the strap onto the wire-wound resistor body. Then, you slide the body through the holder to adjust the resistance. If you are going to do this, wear nitrile gloves, so you don't short out the mess.

I put the volume to low enough to hear the hum, then slide to lowest audible hum.

Believe me, those don't need to be replaced. They last forever.

Good luck! Be careful!

 

[noise415]

Oh wow I would have never guess that. Thank you! I'll give it a shot.

 

[Tom Bavis]

Nobody said easily adjustable... They can be adjusted by loosening the screw and moving the contact, then tightening again. I wouldn't do it while it's powered... you could try just centering both while it's off and see if it's better or worse.

 

[billyz]

You should not need to replace those. They are very robust. Measure the VAC on the outside tabs. Should be close to 6.3vac. The center tab should read 1/2 that voltage. If not loosen the center band and slide a little to one side or the other. But, I doubt that is your problem. Try swapping the preamp tubes from side to side one at a time to eliminate them as the source.
Are your power tubes very well balanced? They need to be balanced to cancel out hum.
There are many other things to cause the hum. Try swapping power tubes . If none of that helps your problem is elsewhere, even a wire out of place. Make sure the cover and bottom are on while listening. I had a scott that hummed with the bottom plate off but quiet when on.

 

[Mr Boochie]

Nobody said easily adjustable... They can be adjusted by loosening the screw and moving the contact, then tightening again. I wouldn't do it while it's powered... you could try just centering both while it's off and see if it's better or worse.

They are not that difficult to adjust. Just don't lose the screw!

Try to adjust with power off, first --- to get used to the feel of the adjustment.

Just be careful with power on.

 

[noise415]

Thanks again for the advice. I have adjusted the Hum balance. I'm guessing this adjustment is for more of a line level hum. This adjustment helped some. What I'm really trying to eliminate is a low level buzz around 190 Hz. Right channel only.
1. I can swap tubes buzz is still on right channel. I've swapped both power and pre. No change
2. Power tubes are nos matched sets.
3. buzzing sound follows when reversing channels with the mode controls
4. I read some adverts about low noise resistors. Could this be an issue? When testing some resistors are very slow to go to spec. Not sure is this is just my cheap multimeter or even an issue.

 

[trobbins]

noise415, perhaps return to square 1 first and confirm what your pot settings are. Afaik the Volume pot is VR5 on the service manual schematic - was that at min setting? Just prior to the output stages is the Balance pot VR10 - what happens when that is turned to each end ?

The Mode switch is S3 and after the Vol pot but before the Balance pot. You comment about the hum transferring from one speaker to the other when the stereo setting is in Reverse. What happens when the Mode switch is set to Monaural for Chan 1 and then Chan 2 ?

You indicate you tube swapped. Did you swap the 12AX7 tubes?

If there is hum in one channel with the Vol pot at min, then does the hum level change with Bass and Treble pot settings, as they are between the first 12AX7 triode and the second 12AX7 triode for each channel ?

 

[noise415]

noise415, perhaps return to square 1 first and confirm what your pot settings are. Afaik the Volume pot is VR5 on the service manual schematic - was that at min setting? Just prior to the output stages is the Balance pot VR10 - what happens when that is turned to each end ?

The Mode switch is S3 and after the Vol pot but before the Balance pot. You comment about the hum transferring from one speaker to the other when the stereo setting is in Reverse. What happens when the Mode switch is set to Monaural for Chan 1 and then Chan 2 ?

You indicate you tube swapped. Did you swap the 12AX7 tubes?

If there is hum in one channel with the Vol pot at min, then does the hum level change with Bass and Treble pot settings, as they are between the first 12AX7 triode and the second 12AX7 triode for each channel ?

T
Thanks trobbins,
1. with volume at min. I can hear slight buzz.
2. with balance to left, buzz is eliminated along with all sound form that channel
3. with mode set to monaural chan 1 buzzing is in both channels, with monaural chan 2 buzz is eliminated
4. swapping 12ax7 did not make a difference. but they appear to be the original tubes

I think I answered everything. thanks again!

 

[Mr Boochie]

Thanks again for the advice. I have adjusted the Hum balance. I'm guessing this adjustment is for more of a line level hum. This adjustment helped some. What I'm really trying to eliminate is a low level buzz around 190 Hz. Right channel only.

Yeah, those 100-ohm wire wound variable resistors are strictly for 60Hz AC hum. 190Hz (or prolly 180Hz) is power supply induced hum.

 

[trobbins]

Can you indicate how loud the buzz is with the vol pot at min relative to at max ?
With vol pot at min, does the buzz change in level with different Selector switch settings?
Can you hear any buzz with the 12AX7 removed from the noisy channel? (Also change Mode switch settings and Balance pot settings).

 

[noise415]

Can you indicate how loud the buzz is with the vol pot at min relative to at max ?
With vol pot at min, does the buzz change in level with different Selector switch settings?
Can you hear any buzz with the 12AX7 removed from the noisy channel? (Also change Mode switch settings and Balance pot settings).

Are you suggesting do a test with the 12AX7 pulled out? Thanks for the advice. Just wanted to verify in case I understood wrong and messed something up.

 

[trobbins]

Well done for asking for clarification. We don't know how experienced you are, and I wouldn't want you to poke your fingers into a live chassis if you haven't had sufficient training or oversight and done that activity before - to that matter you really need to tell people if some fault finding activity is new to you, as you wear the risk yourself. Even if it appears to be a benign risk by some to adjust the humdinger resistor, imho it is a risk at it appears to have 24Vdc on the tapping and if you noticed a tingle then you may react to that if you weren't aware.

Removing that 12AX7 only affects the loading on supply rail B8, which already has some load on it from the other 12AX7 in the other channel. Remove the 12AX7 when their is no power connected, and re-insert it when there is no power connected - there is no need to 'hot-swap' the valves, and unless you are fully aware of the consequences then that is a risk you shouldn't take imho. Removing that 12AX7 aims to narrow down the region of circuitry that is likely to contain a poor part or fault. It is always better to hone in on a portion of circuitry (or exclude portions of circuitry) if that is possible for starters.

 

[Mr Boochie]

Well done for asking for clarification. We don't know how experienced you are, and I wouldn't want you to poke your fingers into a live chassis if you haven't had sufficient training or oversight and done that activity before - to that matter you really need to tell people if some fault finding activity is new to you, as you wear the risk yourself. Even if it appears to be a benign risk by some to adjust the humdinger resistor, imho it is a risk at it appears to have 24Vdc on the tapping and if you noticed a tingle then you may react to that if you weren't aware.

Removing that 12AX7 only affects the loading on supply rail B8, which already has some load on it from the other 12AX7 in the other channel. Remove the 12AX7 when their is no power connected, and re-insert it when there is no power connected - there is no need to 'hot-swap' the valves, and unless you are fully aware of the consequences then that is a risk you shouldn't take imho. Removing that 12AX7 aims to narrow down the region of circuitry that is likely to contain a poor part or fault. It is always better to hone in on a portion of circuitry (or exclude portions of circuitry) if that is possible for starters.

Yeah. I'd turn off the unit, before pulling or inserting the tubes. It's a habit of mine.

 

[Tom Bavis]

Since it swaps channels with the MODE selector, it's upstream of the power amp and tone controls. That's very little... maybe a AC wire close to an input? Is it on all inputs?

 

[kvflyer]

Since it swaps channels with the MODE selector, it's upstream of the power amp and tone controls. That's very little... maybe a AC wire close to an input? Is it on all inputs?

For What It's Worth, I have to agree with Tom. It does not sound like a power supply or basic amplifier issue. More like wiring, repair or placement of wires.

One great thing about Stereo Integrated Amplifiers is that you have (often) the other channel to compare to.

 

[noise415]

Since it swaps channels with the MODE selector, it's upstream of the power amp and tone controls. That's very little... maybe a AC wire close to an input? Is it on all inputs?

Yes buzzing sound is on all inputs. Treble knob boosts/cuts the frequencies in the buzzing sound.