Script

Re: Sansui 1000 massive hum
« Reply #20 on: August 10, 2018, 02:15:01 AM »
Meanwhile I found this page (English). Mentions hum in one channel coming from 6AN8 driver valve.
http://www.thermionic.org/sansui1k.html


abbey road d enfer

Re: Sansui 1000 massive hum
« Reply #21 on: August 10, 2018, 02:41:47 AM »
When top of VR5 is connected to ground via 50R, hum remains.
Whatever the position of VR5? Hum should decrease with VR5 fully CCW... It suggests hum is injected in V6a's grid. Weird...
Does hum change when reversing the mains plug?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

Script

Re: Sansui 1000 massive hum
« Reply #22 on: August 10, 2018, 06:03:41 AM »
Unit is from 1968 or so. All carbon resistors?
Read that they age by slowly increasing resistance.
Better replace them with metal film (?)

abbey road d enfer

Re: Sansui 1000 massive hum
« Reply #23 on: August 10, 2018, 07:15:05 AM »
Read that they age by slowly increasing resistance.
That can be easily checked out; it doesn't explain hum, though.

Quote
Better replace them with metal film (?)
Changes in resistance value are rarely significant. However, carbon comp resistors can be noisy. But they're not everywhere; notice how some resistors (particularly in the phono preamp) are marked with a sign that means "noiseless".
« Last Edit: August 10, 2018, 11:35:34 AM by abbey road d enfer »
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

samgraysound

Re: Sansui 1000 massive hum
« Reply #24 on: August 10, 2018, 06:18:49 PM »
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?

When I ground the top of VR5: hum is mostly gone with pot all the way CCW or all the way CW. Inbetween it is quieter overall then without that point grounded and also loses a couple of its lowest octaves of hum. Volume of hum increases as pot moves CW until it reaches all the way CW then cuts out. Loudness and Tape monitor switches have no effect.

Curiously when I ground the input socket, it acts differently: hum is mostly gone all the way CCW  BUT NOT for all the way CW.

Pot continuity checks:
463k end to end
463k top to wiper all the way CCW
98ohm top to wiper all the way CW
no gaps in wiper connection
Before getting on the forum, I did remove this pot, disassemble it, clean it, and lubricate it.

It is not connected to any other piece of gear. I am listening to it on headphones via the headphone output jack.

It has a 2 pin power cord. The chassis ground is not connected to the power cord at all.
« Last Edit: August 10, 2018, 06:22:42 PM by samgraysound »
Sam Gray
Owner & Technician, Sound Audio Repair
www.soundaudiorepair.com

Recording Engineer, www.samgraysound.com

samgraysound

Re: Sansui 1000 massive hum
« Reply #25 on: August 10, 2018, 06:20:03 PM »
Meanwhile I found this page (English). Mentions hum in one channel coming from 6AN8 driver valve.
http://www.thermionic.org/sansui1k.html

This is not the same unit. My unit has 25E5 output tubes, different chassis arrangement.
Sam Gray
Owner & Technician, Sound Audio Repair
www.soundaudiorepair.com

Recording Engineer, www.samgraysound.com

samgraysound

Re: Sansui 1000 massive hum
« Reply #26 on: August 10, 2018, 06:20:44 PM »
Whatever the position of VR5? Hum should decrease with VR5 fully CCW... It suggests hum is injected in V6a's grid. Weird...
Does hum change when reversing the mains plug?

Hum does not change when reversing mains plug. Hum does decrease with CR5 fully CCW.
Sam Gray
Owner & Technician, Sound Audio Repair
www.soundaudiorepair.com

Recording Engineer, www.samgraysound.com

abbey road d enfer

Re: Sansui 1000 massive hum
« Reply #27 on: August 10, 2018, 07:49:10 PM »
Hum does not change when reversing mains plug. Hum does decrease with CR5 fully CCW.
Does hum change when pressing the Tape Monitor switch?
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

squarewave

Re: Sansui 1000 massive hum
« Reply #28 on: August 10, 2018, 09:10:39 PM »
If you put "full scale" signal in, is it loud relative to the hum? Maybe the hum isn't that bad but you're listinging to it at max with headphones and so the hum just sounds bad. But if you were to play music at that level it would be super loud?

My best guess at this point would be that you have a shaky ground connection somewhere. Check resistance between the PS signal ground and various points around that pot VR5 and measure up stream from there finding whatever ground points that you can. Press on the solder joint / board when you do it and notice any jumps in resistance. Resistance should be 0 but of course compare this to whatever your DMM reads just holding the leads together.

If you have a spectrum analyzer (like a sound card and software like ymec dssf3) characterize the noise. Do an SNR type of measurement by playing 0dB sine wave (2.19Vpp) in and adjusting the gain / output so that the spectrum analyzer software shows the signal peak at "0dB". Then compare that to VR5 full CCW which is just the noise. If you have a huge spike at 120Hz -40dB, that's bad. At -70dB, that's not so bad.

Is the chasis metal? Try grounding it to earth just for kicks. Maybe with the top on it makes a difference? Tube circuits are pretty high impedance so they tend to pickup stray crap like the AC unit just outside the window next to your workbench!

samgraysound

Re: Sansui 1000 massive hum
« Reply #29 on: August 13, 2018, 02:04:12 PM »
Heading out of town for week, but I will try some of these suggestion next week.

Sam
Sam Gray
Owner & Technician, Sound Audio Repair
www.soundaudiorepair.com

Recording Engineer, www.samgraysound.com


 

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