Sansui 2000x: hum, and now distorted output

tubmyk2

tubmyk2

AK Subscriber
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**Full disclosure: earlier in the week I had installed one of the filter caps backwards and I had no audible issues other than I glanced down and realized it was bulging and I shut it off. Replaced the cap yesterday and all is back to normal but I don't know for sure if the hum I hear was there prior, as my shop always has some risidual noise.**

So I have a Sansui 2000x that got the full restoration treatment on everything except the FM/AM circuits: new caps, new transistors, the Conrad Mods... everything is working and the unit sounds really nice. However, I am now noticing this very slight hum coming through my shop / bench speakers even with the volume down all the way. I was hearing what sounded like grounding issues also at first, but after removing and reattaching wires to the tone board and cleaning up the connections a bit, that issue seemed to disappear.

Then I noticed a hum with a buzz and the buzz was most detectable from the right speaker. Turned out the cone was rubbing :oops: (these were a recent Goodwill find) so I stopped screwing around and put everything back on my bench the way it was tested originally. The stupid buzz is gone, but the hum is still there. And while it's extremely low, it's definitely there.

I've read some threads on this, and I've also seen the ECO from the 2000A that sometimes isn't applied to the 2000x, but mine was right (grounding between filter caps). The big question I have is if when I put that cap in reverse, I caused damage somewhere else and I'm pretty sure if I remove the jumpers, the hum will go away. I know the static / grounding issue did so I started tracing from there.

Double-checked that all parts are in the right orientation and again the unit sounds really nice other than this light hum when it's quiet. I'm also going to re-visit the EQ board as the phono channels are extremely hissy even after the parts replacement.
 
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I had my 2000X gone through by an AK member many years ago and afterwards there was still a very low level hum that I was told could not be completely eliminated.
 
It's interesting because the bass tone control actually affects the strength / depth of the hum. I kinda found it to be about 120Hz.
 
None of these should hum. I don’t have a solution if they’ve already been recapped, but they certainly shouldn’t do it.
 
I seem to remember some modifications to the ground scheme being discussed by AK member ConradH for the model 2000 - not sure which variant it applied to (A,X etc), maybe 'all' as it is my understanding that they are very similar. Not sure if he discussed the modifications here on AK or on his own website.
Here is the URL
https://conradhoffman.com/Sansui2000x_mods.htm

To save you searching, here is what he says about hum/noise
  • A small amount of background hum on the headphone output with higher impedance headphones (Koss Pro4AA @ 275 ohms DC) is normal for the 2000x. This can barely be detected with lower impedance headphones. There is zero hum from the power amp alone, so it probably gets picked up on the tone board or is possibly related to how the headphone jack is grounded. Be sure the service bulletin ground mod has been done on the 2000a.
So, maybe I was wrong about a separate discussion for the grounding scheme, I hope the above helps.

Final thought...
It may have a bit of hum with the covers off, if that's when you are hearing the hum, try putting the covers on? ;)
 
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Hyperion said:
Final thought...
It may have a bit of hum with the covers off, if that's when you are hearing the hum, try putting the covers on? ;)
Click to expand...

yes it’s humming ever so slightly with the covers all on. I went through and completely retouched all the solder joints on the EQ board, and pulled the wiring / cleaned the pads and resoldered. Just for peace of mind but that made no difference. I’ll have to bust out the scope and see if it tells me anything helpful.
 
Here’s a question… is it possible when I reversed one of the filter caps, I did any residual damage to the others? I’ll check the schematic again (not that we are to trust Sansui schematics) but I didn’t think they were directly in line with each other, only that they share the same ground points.

I put the unit in my living room to finally sit and enjoy but since I can’t un-hear the hum I’m sure I’m going to back in and see what else I can do. Any and all music will mask it, I’m sure. It’s not that loud, just more annoyed that I don’t know if it was there all along because I work with a fan on all the time!
 
How loud/severe is the hum? It is my understanding, perhaps incorrect, that these units do not run dead quiet. However, it is also my understanding Hyperion knows a lot more about Sansui than I do, so if he does't think a hum should be there he is more likely right.
 
Guido57 said:
How loud/severe is the hum? It is my understanding, perhaps incorrect, that these units do not run dead quiet. However, it is also my understanding Hyperion knows a lot more about Sansui than I do, so if he does't think a hum should be there he is more likely right.
Click to expand...

Well, as it turns out, easy enough to hear from where I’m sitting about 10 ft alway.
 
A couple of new findings:

So the hum that is present is 120 cycle. When I engage the Low Filter button, I also bring in 60 cycle hum. The cap that I originally installed backwards for a few minutes was C822, which has been replaced, but that runs to the power amp board. The voltages to the power amp board look correct at least based on the schematic.
 
Guido57 said:
However, it is also my understanding Hyperion knows a lot more about Sansui than I do, so if he doesn't think a hum should be there he is more likely right.
Click to expand...
Many thanks, but I would defer to those more experienced than me in the model 2000 and their variants like 'ConradH' and 'EastPoint'. ;)

Installing one of the PSU capacitors reversed would have cause high current through the capacitor, and could have damaged other components, you did the right thing to fit a new replacement. The rectifiers would be prime suspects for being damaged, but it would be very rare for them to be 'slightly damaged' as they usually go 'open' or 'shorted'. So, in this case they are unlikely to be responsible for the hum you are describing.

For information, the 120Hz hum will almost certainly be 'rectification noise', and 60Hz hum will be 'mains pickup'. Rectification noise comes from the rectifiers in the 2000 as they conduct in the forward direction, and then get shut off by reverse voltage. The point where this 'forward-reverse' transition occurs can be poorly handled by the rectifiers resulting in 120Hz switching noise. Mains pickup can be from direct radiation within or from outside the 2000 case, or from resistive (or open), ground connections at the input connectors or elsewhere in the 2000.

(Note that nearby LED desk lamps and other electronic devices can emit high levels of interference and noise which is often confused with internally generated noise/hum).
 
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I worked on a 2000A a while back. It took a while but it is dead quiet. Have you removed the preamp to amp jumpers on the back panel? If the hum persists you know it is on the amp board or the output. One thing that helped quiet my unit was shorting plugs for the unused RCA inputs. In my case, shorting plugs on the second, unused phono input was a big step in quieting the receiver.
 
Thanks.
Yeah, I already tested different outlets (on a different circuit), made sure the overhead LED shop lights and fluorescents weren't affecting it. I moved the receiver upstairs to my main area and it was noticeable on my living room setup, so it's definitely in the receiver.

I also noticed that the 2000a engineering change order for the ground point to the 1000uf cap had actually not been done on my 2000x. I was looking at the diagram wrong. I'm going to change that for whatever it is worth.

Also, and this is where I'm not sure it makes any difference or not, but since I upped the value of the 3 main filter capacitors to 2700uf (from 2200uf), is there any reason to change the value of the resistors that connect 2 of them to ground? They are 330 ohm.
 
jerrysstereo said:
I worked on a 2000A a while back. It took a while but it is dead quiet. Have you removed the preamp to amp jumpers on the back panel? If the hum persists you know it is on the amp board or the output. One thing that helped quiet my unit was shorting plugs for the unused RCA inputs. In my case, shorting plugs on the second, unused phono input was a big step in quieting the receiver.
Click to expand...

Yeah, with the jumpers removed it is dead silent. I'll try that with the shorting plugs, thanks for the suggestion.
 
I have to wonder if this issue is actually on the tone control board... it is the only board that is reacting to touch with the volume all the way down.
 
I did notice a hum on mine if the screws mounting the tone board to the chassis were not tightened. Same thing with the the nuts fastening the pots to the front panel.
 
tubmyk2 said:
Also, and this is where I'm not sure it makes any difference or not, but since I upped the value of the 3 main filter capacitors to 2700uf (from 2200uf), is there any reason to change the value of the resistors that connect 2 of them to ground? They are 330 ohm.
Click to expand...
Please tell me the 'R' numbers of these - I don't think you should change their values, but I would like to see which ones you are referring to. (from your description I can't see any resistors on the schematic that match).
tubmyk2 said:
I also noticed that the 2000a engineering change order for the ground point to the 1000uf cap had actually not been done on my 2000x. I was looking at the diagram wrong. I'm going to change that for whatever it is worth.
Click to expand...
Yes, do this before doing anything else.
 
Hyperion said:
Please tell me the 'R' numbers of these - I don't think you should change their values, but I would like to see which ones you are referring to. (from your description I can't see any resistors on the schematic that match).
Click to expand...

I just changed the ground wiring; made no difference.

R843 and R846. They are tied between the ground lug and the filter cap(s).

Also worth mentioning (maybe), I did not re-cap any of the tuner section. I know everything passes through the EQ board but I think I'm correct in assuming this will have no impact on the issue I'm seeing.
 
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Also… any particular reason they chose to eliminate parts on the tone control board but still list them as active parts in the manual?
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