Sansui 5000A Recap

Still trying to solve this problem - also have a thread at Diyaudio. I think the hum is the fault of Sansui engineers. Let me restate that the only things I've done is to recap and put in new transistors - no design changes. 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

preamp test.JPG amp test.JPG driver schem.jpg driver board.jpg
 
Nice analysis John. I was taught that there was a signal ground and a power ground, and "never the twain should meet". My first foray into a 5000- was in 1969 when a Sansui rep came to the base exchange at Clark Air Base to mod all the 5000-s up to 5000As, for all that were still around. I whipped out the schematic from the little black envelope and tried to follow what he was doing. That was when I first noticed that all the grounds were apparently tied together, and I asked the tech about that. He wasn't quite fluent in English, and I can still hear his reply: "No ploblem."
 
John,
What did you use for 2200uf main caps?
Thanks

Still trying to solve this problem - also have a thread at Diyaudio. I think the hum is the fault of Sansui engineers. Let me restate that the only things I've done is to recap and put in new transistors - no design changes. 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

View attachment 812574 View attachment 812575 View attachment 812576 View attachment 812577
 
647-LLS2A562MELC. These are 5600uf 100V that fit the original clamp. They are snap-in so I crimped and soldered on a ring terminal to make it easier to connect the wires.
 
647-LLS2A562MELC. These are 5600uf 100V that fit the original clamp. They are snap-in so I crimped and soldered on a ring terminal to make it easier to connect the wires.

Thanks for reply.
Did you ever get buzz/hum figured out? My 5000a (not the one I'm working on) with same 6013 boards has it too.
What is the reason to change from original value of 2200uf? I always thought value should be kept the same and voltage rating can be changed to accommodate the clamp fitment.
Mouser has 647-LGY2A222MELC30 2200/100v 35mm x 30mm. What do you think of those?
Thanks,
 
I had a 5000a with the same hum problem. I recall reading that people have re-routed wiring in the 5000a to reduce the hum. I don't know which wiring though.
 
I have a 5000a and X with the replacement boards installed. Both are dead silent at idle with good responsive speakers. But I have noticed noise from the internet router through the tuner, have to shut them down to listen to weak stations, and I get a hum if the TV cable is hooked to anything in the stereo setup, even a patch cord from the TV through a VCR or DVD player. But both are totally quiet otherwise.
 
I re-soldered driver board ground strip with higher wattage solder gun and buzz disappeared.
 
I got rid of my hum in my 5000a by isolating the ground from the tape monitor switch to the chassis and sending it through the shield on the input to the driver boards.
 
In the 5000a it says that the ground terminal on the back by the inputs (which I hooked up my turntable ground too) should have a insulated wire with a copper bus bar that is buried underground. In other words, that it's supposed to be earth grounded. I'm not an electrician....but is this mainly because it's only a 2 prong outlet so not actually hooked up to any kind of ground.

Could this be some of the reason for the issues that seem to be ground related?

I was thinking about hooking that terminal to the ground wires in the outlet and seeing if that helps with the slight hum of my 5000a (it can only be heard when nothing is playing and your close to the speaker)
 
I’ve got a loud buzz when I tune in an FM station and the FM Stereo light comes on. Move the dial off of FM Stereo but still picking up the station and the buzz is gone. Anyone ever resolved that one? About to start hitting the MPX board but wanted to resurrect this thread first. Thanks!
 
I’ve got a loud buzz when I tune in an FM station and the FM Stereo light comes on. Move the dial off of FM Stereo but still picking up the station and the buzz is gone. Anyone ever resolved that one? About to start hitting the MPX board but wanted to resurrect this thread first. Thanks!

I have a 5000A on the bench, with the same problem. Still investigating, will be interested to see if you cause of the problem. I will post up here if I find the cause of the problem.
Cheers
John
 
I have recapped the MPX board with all new electrolytics and swapped out TR401-407. Buzz persists. VR works to flip the light off and on appropriately. Diodes will be the next place to look.

Don’t know if I am going to bother any further on this unit, TBH. I have a C33 and an MA5100 sitting behind it.
 
F‐1120 Muting Board has a 2sc458. It could be causing the buzz. My 5000a use to make the buzz, I have changed out all the 458s on mine.
 
I hit the muting board as well, changing out TR251, TR253 and all electrolytics. I missed TR252.

Is it worth getting back in there for this one?
 

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I had look at my notes on the 5000a. Vr 401 on multiplex board F1006 adjusts the fm stereo indicator lamp. If it is turned up to sensitive it will cause a buzz associated with the stereo lamp. If turned up to much i had fm stereo lanp on when the selector switch was on am. I also had the dreaded buzz at low volume and this picture shows where I had to move the wires going from tape monitor to driver boards.
 

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