1. Time for some upgrades in server hardware and software to enhance security and take AK to the next level. Please contribute what you can to sales@audiokarma.org at PayPal.com - Thanks from the AK Team
    Dismiss Notice

8080db recap has hum

Discussion in 'Exclusively Sansui' started by ranukic, Dec 7, 2018 at 9:16 PM.

  1. ranukic

    ranukic AK Subscriber Subscriber

    Messages:
    190
    Location:
    Fox Valley, WI
    hey,

    I replaced all of the board electrolytic capacitors, fixed the R33&R34 resistors for the protection circuit, found a bad transistor and fuse on the main power board and a couple other fuses throuhgout that I replaced. After all of this work and trouble shooting there is hum in the transformer that is resinating through to the speakers. At first I thought it was bad filter caps but when i scoped their and also checked the AC voltage at each cap I found it to be less than 0.89VAC which should be acceptable...

    The only time the low harmonic ham changes is if i press the mute button, adjust the tones and turn the volume up so the music drowns it out. When the mute is pressed the sound completely is eliminated.

    I took the jumpers off the amp out to in, hooked up a bluetooth device to the in and there was no hum. I do not recall if the transformer also stopped humming but I can check that again tomorrow.

    The sound reminds me of the sound cheap rca cables make when hooking a phono to an amp but with just a little more buzz.

    I am not convinced the problem is the transformer itself, i think there is a load causing the hum. Any suggestions on where the issue may be hiding?

    Thanks, James
     

     

    Please register to disable this ad.

  2. tnsilver

    tnsilver AK Subscriber Subscriber

    Messages:
    2,991
    Location:
    Boston
    In my experience the transformer buzzes and resonates a little when it's power hungry, such as when you fire the unit up on a DBT. If it stops doing it when the amp is disconnected from the tone control, like when you pull out the jumpers, it is possible there's a short in the pre-amp/tone control section which is drawing current. See if you did not, by chance, install a capacitor with a wrong polarity or another component (resistor?) with a wrong value. R33 & R34 are located on the driver amp (F2624), not the protection board (F2657) and it would help if you specify what transistor you replaced where and to verify it's pin out is correct.

    Did you manage to bias the unit and adjust DC offset? What's the DC offset showing at speaker terminals with/without the main-in/pre-out jumpers?
     
    Hyperion likes this.
  3. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Messages:
    48,550
    Location:
    Hertfordshire, UK
    Actually I think that sounds like rather a lot for ripple caused by the receiver simply idling. New main PSU smoothing capacitors or originals in good condition should yield ~50-100mV of ripple in my experience. Judging by your comments, and the analysis in Tom's post, it would suggest that something is definitely not right.
     
  4. ranukic

    ranukic AK Subscriber Subscriber

    Messages:
    190
    Location:
    Fox Valley, WI
    Thanks tnsilver,

    Correct R33&R34 are on F-2624 but they were outside of range so I replaced them with 200R 1/2W and by doing so the green power light started to work. The green power light was originally not coming on after protection mode was switched off.

    On F-2656 TR04 was not working so I replaced it with NTE289A. Also R06 was burnt up and outside of range so I replaced it with 1K 1/2W.

    I just checked the bias and the DC at the speaker terminal I adjusted to where it bounced around zero. However when I tried to adjust the mA I am not able to achieve above 6.78 mA on right and 5.80 mA on left.

    After those readings I double checked F-2624 against the readings in the schematic and everything was very close.

    Any ideas?

    I will remove the tone board from the circuit next to see if that eliminates the hum.

    Thanks, James
     
  5. tnsilver

    tnsilver AK Subscriber Subscriber

    Messages:
    2,991
    Location:
    Boston
    DC offset close to zero is very good but bias sure looks like a problem. You may want to replace diodes D05/D06 (red dotted VD1212) each with two 1N4148 rectifier diodes in series, see if that helps biasing... The semi-open trimpots often loose it too, so replacing them is probably a good thing, but the bias limit looks too consistent to be trimmer related in this case. IIRC R33 & R34 on F-2624 are 180Ω so I'm not sure how 200Ω resistors will affect the driver amp, if at all, but why risk it?

    You call it "hum", which is generally a low (mains) frequency disturbance, typically associated with grounding. All grounding with the 9090DB is in one of the corners of the power supply PCB and it may be a good idea to tighten the screw that channels the multiple groundings (via a load of black leads) to the chassis. I have, on the other hand, referred to a "buzz" issue which is a higher frequency disturbance and in case of the transformer sounds like rattling. Just sayin... to make sure we're on the same page.

    TR04 on F2656 is 2SC1211 and according to this, has EBC pinout (which is weird b/c the SM demonstrates ECB pinout). Our best Sansui transistor substitution thread recommends a KSC2383 instead of the 2SC1211 and it appears to have a pinout consistent with the Sansui SM. I'm not saying the NTE289A has the wrong pinout, as it could easily be the wrong datasheet that I've referred to, or the wrong Sansui SM marking, but definitely worth looking into.
     
  6. ranukic

    ranukic AK Subscriber Subscriber

    Messages:
    190
    Location:
    Fox Valley, WI
    Tnsilver,

    I set my meter to diode check and both D05/06 show open in one direction and a good reading in the other. Do you still recommend these getting replaced?

    I did bypass the tone board F-2544 and the hum went completely away. So I am going to pull the board out and check everything over again.

    The hum that I am referring to reminds me of the sound made when connecting/disconnecting a turntable while the amp is on and in phono.

    I appreciate the help.

    Thanks, James
     

     

    Please register to disable this ad.

  7. tnsilver

    tnsilver AK Subscriber Subscriber

    Messages:
    2,991
    Location:
    Boston
    OK, so it's actually a "hum", not what I had in mind at first and probably grounding related. Yes, I'd replace the VD1212 diodes, they are ancient and prone to fail.
     
  8. Tom B

    Tom B AK Subscriber Subscriber

    Messages:
    1,490
    Location:
    Bay Ridge, Brooklyn New York
    Before you pull and disconnect the tone board see if any of the caps are getting hot (a sign they are leaky or installed backwards). Usually you can pull the board and flip it over without disconnecting the wired. Just make sure to put a rag or other insulator under itto keep from shorting out connections. And also check to see if any black ground wires boke from this board, or others near it.
    Also double check TR04 as TNSilver suggested. That supports the +38v used on the tone board. And double check any cap replacements on that main power supply board. I think I found a mismarked board for capacitor polarity on one of them once. If you replaced the caps for the +38v supply, make sure they are in the right way. Verify according to the schematic rather than board markings.
     
  9. ranukic

    ranukic AK Subscriber Subscriber

    Messages:
    190
    Location:
    Fox Valley, WI
    Thanks,

    TR04 is correct, before I replaced it the tone board was only receiving half the voltage it was asking for. The audio traveling through the tone board was getting clipped.

    Not sure what happened but since I left earlier and came back the stereo now won’t come out of protection mode. I didn’t get a chance to make any changes so I am confused. I will start pulling signals again tomorrow.

    Thanks for the help, James
     

Share This Page