Restoring uncle's Sansui 9090DB

Discussion in 'Exclusively Sansui' started by Juan9568, May 5, 2018.

  1. tnsilver

    tnsilver AK Subscriber Subscriber

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    Oh, BTW, you're doing a great job handling this. You're working safe with a DBT and ask before you act. Be patient, you will get this working!
     

     

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  2. Juan9568

    Juan9568 Member

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    Buenos Aires, Argentina
  3. tnsilver

    tnsilver AK Subscriber Subscriber

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    Good reference! It does dim down, perhaps a tad too slow though. Make a note of the time it takes with the 100W bulb. It should be faster. If not, it may indicate slow charging filter caps.
    Lee mentioned a good way of isolating the DC offset measurements of one channel from the other and I think you should follow it. Use the correct DC measurement scale of your meter.
    the 200mV scale is good for a nice behaving channel but if you have a rogue channel with higher DC (very likely) you will not be able to read it and think it's just zero, so measure
    with a higher scale. I already mentioned it could get close to rails voltage so be prepared for the worst.
     
  4. Juan9568

    Juan9568 Member

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    53
    Location:
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    Still testing with a 70w bulb with Lee's instructions. My brain is about to explode.
    Check this out!

    Testing only Right Channel DC



    Testing only Left Channel DC. I start getting that buzzing sound



    While testing Left Channel the sound gets louder



    After 3 minutes of that sound, relay clicks, green light (love when I hear that click...it gives me hope haha). Only left channel fuse connected



    I turn the unit off and start again but this time with both fuses. Still testing DC on the left channel. Nothing happens...

     
  5. Juan9568

    Juan9568 Member

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    Location:
    Buenos Aires, Argentina
    Now I check again if relay clicks. This time without the fuses but driver board still connected. Nothing happens



    Now I check if relay clicks without driver board....and it works. It also clicks with the fuses connected.




    Sounds to me this is a driver board issue...

    Why would it be main filter caps if relay clicks without the driver board??
     
  6. DougBrewster

    DougBrewster AK Subscriber Subscriber

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    Way I do it is to measure/set the offset and bias at the same time. Now you've got two meters you can do this. Focus on one channel at a time leaving out the fuse of the other channel. See if your adjustments are working correctly then do the same thing on the other channel; this way you will be able to identify which channel is causing the issue. It may be you have excessive bias and the current limit is getting in the way. I always use a 60W DBT at this stage and use about 15mA to 20mA for the bias...
     
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  7. tnsilver

    tnsilver AK Subscriber Subscriber

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    The buzz is rattle from the transformer. It's a tradeoff of working wit DBT. It will go away on mains. The DC measurements look OK for both channels. It starts high, gets filtered out, and fluctuates +/-3mV.
    The driver amp provisions for DC offset and bias current adjustment without causing a noticeable current draw so I assume it's pretty much OK. With the driver amp out, the main filter caps have no influence on the DC levels reaching the protection board. I still have the impression they take too long to charge but that could be the DBT. I also caught a glimpse of the bottom of the power supply board. It looks messy and scratchy and the soldering near the driver amp and outputs connections can be much improved. I'd verify the various makeshift jumpers conduct as expected and again, try to spot cracks. I'd follow Doug's advice and set bias current + DC offset simultaneously using both meters, for each channel separately. Like he said, 17mA should be appropriate for a bias current sanity test.
     
  8. Juan9568

    Juan9568 Member

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    53
    Location:
    Buenos Aires, Argentina
    I heard that buzz not from the big transfomer but from the protection circuit board.

    I tried Doug advice and these are the readings:

    Right Channel:

    Left Channel:

    There's no DC offset...So why the protection circuit still kicks in??? Should I give a try without DBT??
     
  9. DougBrewster

    DougBrewster AK Subscriber Subscriber

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    try tapping that relay with a screwdriver handle or something light, see what happens..
     
  10. tnsilver

    tnsilver AK Subscriber Subscriber

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    No, not yet. By all accounts you should have a green light with these settings. I don't think it's a driver amp issue. Likely a protection board problem or a flaky trace/solder pad/jumper issue on the power supply board. Are you sure the buzz is emitted off the protection board? I really think you should overhaul it.
     
  11. Juan9568

    Juan9568 Member

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    53
    Location:
    Buenos Aires, Argentina
    Re-checked the PS board, reflow a few connections, but no cracks...lots of scratches because of all other technicians that have messed with this amp over time...
    Yes, the buzz was from the protection board 100%...it's like if something was charging till (after 3 minutes like in the video I posted) I got the green light..

    I'm thinking of the protection board too, but if the protection board is working fine without the driver board connected then the problem is not there...
     

     

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  12. Juan9568

    Juan9568 Member

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    Question...I'm testing transistors on the board...

    I found out that TR05, TR06, TR17 and TR18 give me a reading with the multimeter while putting Positive on BASE and Negative on EMITER...
    these are PNP, shouldn't I get OL instead of a reading???

    I'm testing them while in circuit

    EDIT: took it out of the circuit and it's okay
     
    Last edited: Jun 10, 2018
  13. Juan9568

    Juan9568 Member

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    Location:
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    Another thing...sorry I'm a bit obsessed with this..

    Took out the relay fuses and the driver board (again)....Relay still clicks, green light....

    Is that normal? why would it give me green light if there are no fuses in the socket?
     
  14. tnsilver

    tnsilver AK Subscriber Subscriber

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    The protection circuit senses DC and trips the relay above a certain threshold (IIRC it's something like 200mV - 300mV). With no driver board, there's no DC at it's input, and no reason for it not to do it's job. This is no big deal but what we really don't know is how it performs with DC present in it's input. So far, based on your measurements, it should have worked - but it didn't, and it doesn't look good for the protection board.

    You say it rattles and it shouldn't, so here we have at least one suspicious aspect of it. I wonder what makes it rattle and I would likely assume it's the relay's electromagnets struggling to make contact. I don't know. I never heard a protection circuit buzz before, but it sure shouldn't. We also know this board (it's been posted in this forum before) to be prone to bad/dirty relay contacts and it's generally sensitive to it's solder joints crumbling and cracking over time.

    That relay is often replaced (or at least opened and cleaned) within the framework of a unit's recap together with overhauling this aging board. Did you try to tap the relay like Doug suggested? It's a good tip and if you manage to release the relay by tapping it, it would mean the protection board is just stuck on a technicality.That would be good news.

    So far it just looks like your protection board is not doing it's job, or that it's doing it's job so well, that it detects high spikes of DC that we don't see on the meter. It's not impossible, but unlikely.

    OL means open loop. You don't have an open loop while the transistors are in circuit. Anyway, if you had a bad transistor on the driver amp, let alone all of the above, the DBT would most likely oscillate, you'd have noticeable current draw, you could expect very high levels of DC on the gray/blue wires and there are good chances you would not be able to adjust bias. You got none of it and it appears your driver amp is good. Be patient, stay on DBT and keep it this way.
     
  15. Juan9568

    Juan9568 Member

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    53
    Location:
    Buenos Aires, Argentina
    Problem solved!!!

    The issue was with the 70W DBT. It didn't let the current flow to kick the protection relay.

    I'm now on a 100W bulb and I get green light! :):)

    Gonna start biasing.
     
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  16. Juan9568

    Juan9568 Member

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    53
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    Thank you guys for your priceless help, patience and advice! I've learned a lot and my 9090DB has come to life again :beerchug:

    Now that the amp section works great I'm thinking on a major recap because there's a lot of noise in the signal and I can also tell that the main filter caps are asking desperate for a replacement.

    I'll keep on posting my progress
     
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  17. Juan9568

    Juan9568 Member

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    Here a short video of the amp working again. Incredible power! :music::music::music:

    Can't imagine how it will sound with a full recap and better speakers!

     
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  18. ccdowenye

    ccdowenye AK Subscriber Subscriber

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    As a happy owner of the 8080 and someone without the skills you (and other contributors have) I am simply amazed at what you have done with this receiver. It’s been so cool to watch your post and to see the video of this beast working!

    Kudos to you man.
     
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  19. Juan9568

    Juan9568 Member

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    Location:
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    Thanks to you for watching! and thanks to the amazing people here at audiokarma that share their knowledge, experience, advice and tutorials; I've couldn't done it without them. I'm just a music producer and musician (with basic knowledge of electronics) and I've always loved to repair things...so this was a great experience and I also learned a lot.

    However there's still a few things to be done with this amp and lastly the aesthetics.

    I will keep on posting
     
  20. Juan9568

    Juan9568 Member

    Messages:
    53
    Location:
    Buenos Aires, Argentina
    While making the list for the next recap there's another issue with my amp.

    Check out the actual condition of my F-2653 equalizer board...:(











    For what I know, the traces started to peel from the flex and touched the chassis...that created a short that burned those three capacitors. I think that short also affected the driver board.
    I still don't know how those traces on the board disappeared.
    An unknown technician did this mess with the inputs on the back and the gray cables trying to "fix" the issue..:wtf:

    I'll try to get a good 9-pin and 6-pin ribbon cables to replace those and reconstruct the traces
     

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