Sansui G-7500 Fried Resistors F2980 Board

Discussion in 'Exclusively Sansui' started by Young46, Oct 10, 2017.

  1. Young46

    Young46 AK Subscriber Subscriber

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    Okay, I see. OL is the reading for infinity on my Fluke 117. I’ll continue with pulling the output transistors.
     
  2. ghazzer

    ghazzer Sansui addict Subscriber

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    If you have an oscilloscope you can make a simple component tester that can test components without removing them, especially components with a semiconductor junction. It cannot give you quantitate numbers, but it can generate a display that represents the signal flow through a junction(s).

    Here is an example:

    P1030890A.jpg
     
  3. Young46

    Young46 AK Subscriber Subscriber

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    I have an old RCA WC-33A that needs a probe. I’m not even sure it still works lol. I appreciate the help!
     
  4. Young46

    Young46 AK Subscriber Subscriber

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    I pulled TR601 and it tested bad. I tested the other 3 in circuit and they don’t show to be bad. Would it be wise to pull them and test? If 602, 603, and 604 test ok would it be ok to leave them and just replace 601, or replace them all. BTW, the NJW3281 is on backorder at most suppliers with a 28 week lead time. Acceptable substitutes?
     
  5. ghazzer

    ghazzer Sansui addict Subscriber

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    I think that the consensus is that you should replace all 4. That way you get the best shot at having "balanced" output TRs. Having unbalanced loads leads to the weaker one(s) getting stressed more than the others, and schit rolls down-hill.

    The G-6700 calls for the same OP TRs as the G-7500 (Sanken 2SA1106 & 2SC2581). The Sanken replacements are 2SA1695 & 2SC4468. I have a set of them but have not taken the time to install them and see how they work.

    I see that the G-7500 mounts the OP TRs on F-2981 boards rather than the F-3058 bds in the G-6700. Mine are quite fragile and I ultimately etched a new set because the tracks on the originals were being destroyed by multiple R&R of the OP TRs. If yours seem to be "older than dirt", make sure that you are very careful with your soldering iron.

    I have a layout for having new PCBs made commercially, but it is another thing I haven't gotten around to. I'd need to learn the S/W for generating files to get the bds made. Oh well, one of these days . . .

    PM sent.
     
  6. Young46

    Young46 AK Subscriber Subscriber

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    I humbly agree with replacing all four. Yeah those little boards are fragile. I very carefully desoldered TR601 from the board and the board looks good. I’m going to go ahead and pull the other 3. Thanks!
     
  7. Young46

    Young46 AK Subscriber Subscriber

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    I had some time today and started pulling the .33 ohm resistors to install the 100ohm resistors as recommended. I tested them as I pulled them and R72 tested bad (OL on my DMM). Could this be part of the problem or the problem?
     
  8. ghazzer

    ghazzer Sansui addict Subscriber

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    R72 looks to be the emitter resistor for the positive side of the right channel. Was it reading OPEN when removed from the circuit board?
     
  9. Young46

    Young46 AK Subscriber Subscriber

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    Yes.
     
  10. ghazzer

    ghazzer Sansui addict Subscriber

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    IMHO that would definitely screw up the signal in the right channel. However, you said the problem was "no sound", not a funky right channel. Not looking at a schematic right now, but if R72 is on the emitter of TR601, I suspect that it was a secondary failure when the TR went. There are probably more 'gotchas'.

    You have to be so meticulous with this stuff. One careless move/action could dork up everything you've done for weeks. Ask me how I know!
     
  11. Young46

    Young46 AK Subscriber Subscriber

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    The “no sound” was probably my amateur way of stating a problem. The unit was not powering up. And then I opened it and discovered the fried resistors. Since that time I have replaced the resistors in the burn area, the 4 output transistors, all the other transistors on the driver board except for the two FET’s, and the trimmer pots. I had a little time yesterday so I started pulling those .33 ohm emitter resistors in preparation for installing the temporary 100 ohm resistors as suggested when I discovered the bad one. I’m going forward with that but have a question. Do the temporary resistors have to be the cement wire wound type or will any 100 ohm resistor be ok?
     
  12. ghazzer

    ghazzer Sansui addict Subscriber

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    Any 5W or higher resistor should work fine for this temporary situation.
     
  13. Young46

    Young46 AK Subscriber Subscriber

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    Thanks! I picked up the 5W 100 ohm resistors today and am going to install them tomorrow. After they are installed what is my next move? Put it on DBT and power up? If that step is ok, what next? Check bias and offset? If so, do I follow the steps in the manual or is there something else better?
     
  14. ghazzer

    ghazzer Sansui addict Subscriber

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    What do you know about "circuit analysis"?

    I have too many years as an engineer to give you a simple answer to what some may consider a simple question. I try to start by establishing (as much as is practical) WHAT is wrong, AND what is right, and any other salient facts that I can establish. Next comes a comprehensive analysis of this datum, including WHERE these conditions exist, HOW and WHY they might have occurred. (WHO & WHEN are not very important for this case.)

    The parts you replaced at the burnt spot (TR20,R46,R56,R58,R68&R70) are all p/o TR20's circuit, and it is easy to believe that something went wrong with TR20, and took the resistors with it. Throw in the failure of TR601 & R72 and I wonder if something else went bonkers and led to secondary failures of these components. You could test this out by replacing all these parts, firing it up and wait to see what happens . . . .

    My approach is to check out the circuits and components that feed TR20 and TR601, hoping to find a primary fault and fix that as well.

    Bottom line: I cannot answer your question without asking you a LOT of questions. AND, since I still don't have my G-6700 repaired I am probably not a good one to be giving you suggestions.
     
  15. Young46

    Young46 AK Subscriber Subscriber

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    I don’t know very much about “circuit analysis”. I thought I made it known up front that I was not an electronics or electrical engineer. Perhaps I didn’t and I apologize for that. I wasn’t looking for a simple answer to a “simple” question. I understand that the problem is complex. I was merely asking for advice on the next move after installing the temporary emitter resistors. Thank you for helping me. I apologize for getting you frustrated. Good luck with your G-6600.
     
  16. Young46

    Young46 AK Subscriber Subscriber

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    I’m sorry, G-6700.
     
  17. ghazzer

    ghazzer Sansui addict Subscriber

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    I'm not frustrated. Well, maybe a little miffed at not being able to help you as much as others have helped me.

    The design of the F-2980 board seems to be on the hairy edge - fine tuned like a fiddle. If it gets off the straight & narrow it can slide down a slippery slope. I am just afraid that if you are not able to identify and resolve the original problem you are chasing the tail of the fox. You would have a better probability of success if you were working on a simpler receiver or amplifier.

    The G-6700 still sits on my work bench, waiting for me. I move it aside as other projects come and go, then move it back.
     
  18. kevzep

    kevzep Its all about the Music Subscriber

    Install the resistors and if you are sure you've replaced everything that burned up, fire it up on the DBT....no other way to find out of its going to work or not.
    The bias setting will be irrelevant because of the higher value of the emitter resistors you've put in. So what you can do it dial in a bit of bias and see if it holds okay, maybe about 10-15mV
    What you are needing to do here is to see if the amp will run first, if something is wrong the DBT will light up and the 100Ω resistors will save the output transistors which are NLA, once the amp is actually running then you can slip the proper emitter resistors in and fire it up then set the bias to the correct specs.
     
  19. Young46

    Young46 AK Subscriber Subscriber

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    Installed temp resistors. Everything seemed ok on the left channel. Took a bias reading on the right channel and got 0mv on my fluke on mv setting, and 34.48V on DC setting. I could adjust a couple of volts up and down on DC setting. I did notice that the right bulb got brighter and dimmer as I adjusted the DC. This would be the bulb that illuminates the tuning and signal meters. Also transistors TR16 and TR18 were a little warm to the touch while TR15 and TR17 were cool. The problem seems to be on the right channel.
     
  20. kevzep

    kevzep Its all about the Music Subscriber

    34.48VDC....this is very wrong if this is coming out of the speaker output.....so, where are you measuring that? At the speaker output, pre or post relay? does the relay click in? Safely operate indicator lit up?

    Where are you measuring the bias? At the test points? across and emitter??

    There will be no bias reading because of the 100Ω resistors, also, you should in this instance have the bias trimmers turned ALL the way down, anticlockwise.
    The DC offset trimmer should be in the middle at 50%.

    You need to be methodical and detailed in your posts otherwise its difficult for us to help you.
     

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