TA-500 with No Phone Stage

Discussion in 'Exclusively Sansui' started by mklein357, Jan 15, 2019.

  1. mklein357

    mklein357 AK Subscriber Subscriber

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    Hello. I picked up a TA-500 yesterday. I really like the aesthetics of this receiver. Excellent condition and the receiver functions perfectly...EXCEPT no phono output. I plan on opening it up tonight and see what I can find. But while I'm at work, I figured I would see if anyone had advice on where I should start my investigation. I've read that fused resistors can be a problem with these receivers?
     

     

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  2. slimecity

    slimecity Super Member

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    So what you mean is that you are getting no sound thru the phono section, when using a phono source connected to the phono input?
     
  3. mklein357

    mklein357 AK Subscriber Subscriber

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    That's correct. Turntable hooked up to the phono source provides no sound through the speakers. A cursory overview of the solder joints and general resistor/capacitor visual inspection provided no clues.

    Thank-you
     
  4. slimecity

    slimecity Super Member

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    Clean and work your switches a few times thru with deoxit - especially the input selector & tape monitor switches.

    Altho this unit is fairly middle of the line - it still has separate boards - including for the phono section - its board F3015 ("equalizer amp") and is mounted underneath the unit. What I would do next is (provided you have the skills & confidence and a DMM) measure for 37VDC between ground and pin 9 of that F3015 board.
     
  5. mklein357

    mklein357 AK Subscriber Subscriber

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    Will do tonight. Thank-you for the input. I'm pretty comfortable checking voltages around circuit boards. My abilities at reading schematics leave something to be desired.:) But if pointed in a direction, I can usually find my way. I did run deoxit through all switches and dials last night but didn't get a chance to test with my turntable.
     
  6. mklein357

    mklein357 AK Subscriber Subscriber

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    Is it fair then to say I can check the condition of the R40 and R42 fusible resistors by checking for 33.5 volts just downstream of the resistors?

    upload_2019-1-16_15-46-18.png
     

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  7. slimecity

    slimecity Super Member

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    Fusibles huh? Are they a different color to the surrounding resistors? Sometimes they are shown in the schematic but not used later in production.

    If they are fusibles, I generally pull them and test their value, or replace altogether with metal film types.
     
  8. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

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    I had same problem on a TA-500 last year, in addition to Blown outputs and one of the IC chips. I seem to recall R39-R42 were all 4 fusible and bad. I replaced with metal films. I also had a bad capacitor and a Transistor. At that point recapped and replaced all 4 transistors. Sounded wonderful after that.
     
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  9. mklein357

    mklein357 AK Subscriber Subscriber

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    Well it sounds like that's the road I'm headed down. The suspect fusible resistors are the two pair in the top right and bottom right of board. In front of R39 and R40, I get 35 volts (not quite 37 as shown on schematic). On the backside of R39 and 40 I get about a half volt. I'll pull the fusibles this weekend and test but looks like that's the problem huh? Is the 2 volt variance from nominal voltage an issue? Voltage at R41 and R42 is right at 37 volts. Might there be a problem on the power supply that would account for this failure or are these resistors just prone to problems?

    IMG_7503.JPG
     
  10. slimecity

    slimecity Super Member

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    Thats looking promising - yes just replace them.
     
  11. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

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    Here is Mouser part number on the resistors I used with success.
    279-FRN25J470R Metal Film Resistors - Through Hole Metal Film Resistors - Through Hole FRN25 5% 470R W

    Transistor Replacements:
    2SA906 ---KSA992
    2SC1845 ---KSC1845
     

     

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  12. mklein357

    mklein357 AK Subscriber Subscriber

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    Awesome. Thank you! I pulled the fused resistors last night. All 4 were bad. 470 ohm resistors huh. I see now that the schematic shows 470 ohm resistors. The parts list as shown below shows 56 ohms. 470 ohms is the correct fused resistor value?
    upload_2019-1-18_8-9-35.png
     
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  13. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

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    I have read on AK that Sansui often made undocumented changes. Or in general, always go with what is on the boards if not replaced previously by someone else.
     
  14. mklein357

    mklein357 AK Subscriber Subscriber

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    OK. Yep, I just checked the color code on the resistors. 470 it is.

    Thanks again.
     
  15. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

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    It might be good idea to check the other components like Caps and transistors on that board. Something else may have crapped out and pulled current through those resistors to burn them up.

    Or, use a dim bulb tester on power up. At minimum watch this resistors closely on power up. If they get hot immediately stop and start testing the other components. I rebuilt my board once found other bad components. Quick and easy.
     
  16. mklein357

    mklein357 AK Subscriber Subscriber

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    OK. I got the new fused resistors in. Powered up with DBT, got a quick flash of light through bulb which quickly faded away (like I would expect), but the speaker relay did not close. Hmmm. So I disconnected DBT and powered up again. This time, the relay closed. However, now where I was getting 35 volts before resistor replacement at the phono board, I only get 10 volts! Didn't see THAT coming. I I tried running my iphone through the auxiliary input to see if I would get output here. No output through aux. In fact, with a source connected to aux, the speaker relay would open after 5 or 10 seconds. I surmize that it is not an overvoltage issue, but instead is not pushing enough voltage to the relay to maintain in closed position. Doing a quick once over on the power supply, it looks like there is at least one bad resistor. My plan is to do a voltage study on the power supply and see if I can figure out what has gone bad. I'm guessing that a power supply rebuild is in order.
     

     

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  17. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

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    I seem to remember I either had to use a 100W Bulb or direct to outlet to get enough juice to power everything up. But always use the DBT first after each repair to see if any obvious shorts. Good starting place to check all voltages off the power supply and compare to your schematic. Mine had multiple issues starting with Blown caps on power supply board (recapped), Output Transistors blown on one side, The main IC on same channel as Outputs, then the Phono Board issues. Work through each problem at a time. Speaker Relay is supposed to wait a period of time the close and stay closed unless sees high DC voltage. Check you DC and bias settings after checking Power Supply.

    You mentioned in original post the receiver worked perfectly other than Phono? Also, do not put phone output into PHONO inputs, to high of signal overdrives the phono amp and possible other issues. (I have been cautioned about this by others here on AK). Edit: I forgot you ran a TT into PHONO input.
     
    Last edited: Jan 24, 2019
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  18. mklein357

    mklein357 AK Subscriber Subscriber

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    Yes. Originally, I had good output from auxiliary and from the tuner. Now nothing. I ran some voltage tests on the power supply. Looks like I have correct DC voltage just past the rectifiers. None of the outputs show more than 10 or so volts. R05 (fusible resistor is definitely bad. I'm going to rebuild the power supply. I have a list of all capacitors, fused resistors and transistors but am unsure of exact replacements for all transistors. Below is my list or transistors and what I can find for replacements. Does my partial list look correct? And for the two in question, any suggestions?

    Thank-you.


    Original Replacement
    2SD313 AL ……... TIP41C
    FS7815M …….. ?
    2SB507 …….. ?
    2SA992...….... KSA992
    2SC1845....…... KSC1845
     
  19. slimecity

    slimecity Super Member

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    I would put the unit on a DBT and take measurements within the powersupply as per the service manual before shotgunning the components in there - you want to see which area is flakey and from there assess how that could have happened.

    Bear in mind that readings on a DBT will be about 2/3rds of that off a DBT, but they should still be relative to each other. If you have anything wildly out of spec, you will still know
     
  20. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

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    Do you have 25V into (@R01 Pin1) and 15V (@R018 Pin3) out of IC01 (FS7815) upload_2019-1-25_21-19-8.png
     

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