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TA-500 Left Channel blown- AN7070 bad?

Discussion in 'Exclusively Sansui' started by jd3rdcoast, Jul 11, 2018.

  1. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

    Messages:
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    Location:
    Houston Area
    I picked up a TA-500 DC Integrated Tuner Amplifier recently that was described as blowing fuses. Sure enough all 5A fuses were blown. Also C01 had completely come off the board but no obvious signs of heat damage. Possibly just deteriorated from glue corrosion.
    I replaced CO1/CO2 330uF/50V Nichicons I had to test. Put new fuses in. DBT showed still short. So pulled Left side Outputs TR05/TR07 and they both tested bad C-E shorts. Now the unit powered up and relay clicks on. Tested Right side with single input on AUX-R and lightly turned up the Right side independent Volume. So that seems to be working.
    I wanted to see if the AN7070 was bad offer reading posts that it is failure prone. I am getting some obvious different voltages on several pins and hope I g someone can help me determine if indeed the AN7070 is toast.

    Here are the discrepancies and pics of circuit below.
    Output to TR05/TR07 should be 0.6 V getting 10v
    P-10 = +10V
    P-14 = -10V.

    Bias circuit inputs.
    P-11 = 7.4V. Should be +1.2v
    P-13 = 4.9V. Should be -1.2v
    I pulled TR03 tested good. VR03 = good, R31/R35 both good.

    P-5 = 3.64. Should be about 5.6v
    Could be a misprint as R side has 5.6v P5 and both have 25v on P6.
    All other voltages match.

    Schematic F-3016 amp section
    016521AF-5E33-4924-BDB4-A435E8C8AD20.jpeg
    AN7070 drawing.
    278B308E-F3F0-4DDD-A7D5-C9C1EE63C842.jpeg

    Any help greatly appreciated.
     

     

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  2. kevzep

    kevzep Its all about the Music Subscriber

    Yeah, those AN7070 Chips are prone to failure, I repaired a Sansui Integrated with those Chips in them recently, you can still get them...
    Slip a new AN7070 in and you'll be away...
     
  3. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

    Messages:
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    Location:
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    Thanks, Looks like the auction site is best source. Ordered 1 for $17. Plan to recap the power supply while at it.
     
  4. kevzep

    kevzep Its all about the Music Subscriber

    Thats where I got them from, I don't buy that sort of thing from there very often, but sometimes you just have to.

    Note that you need to be very careful, make sure the surrounding circuit is perfect, its pretty easy to take those chips out...
     
  5. kevzep

    kevzep Its all about the Music Subscriber

    They are a pretty good sounding little amp when they are running right though, definitely worth the effort....
     
  6. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

    Messages:
    170
    Location:
    Houston Area
    Thanks for that, I’ll make sure to check all components. Resistors, Caps and diodes. I’ll get a set of lights from dwojo as well. Thinking about green or light blue.
     

     

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  7. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

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    I got the new AN7070 installed. Waiting on PNP and NPN output transistors to arrive today. I also replaced both TR03 & TR04 1845’s since had pulled to test. Both tested good w/Hfe app 550. New ones Hfe at 379/374.

    I did power up w/DBT with no problem and relay clicks.

    Checked a few of the points was getting bad readings. I noticed the AN7070 getting hot compared to the other after couple minutes so kept pwr on time to minimum just to check voltages.

    Once I adjusted VR03 (bias) down for -1.2V @ P13 it seems to be stable and other voltages in line.

    Previously had Pulled and checked caps C09, C13, C15, C03. All checked good for value. All resistors and diodes in area checked good.

    Will install new L-Side Outputs this evening and adjust DC 0V and Bias current per spec and test. . Keeping fingers crossed.
     
  8. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

    Messages:
    170
    Location:
    Houston Area
    Great success with the new L-Side outputs. PNP OS FJA4210OTU and NPN JA4310OTU. Was able to dial in Bias at 20mv across both emitters just fine after a 1/2 hour warm up was very stable. DC set to less then 5mv no problem. Tested unit on a set of Sansui SP-200 speakers. Cranked up to 5-6-7 with no issues with overdriving.

    This unit sounds very strong with good bass/Treble adjustments. I cleaned the controls and switches so all quiet and smooth. Tuner is very strong and accurate. AUX input sounds great. Quite nice sounding, better then I expected.

    Couple of minor issues:

    1. FM Tuner has same amount of static between stations with muting on or off. Mono/Auto is working. Stereo light and sound is working fine. Wondering if muting circuit not working?

    2. Phono Output is very bass heavy with little mid/High Output. No noticeable difference in Phono/Subsonic vs Phono. This is testing with iPhone at 3 bars output. Have to turn up volume to get it cranking.
    PHONO SPECS: 2.5mV /47 KOhms (Max input 210mV at1KHz
    AUX 150mV/47Kohms
    Thinking I need to rebuild the Phono amp section?
    2ea 2SA906 (KSA992)
    2ea 2sc1845
    2ea 3.3uf 50V input
    2ea 470uf 6.3v
    2ea 100uf 6.3v
    2ea 47uf 25v
    2ea 47uf 35v
    2ea 100uf 25v
     
    Last edited: Jul 21, 2018
  9. llwhtt

    llwhtt Super Member

    Messages:
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    Location:
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    Phono input is for phono ONLY. When LPs are cut the bass is reduced, takes up too much room on the disc, and treble is boosted for noise reasons. Upon playback the phono amplifier boosts the bass back to normal and the treble is cut back to normal. PHONO ONLY.

    Craig
     
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  10. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    This ^^^ +very likely severe overload into Phono input with the output from an iPhone.
     
  11. kevzep

    kevzep Its all about the Music Subscriber

    Well done on the repair, they can go quite wrong very quickly, so good job there.
    The tuner probably needs an alignment, the TA300 I worked on had the tuner way out of whack.
    The muting will most likely need adjusting among other things.

    Do not plug your iPhone into the phono input, the only thing that should ever be plugged into a phono input is a turntable.
    You'll be overloading the input with the iPod, not good!!
    What you are experiencing is the RIAA compensation equalisation. So if it sounds like that, then its completely normal.
    Have you got a turntable to try the input with?
     

     

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  12. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

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    Got it. Thanks for the good explanation of what sound levels are actually coming into the phono amp. That explains a lot. I knew not to turn up the input volume but wasn't sure about the sound. Will f/u with a TT.
    I did start checking listed voltages on the board. All readings at TR01/02 & TR03/04 E & C were way bad. After checking the transistors on one side showed they were good I found all four 470 Ohm fusing resistors were trashed with heat related damage and reading way too high! (12K on one side and 64K on the other). So the rail voltages were trashed right after these resistors. All other components tested good up to this point so I'm going to get a set of resistors and recheck voltages again. Then test with a TT. Will post what I find.
     
  13. llwhtt

    llwhtt Super Member

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    Damned fusible resistors!
     
  14. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

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    08EE473F-2C04-4824-B184-67D9ED4710FC.jpeg
    This looks like metal film to me. Any special feature I should be looking for? Metal film vs wire wound?
     
  15. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Yes, Metal Film - definitely NOT wirewound.
     
  16. kevzep

    kevzep Its all about the Music Subscriber

    No, those are carbon composite, just slip a metal film in there.

    By the way it would be worth going over the regulated supply board, the one I did, had many resistors that drifted and were causing the voltages to be off....
    I meant to mention that before...
     
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  17. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Actually the fuse resistors as shown 'lifted' in the picture are Film resistors, the rest of the resistors in that picture are Carbon Film.
     
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  18. kevzep

    kevzep Its all about the Music Subscriber

    Oh, my mistake, so they are......Copy that!!
     
  19. jd3rdcoast

    jd3rdcoast AK Subscriber Subscriber

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    Yea, ticks me off as have to pay $7 shipping from Mouser for pennies part. Oh well, at least here in Houston area it's one day UPS ground from their warehouse in Dallas area. Maybe will put a cap list together for one of the 2245's I been wanting to recap.
     
  20. rcs16

    rcs16 Super Member

    Messages:
    4,062
    Location:
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    Wish my shipping was only $7
    I always buy >= $100 worth of parts to get free shipping to Canada. I just ordered parts to fix a Yamaha RX-900/U, it needs about $30 worth of parts, to get it going again. I have stocked up on transistors that go in and out of stock, added common ecaps that are used on most stuff. usually buy 10 instead of 1 since you can get better pricing in some cases.
    I'll get the parts on Tuesday, it is fantastic service compared to the older days.

    It amazes me I get parts from China, free shipping HK post, it takes a month, okay if you are in no rush.
    I guess if Mouser offered USPS it would be cheaper, ask them who knows, they are good people to talk to.
     

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