Want to salvage a Sansui 5050.

Discussion in 'Exclusively Sansui' started by imation_nation, Dec 14, 2018.

  1. imation_nation

    imation_nation Member

    You May have seen me posting here regarding a 9090db. It appears that it needs a Dolby board renovation. Before I do that , before I jump in the deep end of the pool, I thought I should perhaps try resurrecting my dads 5050. Something presumably a bit easier. LBPete suggested I start a new thread so as not confuse the two works in progress.

    This is what I know about the 5050 as of now.

    After powering on, the panel lights come on, the tuner appears to be working.

    There is no sound, no output.

    I do not hear or see the relay click. So I think that means it does not come out of protection.

    At the input to the driver board, I have a good signal on one channel and on the other, the signal is about half and is floating on about -1.5 volts. The good channel has no dc component to it.

    When I remove the input connector to the driver board, there is zero volts dc on the input pin of the good channel. There is -4volts or so on the input pin of the driver board for the bad channel.

    There looks to be a good signal on the bases of the final power transistors of the good channel. No real signal on the bases of the final power transistors of the bad channel.

    I am injecting a 1khz signal onto the aux port and using a scope and DMM for measuring.

    Any thoughts, guidance would be most appreciated. I am putting together an order to replace the electrolytics. I am hoping to get an idea of what has failed so I can order any other replacement parts along with the electrolytics. I am going to replace the fuse resisters also, although they seem to be ok at this point.


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  2. Bob

    Bob AK Subscriber Subscriber

    West coast
    over due for the recap - unit dates back ~50 years.

    download service manual, follow instructions for the biasing.

    determine where the signal drops, use FM/Aux/etc as source and use
    tape monitor to another amp isolate where. if there's a preamp output
    then use that. you want to isolate down to line amp or power amp.

    check schematic to see if headphone circuit is a tap off the power amp
    or if there's a separate headphone amp. if not, then get inside
    and look at the heaphone jack, when phones are plugged in, they interrupt
    the power signal to the speakers.

    I'd also look into the power amp and the protection circuit, you can clean
    the contacts or replace the relay.

    if you have a scope, send a mono signal into the power amp, and look at
    the waveforms as the signal progresses through, you have the good
    channel to compare it to.

    the TCKs will scream at this, but for a temporary test bypass the protection
    circuit: use clips on probes to grab the signal and pass it to cheap speakers.
    that will determine whether its before (in the driver stages) or after at
    the relay.

    at quiescent, there shouldn't be any voltages (DC or otherwise). something
    is wrong if there is. think of a pure AC (music) signal coming in, can't or
    shouldn't be floated up or down by a DC component.
  3. imation_nation

    imation_nation Member

    Thanks for the suggestions. I have made copies of the service manual. For signal source, I am using a CD player with RCA outputs plugged into the aux port. I made a CD with a thirty minute 1khz tone and just play that CD when I need a signal. I do have a scope. It is old HP 1746A, but works perfectly well.

    I had not thought of it, but I have my Pioneer HPM100’s connected to the stereo. That is probably a bad idea. Can I still troubleshoot with nothing connected to the outputs? I do not have another set of speakers.

    I definitely have multiple problems. I am loosing about half the signal as it goes through the F-2646 tone control circuit board. I am going to look in to that this afternoon a little further.

    There is definitely a problem with one of the channels on the F-2647 driver circuit board. I think The other channel is good so I have something to compare with.

    I am hoping the output transistors have not failed, but I need to check those closer.

    One thing I noticed is that the two large filter caps do not bleed off after I turn the unit off. I don’t know if that is normal or if that indicates some sort of problem.
  4. UncleBingo

    UncleBingo AK Subscriber Subscriber

    You can do almost all of the testing without speakers hooked up on soiled state gear.

    Also, you can measure DC offset at the relay inputs (measure with selector on aux; no signal). If you have a high DC voltage there, you have roasted output transistors.

    You can also check for dead shorts between the output transistor cases and ground with the power turned off.
  5. kevzep

    kevzep Its all about the Music Subscriber

    If the output transistors were blown it would blow the mains fuse or the rail fuses.

    Do not connect speakers to it whilst there is 1.5VDC on the output, but then I would have thought the protection would turn the speaker relay off, so if this is happening you will need to monitor the DC offset before the speaker relay.

    Solid State amplifiers don't care if they do not have a load on them. In fact in a questionable state, its best NOT to put a load on it.
    UncleBingo likes this.
  6. imation_nation

    imation_nation Member

    I found one of the problems. One of the channels was about half the other at the input of the driver board. Any guesses?? The balance knob was not centered. :dunno: Ooooooooopsies!!! Smacked my head, then laughed. :D

    So off I go to the driver board.


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  7. imation_nation

    imation_nation Member

    I checked the voltage at the relay inputs. I do not have a circuit board diagram for the board I have so I went off the schematics. So r610 and r611 are connected to each other and on either end is essentially the inputs to the relay. I think. This is what I got.

    R610. R611

    .01v -8.57v. -30.5v

    From your post, I take that to mean that one channel has roasted output transistors?
  8. UncleBingo

    UncleBingo AK Subscriber Subscriber

    Not sure why you have 3 readings. You should be reading with the black meter probe grounded and the red meter probe at the relay signal input coming from the output transistors.
  9. imation_nation

    imation_nation Member


    The formatting did not come out right

    So that diagram was meant to show two resisters r610 and r611 joined in the middle. The three voltage reading coincide with left side of r610, the shared connection of r610 and r611 and then the right side of r611. The left side of r610 is one of the inputs to the relay. The right side of r611 is the other input to the relay.

    Sorry for the confusion. This should be better


  10. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Hertfordshire, UK
    Are you sure you are making measurements on the F-2648 PCB ? - which on this model seems to accommodate the PSU, Protection Circuitry +relay, and the Output Transistors and their associated emitter resistors (R05, R07 & R06, R08). :)

    NB: The service manual is available on HiFi Engine here:-
    Last edited: Dec 15, 2018
  11. imation_nation

    imation_nation Member


    Sorry bout that, I should of mentioned my 5050 has the F-2701 circuit board instead of the F-2648. Although curiously, the output transistors on mine correspond to the output transistors on the schematic that shows the F-2648. According to the schematic with the F-2701 board, the output transistors should be 2SD188A/2SB545, but the output transistors on mine are 2SC1445/2SA765 which corresponds to the schematic that has the F-2648 board shown.


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  12. skippy124

    skippy124 AK Subscriber Subscriber

    Sydney Australia

    R610 is the left channel, and R611 is the right channel. From your readings, you have 0.01 volts on the left channel output, and -30.5 volts on the right channel output. Output transistors on the right channel are suspect, and/or other components in the right channel driver circuit.


  13. imation_nation

    imation_nation Member


    Thanks for your advice. I will check the output transistors. Perhaps compare right to left channels.

  14. imation_nation

    imation_nation Member

    I did some checking on the output transistors. I think I have a bad one.

    I pulled the driver board first. Then used my dmm to check the junctions of the transistors. Here is what I got.

    Pair 1
    ———2sc1445, npn, TR01?
    —————- BE, .536 **** BC, .530 **** EC, OL

    ———2sa765, pnp, TR03?
    —————- BE, .549 **** BC, .534 **** EC, OL

    Pair 2
    ——2sc1445, npn, TR02?
    —————- BE, .541 **** BC, .532 **** EC, OL

    ——-2sa765, pnp, TR04?
    —————- BE, OL **** BC, .536, **** EC, OL

    1) I think that pair 1 is TR01 and TR03. Pair 2 would be TR02 and TR04. Not 100% on that.

    2) This 5050 model has the F-2701 Power Supply Board, HOWEVER, the output transistors are 2sc1445/2sa765’s. The schematic I have with the F-2701 Power Supply Board indicates the output transistors should be 2sd188a/2sb545’s. Don’t know why.

    3) I know Sansui typically used odd numbered components to denote one channel and even numbered components to denote the other. Can someone tell whixh was which? Even = Left Channel or Odd = Right Channel.

    4) Assuming that 2sa765 has failed. Is it possible to get a replacement for that anymore? Would it need to be matched to the 2sc1445 it is paired with?

    5) I am working on testing the transistors on the driver board as I have a hunch something has failed on that too.

    Any input is greatly appreciated.
  15. kevzep

    kevzep Its all about the Music Subscriber

    Normally transistors short-circuit, although I have on the odd occasion seen one go open......Not the typical failure mode here.
  16. imation_nation

    imation_nation Member

    One more peculiararity for the day.

    I was checking resistance on some various components on the F-2701Power SupplyBoard and noticed that the resistance across F604 (250v, .15A) was 28 ohms and the resistance across F605 (250v, .15A) was 58 ohms. I got the same reading when I pulled them out of the holder.

    I would expect that to be zero? Can I assume those are both bad?


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  17. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Hertfordshire, UK
    Yes, fuses do degrade over time, and it looks like these have rather badly. However fuses do have resistance, for a 0.15A fuse, I'll guess at ~2Ω resistance. Best to buy replacements anyway - they should at least be the same resistance, and with resistance that high you may solve some of your problems if you change them. ;)
  18. imation_nation

    imation_nation Member

    Does anybody have any suggestions on where to procure a matched set of 2SC1445/2SA765 output transistors. I have a bad 2SA765. I found someone in the Netherlands who does primarily Vintage Luxman stereo gear who has new matched pairs for $35 plus shipping. eBay has some for $35 from parted out 5050.

    Is that about what one would expect to pay?
  19. imation_nation

    imation_nation Member

    Hi all,
    I was having mixed results testing the transistors. I was doing most of them in circuit so not sure how accurate that is. I decided to write down the voltages at all of the transistors from the beginning of the F-2647 Driver Board all the way through to the Final Output Transistors. The problem is no output to the speakers from right or left channel. Signal tracing indicates a good signal all the way through on the odd numbered channel (not sure if that is right or left). On the other channel, the signal gets wonky once it hits the driver board input.

    I would be interested in anyone’s comments based on the above and the B, E, C voltages of the transistors below. I am not sure what they mean. My is guess is many of them are shorted on the even numbered channel.

    Also, I understand from reading other threads that the 2SA726’s on the F-2647 board are in the top ten list of Transistors likely to go bad.

    Thanks in advance.

    Differential Amp Transistors
    E .590
    B .017
    C -36.4

    E .589
    B .024
    C -37.2

    E -27 to -28 jumping
    B -28 to -29 jumping
    C -31.3

    E 27 to -28 jumping
    B -19.4
    C -32.5

    Predriver Transistors
    E -37.2
    B -36.5
    C -1.2

    E -32.3
    B -31.5
    C -32

    Driver Transistors
    E .633
    B 1.2
    C 39

    E -.63
    B -1.2
    C -41

    E -30.4
    B -30
    C -39

    E -31.4
    B -32
    C -41

    Bias Transistors (I think)
    E -1.2
    B -.54
    C 1.2

    E -31.9
    B -31.3
    C -29.6

    Final Output Transistors
    E .028
    B .623
    C 32.6

    E .011
    B -.631
    C -32.5

    E -30.6
    B -30.1
    C 32.6

    E -30.6
    B -32.4
    C -32.5
  20. Hyperion

    Hyperion Roobarb & Custard Subscriber

    Hertfordshire, UK
    It's up to you, but I would never buy semiconductor pulls from units being parted out - you are paying a lot of money and have no guarantees you are getting good parts. The power transistors in this unit are unexceptional, so you have no need to pursue originals.

    Much better to replace the OP transistors with modern equivalents, Like this to replace the 2SA765 (PNP)

    And this, to replace the 2SC1445 (NPN)

    These new replacements are cheap and are very robust, if you make a mistake and accidentally destroy one, easy to get and cheap to replace.

    NB: I recommend you replace all four OP transistors.
    Last edited: Dec 19, 2018

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