Phono amp problem 1000A

Discussion in 'Exclusively Sansui' started by larryderouin, Aug 8, 2017.

  1. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    I put my 1000A back in rotation last week. While checking it, I had no sources connected to the back and cranked the volume to 11 listening for hum, crackling, motorboating, hiss, etc in all functions except FM/AM. It was quiet in MIC, and AUX/Tape Mon. However in Phono or TAPE HEAD, it now has an inordinate amount of HISS in the Right channel and a rythmic thrumming sound at about 2 times a second in the left Channel.

    When I restored the unit back in 2009, I recapped the phono board and replaced a few resistors on each side that had gone out of spec. The transistors on the board are 2sb381's which are germanium. The only cross ref I can find is to an SK3004 or a NTE-102A. And honestly, I wouldn't buy any of these off E-PREY if they were the last transistors on earth.

    Is there a Silicon pnp that would probably work? I haven't been able to find anything even remotely resembling a 2sb381 datasheet in the last week.
     
  2. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    Unfortunately Larry (as I am sure you are aware), this would require the phono stage to be redesigned to accommodate the different characteristics of the silicon devices, maybe someone has already done this? - I am sure the issue has come up before.

    How about buying some NOS transistors from a reputable seller, known to deal in such devices?

    Like this for example http://markhindes.easywebstore.co.uk/Germanium-Transistors_B2RHQ.aspx - perhaps you know of a similar vendor closer to you geographically?

    From that site 2SB495 (p13) or 2SB116 (p14) seem to be close.

    OC73 (p19) seems to be very close.
    http://alltransistors.com/transistor.php?transistor=44595

    2SB383 (p33) listed as a direct equivalent to a 2SB381 (by Alltransistors) - but alas they only have 1 in stock.
    http://alltransistors.com/transistor.php?transistor=10243
     
    Last edited: Aug 8, 2017
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  3. kvflyer

    kvflyer AK Subscriber Subscriber

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    I understand your concern about th hiss which could very well be coming from the germanium transistors. But how much higher than the "floor" is that noise? Is it loud enough to hear during a rather quiet passage in classical music?

    It might be lower than your typical listening level and therefore maybe not all that objectionable. Of course I am not listening to your receiver and you may well find it unpleasant.

    Please, let us know your solution.
     
  4. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    The hiss only raises it's ugly head when no source hooked up and volume is advanced to full. It's not any more than say my 800c with the controls set the same. Shorting plug does cut it down some. And this is RIGHT CHANNEL ONLY. At normal listening levels it's non existant.

    The Motorboating is in the LEFT Channel ONLY, with the Bass set to center or more, Volume is @ 9:00 or higher, Loudness and presence on or off or any combination of on and off. Engaging the Low Filter will kill the motorboating, as will decreasing the bass to 9:00 and then it comes back in @ 11:30 or so.

    Turntable is 2 ft away from the receiver, ground cable is attached to the receiver chassis (no hum @ full volume, turntable not running but on). Speakers are Minimus 7's about 5ft away ft turntable on Right side and 4 ft away from the receiver (about 6.5ft away from Turntable).

    Turntable is a FISHER Branded BSR (Don't laugh it runs better than most Garrards I have!) with an adjustable tonearm, and a Pickering P/AC-1 cart and a Elliptical stylus. I've gotten the same results with a Technics SL-1200 with a Shure V-15-III and an Elac Miracord 50 with an Empire 2000E.

    I found 2sb382's on your linked site John. I looked at the info on All transistors and this is what I came up with. It looks like the gain is a little higher but probably not anything to really worry about as it more than likely won't be noticeable.

    The periods between columns are to maintain formatting.

    ↓ Type........Mat....Struct....Pc.....Ucb...Uce....Ic.....Tj........Ft... Hfe....Caps

    2SB381.... Ge.... PNP..... 0.27.... 32.... 30.... 0.3.... 85.....0.6.... 45.... TO5
    2SB382.... Ge.... PNP..... 0.27.....32.... 30.....0.3.....85.....0.7.....85.....TO5

    I'll order a pair of the 382's.

    Thanks Guys. I'll let you know how it all turns out.

    Larry
     
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  5. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

    Messages:
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    As I could only get 2 and the board requires 4 total, I figured on one per channel in the 1st position from the input's. Then if the motor-boating stops on the LEFT SIDE, replace the remaining transistor on that side with one of the originals from the 1st position. Leave in whichever sounds best. Then start looking for some more 2sb381's. NTE-102A's are supposedly replacements for the 381, but only as a last ditch effort. The RCA SK3006 may be a better solution however I treat them with about the same amount of disdain as I reserve for NTE's. Any comments on the RCA's?????
     
  6. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    Well I've got 2 2sb382's enroute from the UK. I went hunting tonight and found BDENT.com had 164 of the Toshiba 2sb381's. Ordered 10, so the board will get a full complement of new 2sb381's when they come in. The rest will be put "on ice" in reserve. Crazy thing is the 2SB381's @ BDENT are $0.53 ea. 11.50 for the whole thing incl shipping. And I paid $15.00 with shippng for 2 from a suburb of London. Windsor castle may not be finished renovations after the '92 fire, and we're paying for it! LOL!
     
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