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

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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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  7. rcs16

    rcs16 Super Member

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    I am not sure why you dump so hard on NTE parts, is this from experience or is it hearsay? I have no experience using older RCA Ge type parts.

    With a little work in ltspice simulation, it is possible to re-design or at least verify that the gain stage using modern day low noise Si bjts (ksa992) instead of older Ge types might work fine as it is. Worth a try I'd suggest.

    Based on the sch from hifiengine, the bias V on TR1,3 does not look right if they are indeed Ge types. Since the 2 stages are AC coupled I think that a KSA992 could quiet possibly work fine as it is.

    I see in this thread know one came to a conclusion as to using ksa992 as a sub
    http://audiokarma.org/forums/index.php?threads/sansui-tube-receiver-phono-section.687682/
     
  8. larryderouin

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

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    rick; I've had bad experiences with NTE Transistors in 3-4 different amps and receivers including a Sansui 2000A. Their resistors are ok, but their caps tend to be older than 3-4 years old I've found (at least the stock that was at the now defunct Electronics store in B-more). NTE Products also tend to be a hell of a lot more expensive than other brands too. For instance a pkg of 4 1/2w resistors would go for $3.99, while the same type, value and voltage resistors could be had for as little as $0.10each on mouser. A UA703E OP AMP was like $7.00 4 years ago. Now it's damn near $20.00 at some places although if looking hard enough you can find it for as low as $5.50. Some FISHER Tuners use 4 of these. And they are only going up in price as they are the only place to get them, and they know it. Same with most germanium's and older silicons'. Some Silicons' go upwards of $10-15. while germanium output's are $35.-$40. each. The NTE 121 comes to mind. IT supposedly replaces the TR35144 used in a lot of FISHER consoles from 1964 to 1966. $todays prices range from $27.50 to $39.99. Yes this is a germanium transistor.

    Like I said. For the most part, NTE parts (transistors) would be last resort. RCA SK's after some further research should be a couple orders of magnitude better, but as sensitive as germanium output's are in the 1st place, it's not saying much. 99% of germanium powered gear I have is FISHER and they pushed them to the limits and beyond.

    Now back to the current problem. I received 10 Toshiba 2sb381 transistors today in the mail from BDENT. So I have enough to populate this Sansui's phono amp twice with a 1/2 complement for spares. I CAN do this as a test. Populate one channel with the 2sb381's and the other channel with KSA992's that have been matched for hFe. I'll match the 2sb381's also. Then check for noise floor using my ears. I don't have any kind of analyzers, just a scope which I don't know how to use, a SG-165 I barely know how to use, and a few multi-meters. Grounding plugs, all controls centered or off, Set Selector to Phono, and crank the volume to 11:biggrin:. Then listen for any anomolies, and how quiet the channels are. Then go with the transistor that shows the best results. I may just put 2sb381's in the 2nd channel tho, and keep the KSA992's for when/if I run out of 2sb381's in the future. This 1000A is a Daily driver in the bedroom now that I replaced all the RCA's on the back (flared originals were causing intermittents with newer RCA cables with standard length center probes. The old longer probes never had a problem at the plugs but the wire/casings were breaking down.). It's still got the tubes I put in it 9 years ago, and the output's are holding up quite well, with 100ohm screen stability resistors, 10 ohm cathode samplers, 4 pot bias circuit (originally 2), and replacement of the tall power resistors with Aluminum bodied power resistors bolted to the chassis in various places. Overall the heat output is approx the same but is NOT concentrated in the back at the transformers and output tubes. So which ever side sounds better, I'll probably go with that setup.

    Larry
     
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  9. rcs16

    rcs16 Super Member

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    Hi Larry,

    Good to hear of your experience using NTE parts. I have only used them once in my Pioneer SX-950 ( PA drivers) now over 20 years ago and they have stood up for me. I used them when they were relatively cheap and did not know of better alternatives.
    I agree completely that NTE is a last resort (for semis) if you fail to find anything else and they are over priced as you say. I can not see why one would need to use any NTE passives, since there are so many better choices from regular mfgs/distributors.
    Using your ear as a noise tester is fine.
    uA703 is a very expensive and hard to find IF limiting amplifier. National semi also made the part as LM703L. It is used in the Crown FM1 &2 tuners that I have.
    I do not see why you need to hfe match bjts for the phono section, usually matching is required for differential pairs (lowers DC offset) and parallel pairs( current sharing) The phono amp is a 2 stage singleton ckt with AC feedback
    Up to you to use original 2sb381's if they are genuine and meet your noise test requirements. Would be nice to try out ksa992 as a comparison if you have the time and do not suspect to damage the pcb from multiple repair.
    Looking at the 1000A design construction, it is old school, with lots of point to point wiring, definitely a challenge to repair to say the least, good for you that you have taken the time to modify and re-build the unit as it is a daunting task that I have never tackled myself.

    Cheer
    Rick
     
    Last edited: Aug 19, 2017
  10. larryderouin

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

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    BOY DID I SCREW UP! I ordered 2sc381's instead of 2sb381's.:thumbsdown::no::whip: So much for doing something right when tired. I realized it when I opened up the phono boards from the mount today. So I'm gonna go with KSA992's for now and test them. Apparently I had put a pair of NTE 102A's in one of the channels way back when but had forgot about it. The other has the original Sony's. As the phono circuit is a board mounted circuit (actually 2 separate boards)I'm gonna replace the NTE's and then power it up. I've got a 50-50 chance of it being the left channel. Then I'll hook the phono up to it and give it a whirl. If everything is copasthetic, I'll do the other side.

    Larry
     
  11. larryderouin

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

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    Ok. The KSA992's are in. Hooked up the turntable on both phono inputs and got a boatload of static and noise above and beyond the noise floor. Ground is conncted. This is connected to a SL-1200 with a V15-III. I took a shot after looking at the tape head connection to the preamp and then connected to the Tape Head. Beautiful MUSIC! No STATIC or NOISE! A bit more gain, and the attendant noise from the record, but I can tone it down with the high and low filters. Background noise floor with the turntable off is about 1/2 of it was before. Gain has increased a bunch tho. 2:30 on Volume before is now replaced by 11:30-12:00. Best operation is with PRESENCE ENGAGED, Tone Filters off, Loudness off, Bass @ about 10:00 and Treble about 1:00. Gonna look at the RCA's on the PHONO's later. I don't see any difference between the phono and Tape Head RIAA inputs between the jacks and the base of TR001. So it's probably in the wiring. It's possible that I compromised one of the shielded cables.

    One of the NTE102A's was shorted B-C.
     
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  12. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Those of us that relied on germanium transistors for our guitar effects pedals have lamented the dearth of those transistors for years. All is not lost however, if you spend time looking up classic pedals like the fuzz face and ts-88 you will find there are a number of manufactures in that industry remaking those transistors again. Conversely the ksa992 might be more than adequate and probably quieter overall though you might have to adjust the turn on voltage a bit.
     
  13. Overundr1

    Overundr1 AK Subscriber Subscriber

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    Lol you posted while I was typing :)
    oops
     
  14. larryderouin

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

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    The RCA connectors on the original cables for the SL-1200 (original servo version) are marginal. I could re-create the static in phono on the TAPE HEAD selection by twisting the connection. I replaced all of the RCA inputs on the 1000A 2 years ago and have had numerous phonographs connected to it and had no problems with them. So I'm off to get new cables for the Turntable. I DID get the 2sb382's from the UK y'day. So what I'm gonna do is measure the hFe on them against the hFe on the Sony 2sb381's and see if they are somewhat close. If they are within 50 I'm gonna put the germaniums back in and look for another pair of 382's. I don't think the Sony's are bad, but they are getting a bit long in the tooth. The traces are getting quite brittle as 2 of the holes broke off on different components. Luckily the leads are long enough to bend over and solder bridge with the adjacent part as original.

    The gain on the ksa992's is just too much for the phono. Too sensitive to changes on the Volume controls and Tone controls. Noise floor is about the same, but the pops and other generated noise from the LP's is quite agravating. So the germaniums' go back in.

    I think if we found a silicon with less gain than the KSA992 it would probably be a good fit. Something with about 1/4 the hFe. 80-100 would be good.
    hFe below is on installed parts:
    hFe on the Sony 2sb381 is around 85. The NTE 102A I pulled that was working was 92.
    hFe on the Fairchild KSA992 is 320 for the quad used (of the bunch I tested between 300 and 350 with 2 pair of 320).
     
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  15. Oldsansui441

    Oldsansui441 AK Subscriber Subscriber

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    Thanks for the info Larry, this is great to know not only for the 1000a but any other unit where the KSA992's might present a problem with the high gain.
     
  16. rcs16

    rcs16 Super Member

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    Hi Larry,

    I guess that the phono pre-amp has to simulated to use the ksa992 instead, one day when I have more time in the winter I'll try it out.
     
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