Sencore SG-165

Discussion in 'DIY' started by RocketMonkey, Apr 1, 2017.

  1. Banjo800

    Banjo800 New Member

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    24
    I've just spent a little time in getting my SG165 in working order. Ended up rebuilding the Output Buffer section and the FM RF sections. Seems to be in good shape now. I posted a lot of pics of the calibration process over on the Antique Radio forum. If you're interested.....http://antiqueradios.com/forums/viewtopic.php?f=8&t=320139&start=60
     
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  2. Steven Tate

    Steven Tate AK Subscriber Subscriber

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    All of my stuff is in storage right now due to moving. But I'm over there a couple times a week getting things we need. I'll get that measurement ASAP.
    Steve
     
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  3. amptramp

    amptramp Active Member

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    360
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    Mississauga Ontario Canada
    One bit of advice for this: test and fix everything you can find at once. The mechanical design is like building a ship in a bottle and you don't want to do anything more than once. The regulator board is the only thing that can be accessed easily and it will need upgrades. Beware of the silkscreening error that reverses one of the electrolytics on the regulator board.
     
  4. RocketMonkey

    RocketMonkey Active Member

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    Location:
    Kansas city area
    I didn't really find it that hard to work on. Just a couple knob screws where super crazy tight.
     
  5. RocketMonkey

    RocketMonkey Active Member

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    Location:
    Kansas city area
    Ok for sanity sake, and perhaps the benefit of others here, could somebody post the colors of the clips for the probes? At one point I found a picture of the schematic with them listed but I'm not seeing it now.

    Also update on setup, I finaly got around to doing some measurments and have found that my output is indeed very low on the bands that I can measure anyway(up to the FM IF with a 60mhz scope, could not lock on 88 mhz FM). I intend to adjust it but it is so low I bet there is a problem. My detector is not picking up anything on FM output so I think I'll swap out the supplied diode for a 1n270, the supplied diode had more Vf than my 1N270s.
     
  6. xwarp

    xwarp Active Member

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    411
    Location:
    Southern AZ
    [​IMG]
     
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  7. Banjo800

    Banjo800 New Member

    Messages:
    24
    If you are trying to lock in the FM RF with a frequency counter, it won't do it. There is a max of 10mv RMS on the fm out. Most counters will not lock in below 50 or so millivolts.

    Also to read the voltage correctly will require an RF millivolts meter or a 100 MHz min DSO O'scope that measures voltage. I had a heck of a time with that until I got it sorted out.
     
  8. RocketMonkey

    RocketMonkey Active Member

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    Location:
    Kansas city area
    I was using a 60 mhz scope, not a counter. I looked up the specs on my other scope and discovered its 100 mhz(for some reason I was thinking 70 mhz). I don't use it much because its got some broken controls, stuck on 10 to 1 and its big.

    So with the 100 mhz scope I was reading around 5mv peak to peak using a 50 to 75 ohm adaptor amplitude still rolls up and down accrost the spectrum like its poorly matched.(come to think of it the scope does not have a 50 ohm input duh). 10 mV RMS should be 28ish mV peak to peak so its way off anyway. The scope was not locking on very well and the wave was round but not right, reflection perhaps? Dunno how much I trust the scope being that the trace is unstable. Need to try again with a 50 ohm terminator see how that looks. Looking at the spectrum analyzer the frequency is fairly accurate, stable and doesn't seem to have much harmonics if any. My analyzer is rather low end so dunno how sensitive it is in the bandwith it was set to. I was getting around -40 dbm on the analyzer so if my math isn't totaly wrong thats 2.2mV RMS so 6ish mV p to p. The analyzer is 50 ohms for sure so I'm fairly certain output really is low. Turning up the adjustment pot didn't raise it much, didn't measure the difference, 20 or 30% I'd guess. The frequency drifted off my cursor when I adjusted the amplitude and controls are clunky. Anyway I need to check the resistors, see if drift has killed off gain or something. That or aged transistors, I ordered some replacements.
     
  9. Banjo800

    Banjo800 New Member

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    24
    Lots of things to go bad in the RF circuit. There is one electrolytic cap, check that also. Mine was nearly double in value. Remember that it's a 75 ohm generator, but the calibration guide makes no mention of using a termination resistor. Use RG-59 cable, between 36 and 44 inches. The length of the cable will affect unterminated voltage readings. Also, it will change from 88 to 108 MHz. I shot for 10mv at 98 MHz on an unterminated 36in cable. IIRC it was around 12-13 at 88 mhz and 8 or so at 108mhz. Terminated at 75ohm, my scope read 8.8mv, I think.
     
  10. RocketMonkey

    RocketMonkey Active Member

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    255
    Location:
    Kansas city area
    So tonight if I get the chance I'm going to retest the output and actually measure how high I can get it with the adjustment turned up, properly terminated on both the scope and analyzer (it would be nice to know if the scope voltage is reasonably accurate so close to the edge). I don't have a 75 ohm load but I do have a 75 to 50 ohm adaptor so I'll use than and terminate at 50 for the scope. My transistors are in the mail so I think Ill pull the transistors and test their gain while I have the board lifted. Ill check the resistors also, seems resistor drift is fairly common on these. The electrolytic caps are all new.
     
  11. Steven Tate

    Steven Tate AK Subscriber Subscriber

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    Location:
    Burleson, TX
    Sorry for the delay. Just haven't had a chance to go to storage until today. If you still need the measurement, that cable is 59" long and is clearly original.
    Steve
     
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  12. Banjo800

    Banjo800 New Member

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    24
    Awesome!!! I'm gonna make up a cable now!
     
  13. 70'sredo

    70'sredo AK Subscriber Subscriber

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    17
    xraytonyb on Youtube. Watch the "probes and answering questions" video. He shows how to build the Pad,and the demodulator probes. You don't need the one that let you hook up to a car radio.The dummy speaker loading alone is worth the price.
    He also has a video which goes through every function of this unit. This thing should rightfully be considered a classic. It was built for small shops doing stereo repair.
    There's a few pundits whom have bitch points about this thing. Yes, the RF Frequency dial shaft breaks. But, it's an easy fix Yes, the FM sine output is a bit weak, but only if you try driving a cheap frequency counter. Even a couple of weak/leaky caps if you're really not having the luck.To me, it's a beautiful kit. Whatever work you might have to do in order to get it back to snuff, the freakin instruction manual is excellent for
    helping you meet that goal.
    The prices on the site have become crazy. I was lucky enough to get one for a decent price.
    I REALLY like ths thing.
    It WILL be considered a classic.
     
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  14. Westy56

    Westy56 A symphony of paradox

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    Me x2 :cool:

    [​IMG]
     
  15. smurfer77

    smurfer77 AK Subscriber Subscriber

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    Location:
    San Francisco
    Thanks for pointing out this youtube guy. I just checked both his videos on the SG-165 and they are really useful for me.
     

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