Sencore SG-165

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

  1. xwarp

    xwarp Active Member

    Messages:
    388
    Location:
    Southern AZ
    I've posted the following before, but will do so again.

    I have two of these units. The 1st was an ebay buy that I kind of got the shaft on as the unit was sold as "worked the last time it was used", but showed up without the power supply. I ended up keeping it because the cost of shipping it back was as much as the generator was worth in parts.

    I ended up building a power supply board and recapping it. I also replaced the rca jacks on the front with bnc's. I also went through and pretty much replaced EVERY resistor in it because they were all marginal out of tolerance.

    The 2nd unit is in perfect shape and I've not done anything to it yet.

    Here is a video that might help you in building a probe:

     
  2. RocketMonkey

    RocketMonkey Active Member

    Messages:
    255
    Location:
    Kansas city area
    Well the electrolytic caps are replaced. Now its on to the probes. Hopefully I'll order any parts I don't already have tomorrow. Thus far its been smooth sailing. I didn't measure it but the output on FM is very low. My spectrum analyzer was picking up a fairly distant radio station louder than the FM output on full blast. I guess it doesn't matter if its enough to come in clearly. Perhaps I'll look at the manual tomorrow. Oh yeah, I found two more incorrect markings on the silkscreen. C7 and C4 on the power board if i recall correctly.
     
  3. xwarp

    xwarp Active Member

    Messages:
    388
    Location:
    Southern AZ
    You need to check the output value against the value stated in the manual.
     
  4. RocketMonkey

    RocketMonkey Active Member

    Messages:
    255
    Location:
    Kansas city area
    Thats the plan tonight after work.
     
  5. nerdorama

    nerdorama AK member Subscriber

    Messages:
    2,234
    Location:
    Seattle area
    :lurk:
    Signing up to watch since I've got one of these sitting on the shelf.
    Thanks for posting your work.
     
  6. RocketMonkey

    RocketMonkey Active Member

    Messages:
    255
    Location:
    Kansas city area
    Im getting -28.5 dbm at 10.7 mhz full power going into a 50 ohm spectrum analyzer and -55dbm at about 108mhz. I need to build an impedance matching pad now I guess.
     
  7. RocketMonkey

    RocketMonkey Active Member

    Messages:
    255
    Location:
    Kansas city area
    Ok so now I have a halfway half-assed matched setup. Im getting -50 dbm ish at the bottom and it drops fairly smoothly to -60 dbm as you roll up toward the top. The 10.7 is still stronger than broadcast at about -34 dbm. I don't have the correct resistors so its definitely not perfect. The prior test, bnc straight into the analyzer swept down from 86mhz to about 103.3mhz then started back up for the last couple mhz. I need a matching pad that I trust now lol.
     
  8. RocketMonkey

    RocketMonkey Active Member

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    255
    Location:
    Kansas city area
    Waiting on parts, meanwhile... 20170414_210226.jpg
     
  9. RocketMonkey

    RocketMonkey Active Member

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    255
    Location:
    Kansas city area
    Ok I think tomorrow I'm going to start on the probe and then the impedance matching pad. I do have a question about the matching pad, it uses a 20uF capacitor. That seems huge for such a circuit does it not? Also being all RF what type should that be? The schematic is posted on the first page.
     
  10. xwarp

    xwarp Active Member

    Messages:
    388
    Location:
    Southern AZ
    I've been looking in to the question about the 20uF cap being used in the matching pad but I just don't know as much as I'd like about RF to be able to answer your question as to why it's used.

    I have though found some information regarding matching pads that this might be of some help:

    http://www.ham-radio.com/k6sti/match.htm

    I don't recall reading what the 165's output impedance is though.
     
  11. RocketMonkey

    RocketMonkey Active Member

    Messages:
    255
    Location:
    Kansas city area
    Yeah kinda throws a wrench in the equation. The pad creates a 75 and 300, i guess i can hook it up to the scope and try some different loads but I dont think my Hitachi is fast enough for FM band. Previously I had assumed it was 50 but I'm not sure. Math :(
     
  12. xwarp

    xwarp Active Member

    Messages:
    388
    Location:
    Southern AZ
    It's apparently 75 ohms.
     
  13. RocketMonkey

    RocketMonkey Active Member

    Messages:
    255
    Location:
    Kansas city area
    Guess I need to RTFM. I thought you where suppose to use the matching pad for 75 ohm and 300 ohm. Got tied up and didn't even go into the dungeon.
     
  14. catrafter

    catrafter Marantz Specialist Subscriber

    Messages:
    5,750
    Location:
    Montana
    I haven't used an SG-165 for years, but IIRC there can be audio going out the 'all signals output' and through the matching pad, depending on the settings.

    Tom
     
  15. RocketMonkey

    RocketMonkey Active Member

    Messages:
    255
    Location:
    Kansas city area
    So ending my hiatus on the thread, I've built a detector probe (from a kit on ebay, I wasn't feeling building one from scratch) and built an am/fm radio kit that came with alignment instructions. I was all bent out of shape yesterday messing with XY mode and IF sweep, and not having any success. Tonight I discovered the sweep out jack on the back of the 165. One of my scopes does not appear to have a line sweep and the other was doing crazy things. Tomorrow I shall try to get that rolling using the sweep out on the generator.
     
  16. RocketMonkey

    RocketMonkey Active Member

    Messages:
    255
    Location:
    Kansas city area
    I don't know how I missed this, its got 400 hertz sine and square out on the all signals out line, duh.
     
  17. RocketMonkey

    RocketMonkey Active Member

    Messages:
    255
    Location:
    Kansas city area
    Steven Tate likes this.
  18. Steven Tate

    Steven Tate AK Subscriber Subscriber

    Messages:
    1,888
    Location:
    Burleson, TX
    I have an SG-165 that I want to learn how to use. This radio kit looks like a great educational tool for that purpose. My workbench is down while in the lengthy process of getting two houses ready for a move. But I'm going to get one of these kits to experiment with once I'm back up and running. Thanks for posting!:thumbsup:
    Steve
     
  19. amptramp

    amptramp Active Member

    Messages:
    354
    Location:
    Mississauga Ontario Canada
    For those of you rebuilding an SG-165, check the voltage at the source of TR-316. If it is low, you may be OK, but mine was at 6 volts (well within the MPF-102 spec) and that puts 18 volts across 400 ohms (counting R368). When I got mine, the output was clipped in the negative direction for the 400Hz output. The simple reason was R365 was a 1/2-watt resistor carrying 22.222 mA and dissipating 0.8666 watts. The 1/2 watt resistor had gone up to 2195 ohms. I put in a 1 watt resistor and it looked OK but was running too hot to touch, so I then added a 2 watt 390 ohm resistor and turned down the voltage balance on the power supply so I would get 12.1 volts on the positive output but 11.4 volts on the negative output. There is no reason to balance the power supply voltages - nothing depends on it.

    I replaced the filter capacitors (input and output) in the power supply because the 15 volt rating on the 12 volt outputs means the reliability will be low, especially if the unit is turned on and the output voltage rises before the regulator has a chance to stabilize it. If I was going to be totally anal about it, I would change the rectifiers to fast recovery types as ordinary rectifiers tend to ring at 30 MHz and harmonics thereof and the third harmonic would be in-band for FM. A snubber capacitor may do the same thing, just less effectively.

    I had to replace TR302 and TR316 because they were blown. I also added a 1.5 meg resistor to ground from the base of TR301 because I am not comfortable running transistors with a 15 volt Vceo with the base open when the resistor will bring it closer to the 30 volt Vcbo. I also replaced TR314 because it appeared to be shorted going by the diode voltage test on my DMM, but that was a mistake - it was actually seeing R358 and the removed transistor tested OK.

    I can't complain about the SG-165 I got for $10 with these defects plus one channel of the audio meter was butchered - leads cut and not reconnected for some reason, but I never expect to use that function anyway. It was a piece of test equipment designed to a price to enable the average TV repair and stereo store to have AM - FM - FM MPX repair capability and it succeeded in that mission. I used PN3563 transistors for the oscillators, which are plastic versions of the SE-3002 transistors on the schematic and the 2N3563 transistors that were actually installed. The Vceo goes from the 12 volts of the SE-3002 to 15 volts on the PN3563 / 2N3563, critical in this circuit.

    Checked out the unit I repaired and all functions except the one output meter appear to be OK. They are good enough for repair work but not necessarily for optimizing FM tuner performance since their distortion may be higher than that of the tuner under test - the HP and Sound Technology test equipment may be better for that, but for most tube tuners and low-end solid state tuners, they get the job done. The $10 cost and an extra $15 in parts (mainly replacement electrolytics, a resistor and three transistors) has given me the capability to repair most AM and FM radios and FM MPX decoders and get them set to approximately the correct alignment, so it has earned a permanent place on the workbench.

    There are discussions here with a number of threads dedicated to the topic:

    http://www.antiqueradios.com/forums/viewtopic.php?f=8&t=129016
     
    dlucy, sregor and I LIKE MUSIC like this.
  20. Banjo800

    Banjo800 New Member

    Messages:
    24
    Steven,
    If it's not too much trouble can you measure the RG59 cables that came with yours? I would like to make a set of cables that replicate the originals. The main one I'm interested in is the BNC to F connector cable. Thanks for any assistance!!
     

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