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Dusting off a Sweet Sixteen, everything look ok?

Discussion in 'Amplifiers' started by buickid, Nov 7, 2020.

  1. buickid

    buickid Active Member

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    Got this amp from a very good friend who fell SK several years ago and it's been on the shelf since. Decided to dust it off and perhaps put it in the mobile. Thought I should pop it open and make sure everything is kosher before I hook it up. Looks okay to me, but lets be real, what do I know? So I put it to the readers of WWDX, anything unusual or alarming I should correct? What is that brown round thing with one wire coming off of it? Looks like it used to have something soldered onto the other tab.



    What kind of drive should I be putting into this? Thinking output should key around 200-250W, x4 on peaks.

    Thanks!
     

    Attached Files:

    Shadetree Mechanic likes this.

  2. Robb

    Robb Yup

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    80-100w PEP.
     
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  3. snippits75

    snippits75 Well-Known Member

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    Metal clad capacitor on the output look blackish to me on my screen. The one to the left of the 10 amp fuse especially.

    Also on the first picture, the small resistor to the right of the 10 amp fuse, and right before the output transformer looks to be charred on both ends.

    Not an expert, but that's what I see looking at the pictures.
     
    #3 snippits75, Nov 7, 2020
    Last edited: Nov 7, 2020
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  4. Tokin

    Tokin The Man With No Shoes

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    Several of those resistors look suspect to me. Heres another. If it were mine id have it gone over and freshened up before testing. View attachment 41410
    Screenshot_20201107-082226.png
     
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  5. autotuner

    autotuner Supporting Member

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    The brown disc things with one wire looks like a self resetting thermal fuse. And yes the second tab should go somewhere. I'm pretty sure the base metal disc is isolated electrically. The other guys are right, some of the components look a little toasted.
     
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  6. 999

    999 Well-Known Member

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    That round brown thing missing the wire is a thermo-switch to automatically turn the fans on. It looks like the fans have been wired to the power switch to be always on instead. This is common practice and the mounting location of that thermo-switch is notoriously bad and the fans come on at much too high of a temperature.
    Nothing looks cooked to me besides those two resistors. What you are seeing is flux coming out of the solder in most places. Resistors tend to cook from the middle out, so if the center looks good, they usually are.
    The semi burned looking resistors are in your combiners. That usually indicates and imbalance in the circuit. That can be anything from a capacitor that has gone out of spec to a blown transistor pulling things down on one side.
    To me overall the box looks like it was taken care of, but the way that he had the fuses deleted is not the best. Using zip line is convenient, but even though 12ga is what TS uses from the factory for power wire, it really isn't enough. If it were mine I would swap the 12ga out for 8ga to each board. That will take some of the stress off of things and the amp will thank you for it.
    As far as the fans go....I would leave them always on unless you want to switch them on yourself. They can be loud, but if it's in the trunk, you'll be fine.
    Oh, side note. Mine is still wired to the thermo-switch and when it gets really hot (I'm in CA) I need to unplug the amp from power when I'm at work. The damn fans will kick on in the heat of the day and I don't feel like coming out to a dead battery.
     
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  7. Ranch55

    Ranch55 Sr. Member

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    The discolor I see on some of the resistors is normal. That is just solder flux that flowed and dried.
    Definitely been some hands inside of it before. Not bad though.
    One of the large filter capacitors is missing on the #2 board. I would change all of them to some 3300's to 4800's at 25v ........
    Yes, 80~100 watts PEP drive is plenty.
    I have three of the Sweet Sixteen amps, and I drive them at that and they will all produce 1200 watts plus very easily.
    I think some clean up and some new capacitors are in order.
    I do use the thermal sensor for the cooling fans, but I have devised a method for mounting the thermal sensor switch directly onto the heat sink in order to have better sensing properties.
    I also install a thermal sensor bypass switch in order to turn the cooling fans on directly if I am going to be very long on the key, such as some good DX coming in.
    As far as installing in your mobil, you will need a lot of battery power, and LOTS of alternator charging room .......
    Also, on newer vehicles, the possibility of RF interference with the vehicle BCM or PCM is somewhat likely when running that much RF power output.
    I know this for a fact......

    Nice amplifier, I could always use another one for my collection.
    Good Luck ......
     
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  8. buickid

    buickid Active Member

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    Thanks guys for taking a look. I will certainly examine the capacitors a bit closer and see if they are indeed burnt or just covered in flux. Also, thanks for identifying the thermal switch. Makes sense, as indeed the fans are hardwired to the power switch.

    The filter capacitor, is that the big yellow one? I looked at the schematic on CB Tricks, it looks like there is another capacitor missing on each board, C14/25/36/47? You can see the solder points on each board where they attached before. Is there a reason for this? Should I order replacements and put them back?

    Is there an easy way to check the health of the transistors? I will certainly look into upgrading the power wire to each board. Amp draw at full reasonable output power around 120A?
     
  9. 999

    999 Well-Known Member

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    Depending on drive it can hit 150 amps on AM or 180 amps on SSB.
     
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  10. Ranch55

    Ranch55 Sr. Member

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    Well, the amp would probably work as is, but the integrity and reliability would be questionable.
    It would be way too much typing for me to explain and list all the potential issues that should be addressed and corrected.
    Some components are definitely missing, and some others are workable, but not correct values (large yellow capacitors).......
    As for testing the transistors, there is no shortcut to doing so. They HAVE to be removed from the circuit to do so. Not necessarily from the board, but doing it that way is also a pain in the a$$ .......
     
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  11. buickid

    buickid Active Member

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    So the big yellow ones are incorrect, those are the filter caps you mentioned one is missing? I will replace all four. As far as the four capacitors I believe are missing, those are actually missing right, I'm not imagining things? I intend to have all of the parts that are supposed to be in there, in there. If the schematic on cb tricks is correct, I can definitely follow along and work my way across the board, but I'm just trying to make sure that I'm not seeing differences due to an undocumented revision or something, which I understand happens from time to time.

    I appreciate your help thus far and understand there maybe a substantial amount of work involved in going through this amp. Any pointers you (and others members) throw my way are again, appreciated.
     
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  12. nomadradio

    nomadradio Analog Retentive

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    Someone has upgraded the keying transistor. A good move, since the factory part tends to be a nuisance-failure risk. The larger one in your amp should not cause any trouble.

    73
     
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  13. buickid

    buickid Active Member

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    I'm looking at Mouser, any particular type of capacitor? Mouser lists: aluminum, audio grade, general purpose, high frequency, high temp, low ESR, low impedance, low leakage types for 3300uF 25V capacitors. I'm thinking general purpose is fine for this?

    Also, that value isn't so popular in axial leads, would using a radial cap be a nono?
     
  14. nomadradio

    nomadradio Analog Retentive

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    Radial leads are fine, and cheaper to manufacture. High temp is probably the best choice unless you're conservative about how you run the amplifier and never get it hot.

    73
     
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  15. Ranch55

    Ranch55 Sr. Member

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    I'll use radial or axial lead electrolytic caps. Whichever I have available at the time.
    The last Sweet Sixteen that I put filter caps in were radial leads.
    Currently, I do have some of both ........
     
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