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Knightkit T-60 Problems

Discussion in '"Boat Anchor" & Classic Equipment' started by KN4SMF, Apr 27, 2021.

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  1. KN4SMF

    KN4SMF Ham Member QRZ Page

    I am seeking knowledgeable repair advice on my Knight T-60. These problems are of my own making. I rarely flub a radio service job, but I sure did on this one. It started with a working rig where I decided to put in a 3 prong cord. Upon doing that, I plugged it in to raw 125 house voltage and switched it on to "Standby". All was well. Then instead of switching it off, I stupidly switched it to "AM", with no antenna attached. POP!!! It blew the fuse and burned up the switch contacts. As in instantly. I have been told I may now have a bad power transformer, as in shorted. Shorted to the case? How do I test it with my VOM? And my second question is whether this could be a mis-diagnosis. Attached is the schematic. Does anybody see that this may be an "above ground" rig, wherein putting a 3 prong cord on it with the third prong connected to the chassis might account for this problem? Thank you.
     

    Attached Files:

  2. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Remove the Final, place a key plug in J3, replace the fuse with a 3 Amp fast blow, and if the fuse doesn't blow, measure the B+.

    A three wire power cord, if installed correctly, shouldn't blow the fuse.

    Do check C26 for any shorts.

    Pheel
     
    KE9EX likes this.
  3. KN4SMF

    KN4SMF Ham Member QRZ Page

    Final refers to the output tube I believe, as in remove it? Key plug in J3 i don't understand. That means to short J3? All the other is clear enough. Are you saying you doubt a transformer internal short? I've have had other advice on that, but I don't see how a winding can short to case. I do good work. This entire disaster caught me completely by surprise. My work just doesn't do like that. Thank you.
     
    Last edited: Apr 27, 2021
  4. N8AFT

    N8AFT Ham Member QRZ Page

    I've owned and used 3 of these lil rigs, they are good design. Vy doubtful you hv a xfrmr failure. I've never seen it.
    Yes, ANY of the novice or vintage rigs need to hv a key or just a key plug in place or they are transmitting all the time and as soon as there is power applied in either AM or CW mode...
    Sounds like what's happened is you've cooked the final, 6DQ6 (?) ... AM mode is full power...
    Remove the tube and re-fuse with a key plugged in or just a 1/4 inch plug in place.
     
    W9BRD likes this.
  5. K4CCW

    K4CCW Ham Member QRZ Page

    BTW, I have plenty of switch wafers and would be glad to send you some for the switch repair you referenced in your wanted ad in that sub-forum.

    I must be too old to figure it out, but could find no way to respond to your ad and just gave up.
     
  6. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    Yes, remove the Final tube, the 6DQ6, which is a sweep tube and it is usually pretty tough.

    What N8AFT said: 'I've owned and used 3 of these lil rigs, they are good design. Vy doubtful you hv a xfrmr failure. I've never seen it.
    Yes, ANY of the novice or vintage rigs need to hv a key or just a key plug in place or they are transmitting all the time and as soon as there is power applied in either AM or CW mode...
    Sounds like what's happened is you've cooked the final, 6DQ6 (?) ... AM mode is full power...
    Remove the tube and re-fuse with a key plugged in or just a 1/4 inch plug in place.


    In the Left center of the schematic you will see J3 which is the keying jack. Without a 1/4" plug in there, the second contact down shorts the keying line via J2 pins 4,5. Your rig brings this keying line down to ground when the rig is keyed for transmit, OR when no plug is present, which places all of the cathodes at ground.

    When the rig is not keyed, the cathodes float up to a voltage sufficient to cut off the tubes in the Osc., Buffer, and Final. R7 allows the floating voltage to occur on Keyup.

    Hopefully, the only thing that happened was the Final saw an impedance mismatch which caused it suck enough current to blow the fuse.

    I would also check RFC4 and RFC3, the RF chokes, to make sure they have no burnt windings.

    Pheel
     
    WA1GXC likes this.
  7. KN4SMF

    KN4SMF Ham Member QRZ Page

    I don't recall ever seeing or hearing about the necessity of having a plug inserted into a transmitter at all times, whether the plug is for a key, or just a dummy. If that is so, I take it my Heath DX-40 is the same way. Further I guess it has to be inserted even when operating AM. I just never heard of this, but I'll take it on good advice that this is so. So THAT is all that there was in ruining my T-60? Now I have to repair a burned up switch and reconnect all the things I cut in search of the culprit. I still wonder if I should just treat the whole transmitter as an unassembled kit, and go over the whole thing from start to finish. I don't want a repeat of that ever again. I've seen dead shorts in my lifetime, but this one was a fright.
     
  8. N8AFT

    N8AFT Ham Member QRZ Page

    This is how one learns by doing...
    Sometimes the tuition is a mixture of cash and or pride.
    Welcome aboard OM!
    Yes, BTW shorting the key jack is how you enable the AM to transmit as well, it's not only for CW.
     
    WA1GXC likes this.
  9. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    The DX-35, DX-40, DX-60, Allied Knight T-60, and the Allied Knight T-150 ALL have the "shorting" Key jack.

    In order to avoid just what you did, I replace the Key jacks with Mono 1/4" non-shorting jacks.

    Pheel
     
  10. VE2GCE

    VE2GCE Ham Member QRZ Page

    I use simple way to test power transformers, I put an incandescent light bulb (40 - 100 watts) in series with the power line.

    I would remove all tubes and plug in the transmitter. If the lamp lights at full brilliance, your power transformer has a short (unless something external is shorting the filament or HV leads)

    If the lamp does not light up or does with a dull orange glow, your transformer is OK.
     

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