Yeasu FT-101 fuse blowing

Discussion in '"Boat Anchor" & Classic Equipment' started by AK0EM, Feb 15, 2015.

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  1. AK0EM

    AK0EM Ham Member QRZ Page

    Hi! I am back again with a FT-101 question. I bought this radio in supposed working condition. It was shipped to me in a poorly packed box. I had posted here earlier about the absence of any bias current. I discovered that during the shipping one of the 6JS6C power tubes had been dislodged from its socket, banged around and was cracked. I replaced both tubes. On to my current problem. When I power on the heater switch, the tubes light up but within a minute or two the 5 amp fuse blows. I replaced the fuse once thinking maybe it was old but I had the same result. The receiver section works fine. Is there something I should be doing before energizing new power tubes or do I have another problem?

    Thanks in advance for any help.

    Karl AK0EM
     
  2. K5RT

    K5RT Ham Member QRZ Page

    Karl, do you have the manual for this radio?

    Pull the two 6JS6C tubes, make sure the plate caps are not touching anything, then try the filament switch again.

    The symptom you describe sounds like a short in a tube that shows up once the tube gets hot.

    Paul
     
  3. W3ATV

    W3ATV XML Subscriber QRZ Page

    So it receives ok, but then you put the filament switch on and the fuse blows after the tubes warm up??? Do you hear anything in the receiver when they warm up, right before the fuse blows? Did you notice if the plates of the tubes are glowing cherry red before the fuse blows?
    There is a coupling cap between the plate of the driver tube, and the grids of the final tubes. It is VERY COMMON for this cap to become leaky and put incorrect bias on the 6JS6C tubes, causing them to run away and blow the fuse. Just something to add to the list of things to check out.
     
  4. AK0EM

    AK0EM Ham Member QRZ Page

    Paul,

    I did as you suggested and voila! one of the tubes made a rattling sound. I replaced that tube with the other old one I have and I got the 60 mA resting cathode current that the manual calls for. Do you think it is a problem to have one new and one old tube?

    I really appreciate your help.

    73, Karl AK0EM
     
  5. K5RT

    K5RT Ham Member QRZ Page

    It will work, the risk is the current between the tubes won't be balanced. Having said that, there isn't any way to balance the cathode current in the FT-101, so there's little you could do. Yes, a pair of matched tubes might help the situation, but in your case, go ahead and use the radio and have fun.

    Vy 73
    Paul
     
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