Yaesu ft 101E too much heat

Discussion in 'Radio Circuits, Repair & Performance' started by KM4YTW, May 7, 2019.

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  1. HAMHOCK75

    HAMHOCK75 QRZ Member

    Not sure if this will help because I never measured the temperature of the finals in the FT101EE. I do have amplifier that uses four sweep tubes similar to the 6JS6C. That amplifier only has a bias current per tube of about 10-15 mA but the standby temperature is around 73 degrees C. During transmit with a CW carrier, the tube temperature reaches 95 degrees C. These numbers are with fan cooling. I don't remember if you said your FT had a fan or not.
     
  2. KM4YTW

    KM4YTW Ham Member QRZ Page

    Thanks Ham. Yes it has a fan. Thanks for those numbers . Can anyone explain to me how or whats happening to cause the voltage drop on the cap of these tubes during transmit. I look at the schematic and it comes from the 600 volt off the filter caps thru a coil to the tubes. Is it just the drag on the power supply ?
    Thanks
     
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    There's a couple of things going on.

    - HV power supplies in rigs aren't built to support full key down current for extended periods of time. They could be built that way at much greater size, weight and expense but instead they build the supplies a bit on the light side and rely on the charge held in the capacitors to handle peak current draw. That's especially true in large linear amps but true to a lesser extent in the smaller HV supplies used in 100 watt tube rigs. Basically it's normal to have HV sag during key down conditions which is measurable at the HV metering tap which typically is prior to the anode choke and the tubes themselves.

    - The anode choke has some finite resistance and when the tubes draw full current there's a measurable voltage drop across the choke. So if you do measure the voltage right on the anode chokes you see the combined impact of HV sag and the additional voltage drop of current flowing though the anode choke. Look at the thin wire and number of turns used to wind a typical anode choke and it's easy to see there's some resistance there.

    Some HV drop during full power transmit conditions is perfectly normal.
     
  4. KM4YTW

    KM4YTW Ham Member QRZ Page

    Thanks K7trf. Excellent explination. So having the voltage drop at the tube anode is not "nessary" for the tube to function. Its just a reaction from the draw on the power supply. If all my voltages are correct , which they are, im thinking its safe to say if theres a problem its in the tube. Is that a correct statment ?
    Thanks
     
  5. HAMHOCK75

    HAMHOCK75 QRZ Member

    There is always voltage drop but why did you think it was going to be specifically 80 volts?
     
  6. KM4YTW

    KM4YTW Ham Member QRZ Page

    According to the service manual it indicated 80 volt drop on transmitt. If its like the rest of the power supply its no where near what the manual calls for.
     
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yes, but lack of HV sag is often an indicator that the tubes aren't actually drawing expected current. You should see this in other ways such as your plate current meter (if your meter shunt is accurate and hasn't changed value over time) but lack of HV sag under full transmit power key down conditions can be a clue that the tubes are failing and not drawing as much current as they should or some component(s) around the tube is preventing them from drawing full current which is why checking cathode and screen resistors is so important when you have final stage problems.
     
  8. KM4YTW

    KM4YTW Ham Member QRZ Page

    Rodger that k7trf. I havent checked them yet. Thanks
     
  9. HAMHOCK75

    HAMHOCK75 QRZ Member

    I found it on page 4-19 of the service manual in figure 4-11 where it shows the plate voltage of the finals in receive at +650 VDC dropping to +570 VDC in transmit.

    I did some tests right after replacing a neutralization capacitor for the Sylvania finals I had. The plate voltage measured closer to +750 VDC in receive but it did drop about 80 volts to about +670 VDC under load. The plate current was 300 mA. The test was done into a dummy load. Yaesu recommends no higher than 330 mA on bands 80-15M, dropping to 280 mA on 10M and only 140 mA on 160M to keep spurious modes in check.

    If you have US made 6JSC's have you checked if you can neutralize the final? When I bought mine, it had Sylvania finals but the neutralization capacitor had not been updated for US tubes so the neutralization trimmer was at one end of its range but it was still not enough.
     
  10. KM4YTW

    KM4YTW Ham Member QRZ Page

    Thats right Ham. Mine has toshiba finals, there not the green stripe ones , but yes i did neutralize them . Mine runs 750 volt and drops to 730 on transmit . Should i have it in tune mode when checking this ? How did you check the temperture of the tubes?
     

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