Yaesu ft 101E too much heat

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

1. KM4YTWHam MemberQRZ Page

Actually the metal side between the driver and finals is what's getting hot the spot that is getting hot is adjacent to R37. It reads fine 18K ohms. Maybe a double check of voltages are in order?

2. G3YROHam MemberQRZ Page

Yes . . .

What is the voltage on the top of R37? (ie the Screen Grid voltage to the Driver valve)

Roger G3YRO

3. KM4YTWHam MemberQRZ Page

I have 350 volts. Now a new problem has developed. Pin 7 has 350v on receive and 350 volts on transmit . Its suppose to drop about 70 volts on transmit . My question is what componet drops the voltage on pin 7 on transmit?. Not theres very little output. 10 watts .

4. G3YROHam MemberQRZ Page

Well the driver HT voltage should only be 300V ! What does that measure?

And the screen voltage should be around 220V (on the top of R37).

What is your AC Mains voltage?

Roger G3YRO

5. KM4YTWHam MemberQRZ Page

Well the line voltage is average around 123vac . Every yaesu 101 i have owned the -100 volt is always -180 . The the 600 volt is always 745 volts . Everyone i seen on youtube is also same voltage. Maybe it was measured with a different meter. The voltages are high but i think i should still see a drop on transmitt proportional to the ones listed in the manual.

6. HAMHOCK75QRZ Member

My line voltage is about the same as yours. Just under 121 VAC. I also have +350 VDC on the driver plate but it drops to +285 VDC in transmit.

You should never have +350 VDC across R37. R37 is a 5 watt, 18K resistor. If you have +350 VDC across it, the power dissipated is (+350 **2 )/18K = 6.8 watts. It will be extremely hot at that wattage.

Looks like the only way to get +350 VDC across R37 is for R5 ( 5.1K, 3 watt ) to be shorted.

Also, the -100 VDC should be about -180 VDC. The -100 VDC is labelled in error in both the manual and on the rectifier board ( PB1076B ). The way you can tell is that the +160 VDC is derived from the same 120 VAC transformer winding. The only difference is that the half wave rectifier diode D6 is reversed.

7. KM4YTWHam MemberQRZ Page

Thanks for the explination on the -100 volts . I knew something was a skew.
I'll check those a little closer when I get home . I burnt R40 up by accident . So I replaced it I also replaced the 5.1k resistor . Solder bridge. I'm waiting on the 18k resistor to get it replaced. Thanks for all the info. 73

8. G3YROHam MemberQRZ Page

Yes, as I said before, if the Screen Grid dropper resistors are the correct value (which you said they are) , it makes no sense that you could have such a high voltage on the screen. (and that's why the resistor is getting so hot)

Roger G3YRO

9. KM4YTWHam MemberQRZ Page

R37 reads 4k ohms when hot . Its shot. When it cools it reads close to normal. Ill change that thing out and see how it works out. Ever heard of that ? I didnt think it would return close to normal ohm value.

10. KM4YTWHam MemberQRZ Page

Well. I got the resistors replaced, everything checks out voltage wise. Im wondering if the tubes are shorted internally. The bases of the tubes get really hot. The cap of the finals only drops 38 volts on transmit. They should be dropping 80 volts. As soon as i turn the heaters on i hear a noise like glass getting hot and cooling off quickly. Makes a clicking noise , like a hot muffler cooling off.
Anyone heard of such a thing. What could it be? I get good output on all bands , but little less on 15-10 meters.