FT-101ZD sizzling sound on tune up into dummy load.
 

Tuning my FT-101 into a dummy load today I heard a sizzling sound come from the radio...this is the second time I have heard this...made me jump both times and I very quickly turned the MOX off to ptt.

I think it occurred after turning up the drive control had peaked the meter..I was getting no more increase in output power ...but the drive was only just over 50% open...

I carried on opening the drive control past the point I was getting no more power out on the meter getting to full clockwise on the drive control and kept rotating load and plate to see if I would get any more out of the radio...I didn't.

The manual does say continue till the drive control is fully clockwise...but is there any point after the power out has peaked...would I be better stopping then....just wonder if push on with the drove control after the power has peaked could have overloaded something.

As you can probably tell I am a newbie with valve transmitters...and am terrified of damaging my 101ZD.

Nick 2E0LPL in the UK.

Re: FT-101zd sizzling sound on tune up into dummy load.
 

Hi tuning valve transmitters is straightforward, start with loading cap fully anticlockwise drive control ajusted to give around 50MA anode current dont forget to peak the preselect control for max indication on meter keeping below 50ma Quickly
rotate plate control for a dip in pa current as indicated on meter within a few seconds ON NO ACCOUNT ALOW THE PA TO DRAW LOTS OF CURRENT as this WILL distroy the pa valves/mains transformer i have had many 101s for repair due to misuse, continue to load and dip untill no further increase in pa current If memory serves max IC to pa is 100ma for 100 ish out. Just for info C13 in the pa driver a to g of pa goes s/c this also will destroy a 101/102/902 radio.
PM me i will send you a copy of operating manual hope this helps nice radio by the way i use the other classic rig ft902 Mick
https://www.youtube.com/watch?v=LUjvtY-h77I

Re: FT-101ZD sizzling sound on tune up into dummy load.
 

The other thing to worry about is the preset pot that sets the PA bias voltage. If this goes open circuit (and it can) then the PA will go into overdrive.
A simple fix is to put a bigish fixed resistor to pull the bias to maximum negative if the pot fails.

Re: FT-101ZD sizzling sound on tune up into dummy load.
 

As mickm3for says, the coupling-capacitor from driver-anode to PA-grid is a known weakness in these radios - you could say it's the equivalent of "that capacitor" in valve broadcast-radios!

If it goes a bit leaky it can cause all sorts of odd and inconsistent behaviour of the PA-current during tune-up. If it fully-fails you'll be needing a new set of PA bottles and possibly a new mains-transformer too.

Re: FT-101ZD sizzling sound on tune up into dummy load.
 

Nick
That sizzling sound is HT arcing somewhere and not good. I have the same radio and find the drive doesn't need to go much beyond 11 or 12 o'clock for full power out, so I have always left it at that point.
You need to take the covers off and give it a good visual inspection, with a strong light, in and around the PA box and driver valves. Look for burnt resistors.

WARNING!! the 900v HT remains for sometime even when the main power has been switched off. There are no bleeder resistors!! Also HT is present on the PA valves even when in RX mode and Heaters switch "OFF"

Check the 6146 anodes with a testmeter set at 1000V before touching anything.

If you're lucky it could just be a spider web shorting something. While you're in there look for any signs of disturbance of the crystal mixing board. Any hacking about in there will indicate it may have been "adjusted" for use on 27 MHz at some stage.

Did the power output vary when you heard the sizzling noise I wonder? If so it could have been flashover in the PA bottles.....once again, not good. However, once that happens its usually game over for the 6146's and the EHT tripler board.

Also worth knowing is the "PO" meter on the rig is only a relative power output indicator, basically a sniffer via a diode rectified to the meter and is adjustable via a pot on the rear apron, so needs checking with an external power meter.

Finally, my tip is to count up to 10 while tuning up. :-)