[Yaesu] FTDX-400

Thu May 5 21:28:15 EDT 2005

Hi Dan,
> I need a plate choke for this radio. does anyone happen to have
> one or know where I can get one? the windings are loose on mine
> and I can't seem to neutralize or get any more than 40 watts. 

Inability to neutralize is probably a second consequence of 
whatever is causing the low output. 
I'm sure you know to use great care troubleshooting the PA section 
of this TX -- 800 volts is plenty to 'do the job.'
Does tuneup go normally (normal plate current behavior) or is that 
messed up too? For example if you can't get enough plate current 
when tuning up, that suggests a driver problem or weak PA tubes. 
A sharp peak in plate current when resonating the driver but little or 
no dip in plate current when tuning the PA plate suggests a partial 
short -- something like arcing in or around a PA tank part (could be 
within the plate choke) -- while a very sharp dip suggests a good 
hard short circuit. 
Does the problem occur on all bands? Are there abnormal 
smells? Any traces of smoke, either during operation or inside the 
PA cage?
I'd be surprised if loose windings on the plate choke caused these 
symptoms unless associated with other choke problems. Check 
resistance when cold, if that's okay then power on, try to tune up, 
power off, *SHORT HV POWER SUPPLY FOR SAFETY* and 
carefully check for heating of one or more pies of the choke to 
eliminate a short when RF is applied. Such shorts usually leave 
visible signs -- discolored windings, smoked area on the top of the 
PA cage, and so on. 
Do a careful visual exam of the various fixed caps in the PA tank 
circuit; they may be bulged or discolored if there's an internal arc. 
The fixed capacitors on the output side of the pi net are a frequent 
source of problems, ditto the fixed cap in series with the 
neutralizing cap. These parts often fail when an attempt is made to 
tune up the TX without a proper load connected. Burned contacts 
on the PA section of the bandswitch can start when the 
bandswitch is turned with key down and will get worse rapidly after 
that. 
I've bought an FTdx-560 parts set on eBay and if it eventually 
happens that I can get the seller to ship it, then I may have some 
spare PA parts available. Keep us posted!
Walt Hutchens
KJ4KV