VE7RF commented
## did yaesu ever fix the problem..or are they still building it wrong
?
Sorry, I don't know.
## the simple fix is to replace the step start resistors with real
ones, even
the metal finned arcol types should work.
Metal cased resistors also have quite poor short-tem surge performance
because heat conduction to the case is very slow. Even worse, the
internal insulation is an organic resin that decomposes when overheated,
building up gas pressure inside. So if the startup circuit should ever
'hang', and there is no fuse, you're going find out what that metal case
is really good for: it makes an excellent smooth-bore gun barrel!
## 240 vac / 50 ohms = 4.8A =..... 1152 watts
240 vac / 20 ohms = 12 A..... = 2880 watts.
240 vac / 10 ohms = 24 A...... = 5760 watts.
## as u can see from the above step start resistor values, this is no
place for a cement cased resistor. When T=0..... u have one helluva
lot of instantaneous power being dissipated in the resistor.
It is OK for resistors to be overloaded *by design* if they are the
right kind of resistor that is capable of handling these conditions for
a small fraction of a second; and of course that overload period *must*
have a guaranteed end-point.
Most cement coated resistors don't possess a surge or overload rating of
any kind. The best type seem to be the glass-coated wirewound resistors
on a long ceramic tube. It is no coincidence that these are among the
best for handling HV surges too.
It also needs pointing out that the large majority of step-starts are
way too long. Modeling with typical values shows that the surge into the
electrolytic capacitors only lasts about 5 mains cycles (about 100
milliseconds) and then you're ready to step up to full mains voltage.
There is no advantage in stretching out this period any longer. Some
people seem to imagine that a step-start period of several seconds is
"more kind to the components" - but in fact it is the exact opposite.
All it does is stretch out the overload time for the step-start
resistor, and increase the risk of it failing.
73 from Ian GM3SEK
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|