: [Amps] SB-220 parasitic suppressors
One reason that an SB-220 can develop VHF parasitics is the
following: the original suppressors use 2-watt carbon composition
resistors. These resistors tend to increase in value when they age,
and especially when they are heated. If an SB-220 has been used
heavily, especially on 10 meters or -- worst case --
on 10 (or 11!) meter AM, the resistors will have gotten quite hot
and may have changed significantly in value, or even opened up.
If the resistors are discolored, they should definitely be replaced.
Sage advice, Vic....
My standard procedure here is to check the value of the suppressor
resistor on a time schedule, along with cleaning the components and
chassis, and a thorough visual inspection throughout the amp. As Rich
and several others have pointed out, the Rsupp can change value
without any outward signs whatsoever. It can be the result of age,
over-heating, glitches, or _____.( fill in the blank) It is a chore
because one end of the resistor must be lifted to measure it. I have
had several Rsupp's over the years that appeared pristine, but broke
when gently unsoldered. Those of us that use Globar resistors need to
make sure the clips are clean and good contact is made between the
clip and resistor body. With the price and availability of tubes
nowadays, a maintenance schedule makes sense to me, and is cheap
insurance. While you are at it, don't forget to clean the fan/blower
blades and oil the motor bearings.
(((73)))
Phil, K5PC
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|