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write your own review of the Barker & Williamson Model 425 Low Pass Filter.
Feb 19, 2012 13:32
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Great After Modification
Time owned: more than 12 months
Gave this a 4 instead of 5 for the reasons below.
Purchased this unit at some time in the early 80’s and used it for years at the 200 watt level.
Then purchased an amp and ran it at 1250 watts for about a year until I selected the wrong antenna one night in the “heat of battle”.
It made very bad arcing noises then dead shorted.
After sitting in a drawer for a couple of years I opened it up and repaired it.
The guts are 5 coils and 4 caps. The caps consist of two washers each, mounted on either side of a divider panel and insulated by some kind of very thin plastic.
One of the caps had burned through the insulating plastic and through the aluminum divider.
Close inspection showed that the reason for the failure was the washers were not de-burred and were mounted sharp edge to the plastic, and the holes in the aluminum were not de-burred either.
This is bad for two reasons. One it cuts the plastic, and two, sharp edges severely lower breakdown voltage.
I completely disassembled the unit (the caps are held together with 10-32 brass screws) and cleaned up all the burrs and buffed all the edges smooth.
I threw away the plastic sheet insulators and replaced them with 4 mil thick Teflon insulating tape.
I then re-assembled the caps with the rounded edges of the washers against the Teflon to prevent any chance of the edged of the washers ever penetrating the insulators.
Teflon is rated between 1000 to 2000 volts/mil depending on which source you check.
The tape I have is rated 1500v/mil which puts these caps at around 6kV.
I then swept the unit with an HP Spectrum Analyzer and tracking generator.
1.8 to 14 MHz showed virtually no insertion loss, from there to 37 MHz it was < .3dB.
At 38 MHz it went to 2.5dB and by 55 MHz was over 50dB.
Not bad for a unit from the 80’s, and it should hold up to anything I throw at it now.
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