[Amps] Advice on instruments for ham use

[Bill Turner]

On Wed, 24 Nov 2004 22:52:12 -0500, Bill L. Fuqua wrote:

>455 and 465 scopes are fine except when the attenuator switches wear out. They consist of cams pressing small gold plated fingers onto small gold plated pads on the little circuit boards. Once the gold is worn off the switches corrode and they are no good. 

_________________________________________________________

So many people have requested my fix for the attenuator switches, I
thought I should post it to the group:


The 4xx series of attenuators are switched in and out by a tiny set of
gold plated contacts in the shape of a "U", about 1/8" square.  A
series of cams raises and lowers them onto pads plated on a circuit
board.  The "U" is insulated from the leaf spring to which it is
connected.  All the "U" does is connect two pads together when it is
lowered onto them.  In nearly all cases of intermittent contact, the
problem is insufficient pressure on the pads.  The tension of the leaf
spring and the downward angle of the "U" were designed to place
minimal force on the actual contact surface.  Sometimes the force is
so minimal that the tiniest amount of dirt or corrosion prevents solid
contact.  The cure is to increase the amount of force, but only a very
small amount.  Both the "U" and the pads are plated with a copper/gold
alloy which is harder than gold alone, but still relatively soft.
Light pressure ensures long contact life.  

Here's the fix:  You must increase SLIGHTLY the downward angle of the
"U".  

1.  Rotate the attenuator switch until the offending "U" is lifted off
its associated pads.
2.  Using a small flat-blade screwdriver or similar tool, gently push
against the "U", placing the blade of the screwdriver at the bottom of
the "U".  This bends the "U" downward a small amount.
3.  DO NOT try to bend the "fingers" of the "U".  Bend the entire "U".
4.  Go slowly and make only a small change at a time.
5.  The object is to increase the downward angle of the "U".  Almost
any visible increase is enough.
6.  Do not try to bend the "U" with needlenose pliers or any other
tool.  It is very easy to ruin the contacts.  Use only the screwdriver
pushing technique.

You may wish to use a contact cleaning liquid in addition.  Beware of
any strong solvent, however.  The attenuator circuit boards are not
epoxy like most other boards.  They are made of polysulfone, a very
low loss material, but one easily damaged by heat or strong solvents.
Isopropyl alcohol is recommended.  Flush with distilled water and dry
thoroughly.

This should do the job.  There are similar contacts in the trigger
circuit, but they are on a regular epoxy board instead of polysulfone.
The same technique applies.

Good luck!

--
73, Bill W6WRT



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