Bandswitch Repair

How do you tell when a section of the band switch is giving you problems?  If the associated coil adjustment for that band and portion of the switch doesn’t do anything, then, most likely, the band switch is not making contact for that band.  For example, say you have no ALC on 40m.  You need a vtvm to monitor the drive output, and if turning the slug in the coil makes no difference to signal level, time to repair the band switch.

We’ve been repairing the band switch here for quite some time. We have taken a different tack than some others you might read about on the web. Others have recommended using the paint on silver solution. I’ve had experience with that stuff, over many years, trying to repair traces and connection in various old pieces of pro audio mixing consoles. The problem with it is that when the solution dries, it only loosely adheres to the copper metal surface of a foil trace.  Basically the only thing holding it to the surface is the glue like action, of the dried solution.  Glue doesn’t stick to metal very well.  It doesn’t bond with it, it is only stuck to it by the action of the polymer liquid it is suspended in. After some years, the copper will gradually oxidize underneath, and the connection will go bad again.

Our solution with the bandswitch is very simple. It’s the fingers, the spring loaded contacts, that are riveted to the fiberglass circuit board that is each wafer of the switch, that cause the problem. The real fix is to solder that finger to the trace. You can’t do this directly, but you can solder a short piece of wire to the end of the finger, farthest from the center rotor part of the switch, and then solder that wire to the trace.

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This is the RF drive coil wafer. Note how the wire is soldered on the end of each contact finger, and that wire soldered to the trace. Click to enlarge.

 

   This is the last wafer, the one nearest the cage, and it switches the RF drive coils, and carries quite a bit of voltage on its contacts, the plate voltage for the driver tube.   Notice how I have soldered on a wire to the very top of each little contact finger, and then have soldered that wire to its respective trace.  Bingo, no more corrosion to deal with, and its a fairly permanent fix.  In the future the only thing that can go bad is the  connection between the rotor and the contact finger.  The rivet is bypassed and fixed!!

  Now a word of caution.  Only attempt this if you have quite a bit of experience soldering.  First scrape of the trace til you have bare copper.  Then solder your short piece of wire to the trace.  Then clean off the end of the finger contact, and get the little piece of wire soldered to the end of it as quickly as possible.  You have to be able to look and tell the difference between a good solder connection, and one that just has the solder laying on the surface.  If you get a good connection but it’s not pretty, leave it alone. 

  The reason you want to do it quickly, is that the contact is made of some sort of alloy that  is springy and has a temper to it.  If you apply too much heat, for too long, you could destroy that temper and then it would quickly lose contact with the rotor part of the switch.  We have successfully done this repair many times.

  Of course, first you have to remove the RF board, and then loosen up the long screw and spacers that hold the switch together.  Unsolder the wafer you want to work on, and then do the above.  None of the process is especially difficult, it’s just tedious work and you have to pay attention to detail.

  You can get the last wafer out, the one that does the RF drive coils for the final, without much dis-assembly at all.  This is the one most often causing problems.

Here, just the last wafer, the RF drive coil selector has been removed.  This requires a minimum of disassembly.

Here, just the last wafer, the RF drive coil selector has been removed. This requires a minimum of disassembly. Click to enlarge.

   In the above picture, you see the next to last wafer.  There are only 4 connections on it that are switched.  I have soldered them with a jumper wire, just like the other section.  I was able to do this without removing the wafer from the board, once the end one is out.  This is our standard repair when the RF drive will not work on some bands, as so often happens on these old radios.

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Click to enlarge.

 

 

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