95db Attenuator Kit

There are a few changes to the attenuator kit. The top panel is one piece now, making it easier to assemble.The small circuit boards on the back of the switches are being hand made at the present time, pending receiving a quote for boards from a commercial supplier. 

 

New photos and assembly detail to follow as soon as possible.

 

Plans and assembly photos that come in the 95db Attenuator Kit.

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95db Step Attennuator plans

Please read thru this document completely, at least once before starting construction, some of the steps will make more sense , as you start assembling the attenuator sections.

Installed means to solder in this document.

Before you start to put this kit together make sure you have an adequate heat range soldering iron. A wide chisel tip 25 watt Iron ( 700 degree) is what I used, only in some of the joints where there was a large fillet did I notice the solder didn’t want to flow as nicely. A 40 watt has plenty of heat.

Make sure you use a quality rosin-core solder. Good clean solder joints are easy with this pcb material.

The success of putting this project together with a minimum of problems latter on is keeping all edges square as you tack solder the joints prior to final soldering of the joints. If necessary use pieces of masking tape or a small spring clamp to hold pieces in place as you tack solder the joints. Double check all pieces for the correct fit when you are tack soldering joints and before you final solder pieces in place.

Start by mounting three switches to the top strips with 4-40 screws, by mounting a switch at each end of the top strips and then the third switch next to either one, the top stays square a little easier as you are handling it. Use a flat surface to square the ends or a Square along both edges of the top strips. When you are satisfied that the top panel assembly is square, tighten all the screws holding the switches.

After tightening the screws holding the switches, look at the front and see if all the switches are in line with each other equally.

Make sure that the side pieces are flush along the edges of the dividers (Photo 1 & 2). Make sure there is no gaps when you tack solder the seams of the dividers to sides and top strips.

When you have done this, check along the length of the outside edge of the side pieces. You should have a small gap to the outside of the top strips (see photo 1 & 2). Install a few small solder tacks ( photo 1 & 2)to start holding the sides to the top strips together, the ends will be installed latter. ( You will attach the ends after the bnc connectors have been mounted to the end pieces, and all the soldering has been finished inside the attenuator).

With the top strips holding two or three switches ( this is after you tightened the switches, squaring up the top section).  Place a divider piece against the side of the switch (not on the end side of the switch). Use a small spring clamp to hold it against the side of the switch ( this will help hold it vertical), make sure that the small hole on the divider is in-line with the center set of terminals on the switch ( photo 1 & 6 shows this clearly), put a small dab of solder to just hold the divider to the top strips,  (Refer to photo 1 and 2.) When you tack solder the dividers  to the Top strips, make sure all edges are flush and square, do so with a minimum of solder(a small bead). If you have too much solder in this area your switches will not lay flat. You can see this clearly in photo 2, the left side of the divider solder job is correct, the right side of that same divider has too much solder on the divider, this solder fillet needs to be reduced or the switches won’t lay flat against the top strips when mounted. Refer to photo 2. 

You should check each section as you assemble it to make sure the dividers are all flush along the top edge of the side pieces ( the divider pieces should not be above the edge of the sides or when you attempt to fit the bottom cover on, it wont be flat and you will get gaps along the joint. If some corners of the dividers are above the top edge of the sides, not to worry. This can be corrected, you can use a piece of sand paper, laying flat on a table top, sand the entire bottom surface edges until all the edges are smooth and flat. This is important, so that the bottom will seal along all edges on the inside.

 In photo 1 you can see where I put a switch and divider at each end of the box (small solder tacks to start with. Putting in a switch and divider at both ends at the beginning helped to keep the box square as you start building the sections. After installing two switch/dividers, you can remove the far end switch/divider, so that it is easier to get good solder fillets when installing the dividers to the side pieces.

Photo 2 shows where I removed the last section divider and switch to make soldering the remaining divider/switches easier to install.

Photo 3 shows how the resistors were installed with the switches mounted to the top strips.

At this time, I removed all the switches and attached the resistor network to the switches.

After re-installing the switches/resistor combo back on to the top strips, solder each of the resistors legs   coming from the switches to the copper ground of our enclosure ( see photo 4 ).  NOTE- save the resistor leads that were cut, you will need pieces for the internal wiring of the switches.

Removing the switches and installing the resistors and jumpers to each switch eases the difficulty of trying to do this wiring in a small space and possibly breaking the resistors.

Using some of the resistor leads that were trimmed earlier when you were making the resistor/switch pieces. Push a piece of the small clear teflon sleeving ( approximately ¼” long) on a piece of resistor lead and thread it thru the feed-thru hole at each divider and solder to the middle terminal on the switches on each side of the dividers. ( Photo 8) Install the jumpers on each switch ( photo 3 & 8 ).

Install the top filler pieces between the top strips at each end. A large solder fillet can be seen under the BNC connector threads in photo 8. 

In photo 3 you can just see under the bnc connector where I tinned the area next to the switch, for a small piece of pcb material ( top filler piece) to fill the gap from the switch to the edges of the top pieces.

When you install the filler pieces to the top strips, the end’s edge should be flush with the ends of the top strips ( photo 9).  Place a large solder bridge on the filler to top strip joint  on the inside seam. After soldering the filler pieces in, check the end of the top strips and see if is flush.  If it isn’t sand or file it down until the end piece fits flush on all edges with no gaps.( Photo 9)

Mount the BNC connectors to the end pieces. Make sure they are tight, the ends get a lot of action, connecting and disconnecting cables will loosen the connector if it isn’t good and tight.

Mount the end pieces (photo 8 & 4).

Solder the BNC connector center pin to the center terminal of the end switches (Photo 8), that completes the electrical components installation.

Use an ohm meter to check your solder job and wiring of the switches. Connect each lead from your meter to the center contact of the each bnc connector center pin. You should get the following readings.

0 ohms with all the switches in the OFF position.

103 ohms with all the switches in the  ON position. ( 95Db)

84 ohms with 1 end switch ON. ( 20Db) The next 3 switches will read 84 ohms individually.

34 ohms with 1 end switch ON (opposite end of the attenuator) in ON position. (5Db)

After the above test, the next switch should be 54 ohms in the ON position. ( 10Db)

If you get any ohmmeter readings other than those listed, check your resistor networks at each switch, to make sure all resistors soldered to ground have a sound solder connection.

If you make an error or a resister broke, take the resistor/switch apart to correct the error or replace any broke resistors, don’t worry I will replace the damaged resistors at no charge if you will come by my QTH and pick them up. Same goes for the pieces of Teflon sleeving.

If you are not in the Las Vegas area, send me a self- addressed stamped envelope and I will send you the resistors or Teflon sleeve pieces you need at no charge.

If you want any other components, send your request and I will notify you what the charges will be and lead time for components not currently in stock.

You can see in photo 4 how the copper sides and end pieces are sealed together, just like how the dividers were installed.

Photo 7 shows how a small piece of .005” brass foil is shaped on the edge of a piece of pcb material ( I used the bottom piece), While holding the u-shaped piece on the edge, I took a rounded screwdriver shaft and ran it along the top length of brass which gave it a flat edge, after bending and flattening the brass strips, I squeezed the length in so that when placed on the edge of each divider ( photo 6 ), it fit snug along each side of the divider, Tach solder on each end, and on one side only. If you solder on both sides of the divider, the brass won’t be able to flex and lay tight to the bottom. This step is important, because it is what makes the bottom cover connect to each section electrically, isolating each section from one another.

Photo 8 shows how the brass pieces and the over all attenuator should look at this time.

Placed the attenuator on the bottom piece laying flat. Adjust for an equal edge on both sides ( this edge should be the same as the seam of the top strip to sides ) Approximately 1/16”.

Check edge all the way around before tack soldering the bottom to the sides.

NOTE- I used a small desk vise to help squeeze the bottom edge tight to the side edge ( a small “C” Clamp or  a pair of locking pliers ( not too tight) will work just as well.

Move the clamp along the length of the sides as you tack solder to the bottom), Check to  keep the edge tight to the sides. After getting both sides tacked to the bottom ( every ¾” or so)  Fill solder over the tack solder points along the entire length as shown in photo 8 and 9, use lots of solder to fill the seam.

The final step is to add two pieces of the .005” brass on the bottom at each end( see photo 10 ). Photo 10 has the necessary details to install these strips of brass.

Using a spectrum analyzer and a 110db rf source, we found that each section was isolated very well from one another and the leakage and shielding from an external rf source was comparable to a commercial Hewlett Packard attenuator I had on hand.

This is probably the single most used piece of equipment you’ll use when it comes to T-hunting, and it makes an excellent addition to your radio service test equipment list.

Good luck with your project.

 

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