INSTRUCTIONS TO CONVERT THE MICOR EXCITER HELICAL BAND PASS FILTER TFD6112A TO 220 MHZ
By David Stanford K7IOU

Or buy a Micor mobile filter Z501 as the cardboard forms / slugs aren't glued in place. 
For ease of adjustment check Step 11.

Steps 1 through 7 came from: Exciter 220 MHz Band-Pass Filter Conversion:
http://www.repeater-builder.com/micor/222z501.html

There are 5 slug-tuned coils in this filter. It is symmetrical.  

Step 1: Remove the filter from the casting. 

Step 2: Remove the cover from the filter. 

Step 3: Remove 1 turn from the outermost coils in the filter (coils connected to the 
input and output). 

Step 4: Remove 1 turn from the center coil in the filter. 

Step 5: Remove 2 turns from the remaining 2 coils. 

Step 6: Tune the coils for minimum VSWR at the operating frequency. Tuning will change 
when it is assembled, so make an adjustment, then put the cover back on to check it. 

Step 7: Re-assemble and re-install in the casting. 

Step 8: Use a small screwdriver and loosen the coils on the cardboard forms by pushing 
up and down on the form. This will make for easy removal when you remove the cardboard 
forms.

Step 9: Use a very small wire cutter and cut the base of the cardboard form on the 
housing. Note: there is a step up on the housing the form sits on. Twist the cutters 
side to side as much as possible, until form is free from the housing. Remove the tube. 
Clean out the remains of the tube and epoxy in place. The step in the casting will 
re-align the new tube.

Step 10: Using a similar form (we had plastic), glue it in place. I used a commercial 
hot glue gun. WD7F removed the wire coil on the form, I cut off the washer end and 
trimmed the slug to 1/4 inch as our slugs were too long. I trimmed the slug with an 
Allen wrench inserted up to the trim point. Using a small wire cutter I cut a thread 
or two at a time. Note: Have extras on hand just in case you break one.

Step 11: Drill 1/8 inch holes in the cover. Measure, mark, and drill out over the slugs 
for adjustment. WD7F suggested this and pointed out there would be no loss at the 
frequency with that hole size. Re-assemble and install the cover.

Step 12: We took an HT on our transmit frequency, 1 watt, into a dummy load. The scope 
was connected with a "T" into the output of the filter. We keyed the HT and watched the 
pattern and adjusted it with our plastic hex head tool for the widest pattern we could 
get. Do one slug at a time and repeat.

Step 13: Then install a watt meter and check the power through the filter, and then 
with an RCA female union in place of the filter. We picked a union that had no or low 
loss. We measured 1.02 dB loss through the filter.