As you all know, Maxtrac filters for the HAM band have become very hard to find! Thanks to Allen WA6IPZ taking the time to poke around a bit, he has discovered that there is a way to "tune" them for RX at 927 without removing the board!
Below is a before and after sweep shot of the filters to show the change in response at 927.500.
As you can see, there was more than a 6db improvement over the unmodified stock filter at 927.500! This was just ONE filter. My tests showed about a 1.8db higher loss at 927.5 than the stock filter did at 932MHz, but that is to be expected after you see how the tuning is done. Now....how to do it!
It is done by inserting a steel rod into the center hole of each filter. I used the bottom of a broken Larsen UHF antenna, which had a diameter of exactly 0.10" or 2.58mm. It does NOT need to be insulated from the inner walls of the filter itself. To hold it in place, I wedged a piece of saran wrap between the rod and the filter. This will give it a bit of "resistance"....BUT...BE CAREFUL!! If there is too much resistance, or you force the rod in too hard, you will crack the porcelain in the filter and it is now useless! I know this from experience!! The trick is to push the rod all the way to the bottom and begin backing it out VERY SLOWLY! Just off bottom is where the tuning is. You should grind the end of the rod flat, as shown below, as it is the capacitance with the bottom that does the tuning.
The rod with the end ground flat (leaning on the left side of the filter) before being inserted into the filter.
The rod inserted and tuned. If you look closely, you can see the small piece of saran wrap sticking out on the left side of the rod. The fit is pretty "snug" but not too tight. Can just barely move it with my fingers. Used a small pair of needle nose pliers to "manipulate" the rod for tuning. The best way to tune is by injecting a weak signal at 927.500 and tuning for best RX. Normally we have seen around 0.25uV after tuning, which is about what you would see with 915 filters! Touching the rod with your fingers or pliers has little to no affect on tuning.
After tuning is complete, you may want to drop a bit of epoxy or superglue on it to hold it in place if you are not pleased with the "grip" of the saran rap!
This tuning is only actually changing the center portion of the filter, so the process is not exact, as can be seen in the overall shape of the filter after tuning for 927.5. The shape may also be affected by the test setup as it was tested in a "dead" RF board by soldering mini coax to the in and out connections of the board with the filter in it.
There may be a way to add some capacitance to the first and third part of the filter for that extra 2db, but is it really worth the effort?
Also, it will not tune much below 926, so for a repeater receiver, you will still have to remove the filters. This should also work for the Kenwood 931 & 941 radios that use the same filters!!
Thanks again to Allen WA6IPZ for taking the time to experiment and figure this out!