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Icom IC-756pro2 filter board clearning and testing
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 I bought a dirty Icom IC-756pro2 with water damage as a project. One of the repair steps was to clean and check the filter board. The events of cleaning and testing is described below.
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The filter board consists of a common mode input power filter and a switchable RF filter section.
Power filter
The 13,8 VDC power is filtered to prevent radio frequent noise getting in the rig and above all preventing radio frequent noise getting out of the rig to the power supply. This is a rather simple passive LC filter.
Switchable RF filters
The other section has eight switchable LC radio frequent filters. Each filter is a parallel band stop filter for suppressing higher order signals (harmonics). If for example, an 3,5 MHz signals has impurities, there will be harmonics at 7/10,5/14/17,5/21/24,5/28 MHz and so on. You don't want to sent out these spurious signals so your transmission can be heard on other frequencies. The parallel band pass filters are great for harmonics suppression. A set of low pass filters would also work, but these band stop filters are more effective. The first harmonics (f*2 and f*3) are usually the strongest signals. It seems that the band stop filters are designed to block these two harmonics. Higher order products are also suppressed, but the damping is less. There are eight switchable filters for 1,8/3,5/7/10/14/18+21/24+28 and 50 MHz. By selecting a certain frequency on the rig, the corresponding filter set is activated. If the rig is turned off, all the filters are deactivated. The damping is therefore more than 70dB (up to 30 MHz). Thus if the rig is turned off, the receiver is rather well protected against strong signals! This is a nice feature; well done Icom! The board is also rather clever designed. Each filter set has two relays that switch simultaneously. So both relays are connected in parallel. To simplify the board, a not used filter is used to pass trough the DC voltage for the relay of another filter. This reduces an additional connecting trace between the relays. This is a nice efficient design!
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There are traces of water on the boards. There's also some rust residue on the boards. It seems that wet cleaning with baking soda is needed. I got a glass oven dish and put the board into it. I made a 1:3 solution of water and baking soda. The result is a thick white paste. I scrubbed the board with the paste using a set of paint brushes. The baking soda works as a abrasive solution that removes the residue from the board without damage. After some scrubbing, the board is as new. After the scrubbing I rinsed the board thoroughly using tap water and then demi water to remove possible impurities from the tap water. The board is thoroughly dried using compressed air. Heating the board at 70'C was also possible, but I didn't want to heat stress the board and the compressed air removed all the water. Some images of the scrubbing is shown below.
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Since the working of the original rig was questionable, I wanted to eliminate as much possible failures as possible. Testing of the board is rather simple so I tested the board fully. I connected the spectrum analyser to the filter board using two TMP/BNC cables. By using the tracking generator I got a nice plot per filter set. By applying a 10...12 VDC signal to the filter relays, the corresponding filter set is activated. The test setup and results are shown below. As you can see, the filter board performs great! Everything switches as desired and the signals seems perfect. So this board is ready for use again!
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