Replacing Yaesu FT857 IF filters
Recently my Yaesu FT857 became deaf. It turned out that the plastic enclosure of the FT857 filters over time consensated moisture which ruined the filters. In this post I’ll show you how I replaced the filters and brought the radio back to life.
I own a Yaesu FT857 for some years now. I haven’t used the radio much. A few weeks ago I wanted to do some tests with a new antenna, but when I took the radio out of the box and connected it to the antenna it felt pretty deaf and all I heard where static crashes. Even after disconnecting the antenna the “static” crashes remained. So obviously something was wrong.
I was a bit surprised, because so far the radio spent most of it’s life in the OEM’s box.
So it was time for a visual inspection of the radio’s inners. My eyes felt pretty soon on the IF filter stage. On top of the plastic housing of the 455kHz filters (CF5, CF3 and CF2) sweat beads and some sort of cristals were clearly visible.
The pictures above shows the “sweat beats” and cristals formed on the filter’s plastic enclosure.
Fortunately, replacement filters are inexpensive (less than 10€) and commonly available from large eletronics distributers or ebay.
I decided to replace the three TOKO filters with pin compatible filters from Murata.
- CF3: TOKO A55GK -> Murata LTM455GW
- CF2: TOKO A55EJ -> Murata CFWM455E
- CF5: TOKO A55HL -> Murata LTM455HW
How to replace the filters
Just a word of caution: Soldering on the backside of the PCB is a bit tricky since small SMD components (mostly 0603) are located nearby. You need good magnification glases or a microscope. Below is a picture of the lower part of the PCB, marking the solder points of the three filters.
Since all modern eletronics is now soldered with lead-free solder it’s a bit cumbersome to remove it. I usually add some 60⁄40 solder and then remove it with solder wig. Adding flux also helps.
After removing the solder I was able to pull out the old filters.
The position of each filter is marked with a letter which corresponds to the appropriate filter.
- G: 455GW
- E: 455E
- H: 455HW
After soldering the new filters, I removed the flux with Isopropanol and waited a couple of hours until the alcohol was evaporated. The las thing you want to have is a shorted circuit somewhere on the PCB ;-)
The picture above show’s under high magnification the lower part of the PCB, cleaned and with new filters equiped. The whole procedure took about 90 minutes.
So what after all is the root cause of the failure? Well, you can find different speculations on the web. Some say that the plastic of the filters was just bad, others believe that there is a design flaw in the ciruit and that the DC bias voltage killed them. There are even modifcations available where folks break out vias and insert additional DC blocking caps.
I personally don’t think that such drastic modifications are necessary. As The Radio Mechanic pointed out in this Youtube video on this topic, non of the datasheets actually tell that DC voltages will harm the filter. So probably the bad plastic theory is right. Time will tell… I got another spare set of filters, just in case.
Finally: Here is The Radio Mechanic’s video, which is definitely worth watching if you are planning on changing the IF filters.