Good news: YAESU has added the DC blocking capacitors at the IF filters of the FT-857D!

Fig. 1. The new FT-857D PCB.

Sometimes I think it's mothing short of a miracle that we can buy new technology products (like our transceivers) at VERY reasonable prices (not exactly so when I got into the hobby, 30 years ago - a handheld transceiver was a very expensive item in my country, it cost more than the monthly wages of a public employee).

In sharp (and delightful) contrast, recently I bought an Agilent E4406A Vector Signal Analyser,  an HP3586A Selective Level Meter and a Hagenuk Digiflex LAN Time Domain Reflectometer from the surplus market, at what must be a tiny fraction of their original cost! Mind you, the E4406A was about 45000 euros less than a decade ago! Long live companies like NOKIA (that's where my wonderful E4406A came from) and the ever-faster changing industry standards!

Fig. 2. The Serial Number.

But I digress... It seems that the nasty filter problem has caught the attention of the manufacturers! Tomi from Romania has sent me a photo of the PCB of the FT-857D he bought recently (Fig. 1 - click on the photos to enlarge), in which we can see that the DC blocking capacitors have been added! Thumbs up for Mama Yaesu! 

Maybe they do that for all their products now. I am sure other manufacturers have also taken steps to cure the nasty filter plague, but I haven't seen any concrete proof yet.

Tomi also sent me the serial number of his rig (Fig.2) - to save those that would like to add the capacitors in their recently-built rigs from the trouble! Thanks, Tomi!

The withering filters - adding a blocking capacitor when space is tight

Adding the DC blocking capacitors to protect the ceramic filters in your  rig may sometimes prove a bit difficult with the newer, extra small SMT components. Sometimes, there just isn't enough space or PCB trace length to accommodate the new part. In such cases, alternative methods might help, like the one in the photo (click to enlarge).

Here, the capacitor has been added only at the output pin of the filter (top right in the photo), because the output side of the filter is the most vulnerable (owing to the thinner ceramic resonator there). Due to the very small dimensions of the cicuit (the distance between the input and output pins of the filter in the photo is 8.3mm) there wasn't enough PCB trace length to comfortably add the capacitor by cutting the trace. So the solder was sucked out of the pin's plated-through hole, which left the pin standing in the center, unconnected. Then, the size 0603 capacitor was soldered at an angle, bridging the distance between the top of the filter's pin and the PCB trace. Problem solved, without PCB cutting (which may also sometimes prove dangerous to nearby SMT components)...Be sure to check whether the filter's pin actually stops making contact to the plated-rhrough hole, after you suck out the solder!

(The photo is from a President brand marine portable VHF tranceiver, that had gone completely "deaf" due to filter deterioration - after reviving the filter and adding the cap, it's back singing and dancing. The manufacturer [what a surprise!!] had omitted the DC blocking capacitors between the IF IC and the 2nd IF ceramic filter.)

Lift the withering spell off those ailing ceramic filters!

As promised, here is the solution to the "withering ceramic filter" problem. The mod here was performed in a TM-D710, but the concept presented holds for all similarly affected rigs.

First of all, take a look at the photo on the left (click on the pictures to enlarge). There are four ceramic filters in the '710, two (wide-narrow) for each receiver.

Our first step is to cut two PCB traces converging at each filter's input and output pins, bridge them, and then solder a size 1206 or 0603, 0.1μF / 25V or 50V SMD ceramic capacitor from each bridge to the filter's input and output pins (8 capacitors in all for the '710). This way, the DC switching voltage is blocked by the capacitor, that passes only the small AC (signal) component into and out of the filter. Removing the DC bias from the filters is all that is needed to avoid filter failure.

The first photo shows (with the red arrows) the eight traces to be cut with a very fine-tip grinding tool. Notice that there are 16 cuts for the 710, eight are shown completed, the other eight are in the red circle on the left (the circuit topology is identical).

Take a look at the second photo (click to enlarge), showing  the completed job for a filter set. As you see, fine wire bridges have been soldered, bypassing the cut-off portion of the circuit that was going to the filter pins, and the ceramic capacitors are also in place, soldered between the wire bridges and the filter's input / output pins.

The third photo (click to enlarge) shows the completed job for all four filters.

I must say, that although very simple, this mod is a bit difficult because of the very small dimensions of the traces and SMD components. Extreme care is required, along with the proper tools and ability to work with them. It's easy to make a disaster out of the job, so please be careful!! If you don't feel up to the job, have somebody qualified do it!

The same concept holds for any transceiver that has DC bias voltage applied to ceramic filters. The objective is to block the DC bias with the capacitor, but otherwise maintain intact the circuit's filter switching functionality (if of course there is one), which in most rigs is accomplished by properly biasing switching diodes.

Please note: If your filters have already deteriorated, they must be replaced or repaired before the mod. The symptoms of deteriorated or failed filters are: "deaf" receiver, crackling noises heard during reception of otherwise full-quieting level signals in FM receivers, crackling noises with no antenna connected in SSB receivers.

Good luck!