25 October 2010

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!

20 October 2010

Does your FT-857 display look like a zebra?

I have come across several cases of malfunctioning display in the control head of the FT-857, where several "scan lines" in the dot-matrix display cease to function, so the display seems "streaked" at places. In extreme cases, the display becomes difficult to read. The funny thing is, the streaks come and go, or seem to move about the display with time! I finally got to open a control head belonging to a friend to see if there was something that could be done. The photo (click to enlarge) shows the display module ribbon, which is soldered directly to the PCB. The connections on the PCB turned out to be OK, so I checked the LCD side. I couldn't spot anything there either, but I saw that flexing the flexible ribbon near the LCD glass made some of the streaks come and go. The flexible ribbon is attached to the LCD glass with epoxy material. This reminded me of my experience with my Sony F-717 camera, where the epoxy material holding the CCD sensor on its carrier had softened due to heat and humidity, making the connections fail - the CCD had to be replaced. Perhaps the control head of the '857, when subjected for a long time to the temperature extremes and humidity inside a car, suffers a similar degradation of the epoxy material at the LCD display, eventually causing those streaks to appear. All of the affected units I know of were actually being used in cars! For the time being, I can't think of a solution, except of course replacing the whole display module - a bit tough.
So, my advice is, don't leave the control head(s) in your car exposed to the sun, humidity and temperature extremes, the dot-matrix display might become "streaky"! I, for one, have always covered the FT-857 control head in my car with a piece of black cloth, protecting it from strange eyes and the deleterious effects of sunlight. After five years in the car, it still operates fine.

I'll keep my fingers crossed - and my control head covered!

18 October 2010

More on the withering filters case: The TS-2000 disappearing noise conundrum

A few months ago, a friend gave me his TS-2000 to check, because occasionally there was a crackling noise coming out of the speaker (and showing on the S-meter) during SSB weak signal reception (SW8KOU operates EME on 2m).
However, when I checked the rig, the noise just wasn't there. I checked for bad SMD components or soldering, but found nothing. I returned it, the noise eventually came back even louder after a few days, he gave it back to me to check it again, and guess what - the noise was absent again. I gave it back to him again, and he reported there were sporadic outbursts of noise now and then.
Eventually, the noise came back to stay, and loud it was!! It resembled lightning noise, heard during a storm on a low HF band and registered S2 ~ S5 on the S-meter. This time the noise was kind enough to be there when I powered the rig up on the test bench. It could be heard on SSB only, not on FM (with the proper test setup). I had already previously checked the rig thoroughly for bad SMD components or bad solder connections, and, having seen many "withered" ceramic filters in the meantime (in other rigs), it suddenly dawned on me that the noise could very well be emanating from a faulty ceramic filter! The schematic diagrams showed two such filters in the SSB signal's path, CF3 and CF4 in the TX-RX unit.
To make a long story short, the culprit was CF4. The filter had suffered the same degradation I describe in another post, but in a more gradual way, due to different conditions. The other cases I have examined had about 7V applied to the filter's pins, but the circuit around CF4 applied about only 1.5V to it. The degradation was there, but the symptoms and time scales were different. I measured about 1MΩ from the output pin to the nearest ground pin, which was far more resistance than the usual 20 ~ 100 Ω I had previously measured in other malfunctioning filters. That explained the crackling noise and the fact that the receiver didn't go totally deaf, like in the other cases. The lower voltage was slower to act on the filter, and the degradation was milder. I opened up the filter, and, there you are, I could immediately see the effects of oxidation on the phosphor bronze spring plate. I removed the thin element at the output side, and sure enough, it showed the telltale signs clearly (see the photo, click on it to enlarge). After carefully cleaning the edges in the way I have described, the resistance reading was more than 40ΜΩ. I soldered the filter back in place, and the noise was gone. My friend tells me that the receiver is very quiet now (but let's see what happens when eventually CF3 (with 7V applied to it) breaks down, too!).
 I will shortly publish the solution to this vexing problem, consisting of just adding two DC-blocking capacitors at each filter (the example mod will be performed in an ailing TM-D710). The same simple concept, however, holds for the modification of all similarly affected rigs.