11 December 2012

A proposed root cause for the "Withering Filters" phenomenon

Fig. 1 Photo by Jerzy,
(Click to enlarge)
After having examined the failure mechanism of more than 100 ceramic filters that have succumbed to electromigration, I think I have collected enough data to propose a root cause for the phenomenon that has caused failure of a very large numbers of filters. I should gratefully thank the many fellow radio amateurs that have sent photos of their filters to me, adding to my database on the subject. Manufacturers might want to consider changing their production methods to avoid the chain of events that I propose that leads to this phenomenon.

So, let's look at the data I have collected and a proposed theory that explains the facts:

Fig. 2. Note the water droplets.
1) In all of the ceramic filters I have examined, there was a surprising quantity of water inside the plastic case (take a look at Figs 1, 2 and observe the signs of water presence on the elements and inside the black case, also the whitish flux residue on the case of the adjacent filter in Fig. 1, which shows that a quantity of water evaporated from that surface, leaving it behind). The water inside the case causes the filter to fail soon after the transceiver circuit applies DC voltage to the input, and mainly the output pins of the filter. The quantity of water is such, that can't be attributed to a filter manufacturing flaw: it's just too much. I can't imagine that the filter manufacturers would have ignored it. Also, I haven't found water inside some new filters I have "dissected". So, there must be another reason for this quantity of water inside the filter.

2) Where does the water come from, if not from a filter manufacturing flaw? I think the answer lies with the newer technologies and materials used for the fabrication of populated printed circuit boards (PCBs). Briefly, the new methods of wave soldering with non-Pb solders extensively use organic acid (OA) fluxes. Those fluxes are removed from the PCBs after wave soldering by pressure rinsing with hot water. The rinsing process takes some time to complete.

3) I think that the water ingress can be explained thus. The rinsing process uses hot water (about 70 degrees Celsius), sometimes with ionic additives. This (as well as the previous soldering process) causes the (so far dry) air inside the plastic filter case to expand, creating positive pressure inside the filter case. This results in air escaping from inside the filter case, as the temperature softens the plastic case and compromises the (not so great to begin with) sealing at the bottom of the filter case.
When the PCB exits the rinsing process, water is held by affinity under the filter case, between the filter and the PCB surface. The filter starts cooling down, and this causes the pressure inside the filter to drop under the atmospheric pressure. This negative relative pressure tends to draw a small quantity of the water under the filter inside the filter case, through the very fine cracks at the filter case seals that the positive air pressure (when the filter was hot) created. The quantity of the water is some micrograms, but it's more than enough to cause trouble. The board is then dried, but the accident has already happened. The water is already inside the filter.

4) Once inside the filter, the water can't escape and remains trapped inside the case. Some of it condenses or moves on the filter plates, starting the electromigration process where the electric field (due to the externally applied voltage) reaches sufficient value. This most often occurs at the corners of the thin output ceramic plate (element) at the output of the filter, which is only about 0.35 mm thick. This leads to the failure of the filter after some time.

5) What might be a solution? A slight modification of the rinsing - drying process might help in avoiding this phenomenon.
If the PCB is dried IMMEDIATELY after exiting the rinsing chamber with pressurised HOT air, then the water might not get inside the filter case, as the pressure difference doesn't occur this way before the water has been removed from under the filter.
I have no further knowledge of the exact conditions used in any particular case, so the above proposal may not always apply. But each manufacturer could certainly review the method used and modify it accordingly, so as to avoid the conditions that drive water inside the filter case (and possibly inside other "sealed" components, such as miniature relays).

If someone with more knowledge of the PCB processing methods wishes to add (or correct!) something in the above, please send an e-mail to sv8ym@raag.org.


Addendum: The above suggest that if you repair (clean and dry thoroughly with hot air) or replace a filter that has gone bad through water ingress during the rinsing procedure with a brand new one that hasn't come in contact with water, you may not have problems in the long run, even if you don't also install DC-blocking capacitors. That is so because you need BOTH water AND a voltage gradient to start electromigration. A completely dry filter probably can withstand 7-8 V indefinitely (as experience with older rigs shows - the PCBs were washed with organic solvents a long time ago, in the paleolithic Pb era!).

08 October 2012

A scorching hot summer in Zante

No, the airplane in the first photo didn't get entangled in my antennas!   It flew over the house and literally rescued our property from going up in smoke, like the pine trees in the second photo, where the plane drops 6.5 tons of water on the rapidly approaching fire front, less than 100 meters from the house (click on the photos to enlarge them). It was the 28th of August, and had the firefighter planes not arrived for another 10 - 15 minutes, all bets would have been off. It was the second close call this year for us. Such scenes have been extremely commonplace in my island during the last 25 years.
It's a sad reality that a vast pine forest area has been burnt to the ground during the last years, depriving Zakynthos of one of its greatest assets. Unfortunately, the great damage to the flora and fauna of the island will take many years to mend (if ever!). The planes and the firefighters on the ground (among them several volunteer radio amateurs) have saved many homes and possibly lives. We owe them all a big "thanks", perhaps more so to the pilots, who perform extremely dangerous aerobatics to deliver their payload with pinpoint accuracy.

05 October 2012

The GaAsFETgate scandal in Zakynthos

Click on photo to enlarge.
GaAsFETs have really become ubiquitus in every situation one wants high gain and low noise figure in frequencies ranging from the low VHF to microwaves. The picture on the left is of a very popular transistor (although a bit dated now), the ATF 10136 from Agilent Technologies. The ceramic cap has been removed to take this photo (about 200X magnification). 
The chip is in the center, attached to the source strip. The diameter of the white ceramic material is about 1.5 mm. The drain is on the right and the gate on the left. Observe the thin gold wires that connect the chip to its carrier contacts (and to your circuits!). This particular transistor (from a low-noise 2-m preamp) had a cruel death: it was accidentally (and quite scandalously!) bombarded with high power on 144 MHz from the amplifier of an EME setup in a friend's station. The gate short-circuited to the source, and you may see that even the gold wires almost melted by the heavy current that destroyed the transistor (see the red arrow). Fortunately, nowadays those great transistors are fairly cheap to replace, and can be easily procured.  Thumbs up to Agilent (now Avago, and the other semiconductor manufacturers) that make our beloved toys!

03 October 2012

Does your IC-E92D have any loose screws?

"It's the loose screws in my head!"
The IC-E92D is a very well constructed handheld transceiver, with a really solid feel of quality. I bought mine in 2008, in order to wet my feet in the new D-Star ocean. I also have the external speaker - microphone adaptor, which I think makes the rig a lot more pleasant to use.
The transceiver performed flawlessly, no problems at all, up to a few days ago, when I noticed that when I keyed the transceiver in medium and high power output on UHF using the provided rubber antenna, the display backlight would consistently and magically turn itself on and strange beeps and noises would emanate from the external speaker. The phenomenon was absent in the low and super-low power settings, so I reasoned that some kind of RF intrusion into the logic circuits was to be blamed. Removing the external speaker/mic cables changed things a bit (only full power produced the strange effects), but the goblin was surely still there. A quick search on the Internet revealed that other users have also reported similar problems with the display going blank or even getting inverted (!), so I was not alone out there. 
But what had changed to produce those effects? I hadn't changed anyhthing, the rig functioned with exactly the same accessories from day one. So, it was logical to think that the reason was internal.
Before opening the case, I studied the service manual a bit. I observed that the screws holding the transceiver together also had another very important mission: (especially the top two) pressed the main printed circuit board grounding spring contacts on the chassis. So these (and of course the other internal screws) are cleverly forming RF-tight enclosures within the transceiver. Could it be that some of those screws had gone loose, compromising this important function?
So, to take a first shot at it, I proceeded to carefully tighten the six screws that hold the transceiver together (they are at the back side, two of them near the top and four of them under the battery - you have to remove the battery to see them). Indeed, they took about 1/4 turn to feel nicely tight again (BUT DON'T OVERDO IT WITH THAT SCREWDRIVER, PLEASE!).
I am happy to report that the problem vanished immediately. I loosened the screws a bit again to see if it would come back, but it didn't, obviously because the contacts were not disturbed enough this way. My theory is that the screws gradually become a bit loose with normal use and thermal cycling, making the shielding / grounding contacts unreliable and "leaky". A bit of tightening and everything is back to normal again. Perhaps some day I will also have to open the case and tighten the internal screws, too. If the problem doesn't go away by tightening the externally accessible screws (especially the two ones at the top), that's the next logical step. This reasoning may also hold for other transceivers with the same shielding method. I must say I have certainly seen a lot of strange problems in other modern VHF/UHF transceivers go away when I tighten the screws that hold the printed circuit boards on the cast aluminum chassis!! They have a tendency of coming loose, mainly due to thermal cycling action.
Enjoy your IC-E92D and see you on D-Star!

14 June 2012

Removing the filter case without desoldering it from the PCB

Desoldering the ceramic filters in order to fix them can be a tough proposition in many cases. So, I came up with a different way of removing the plastic case - without removing the filter from the PCB..
Take a look at the picture (it's from the repair of an TM-D710E). Using long nosed pliers, grab the filter case exactly as shown and, gently but firmly, pull the case by - very carefully! - rocking the pliers in the direction of the red double arrow (perpendicularly to the longitudinal axis of the filter). The case usually comes off relatively easily and the offending elements can then be easily inspected, removed and cleaned, restoring proper filter function. Be careful not to lose the bronze tensioner spring plate that (usually) comes off with the case. This method of case removal has worked every time for me (I have fixed more than 50 filters up to now), saving me considerable time and work. Of course, after repairing the filters, don't forget to add the DC blocking capacitor at the output side of each filter, so that you won't have to remove the filter case again!

***ADDENDUM: Carefully heating around the base of the filter case for a few seconds with a hot-air soldering gun set at about 200 degrees Celsius usually makes the case come off much more easily. I said "Carefully", please take note!

01 June 2012

Sorry, Baldur

A well-known and widely respected fellow radio amateur and DXer, Baldur, DJ6SI, was arrested for "spying" (!!!) and put through an ordeal by the incompetent and ignorant Greek "authorities" in Kos island yesterday. As a radio amateur, I would like to express my sympathy to Baldur and say how sorry I am for the unfortunate event... Sorry, Baldur.

07 April 2012

The ailing ALC of the venerable FT-ONE - a gooey story

I recently got to restore an FT-ONE to full working order.   This particular FT-ONE brought back memories from 25 years ago, as it used to belong to a good friend in whose shack I drooled all over it, as it was the "Yaesu flagship" of the mid-eighties. That friend sadly passed away a few years ago, and the rig, after staying idle in storage for some years, was bought by another friend who sent it to me to shape it up, as it was quite unusable.
After fixing several problems (among them xtal oscillators that didn't want to go where they were supposed to and also an obnoxious general case of acute screwdriveritis the rig had gone through) and proceeding to full alignment, I stumbled across a most peculiar problem: The ALC was acting up. Although I could balance the directional coupler on 28 MHz (as indicated in the service manual), and then I could complete the ALC alignment transmitting into a dummy load, after a few hours, when first switched on and put to TX, the rig had a very low RF output (10 - 20W) and showed a large SWR indication for no apparent reason (transmitting into a dummy load). If I went on transmitting a carrier for a minute or so, the power would slowly come up, but even then it never reached more than 60 W or so on 1.85 MHz, and various other levels on the other bands - also, the ALC indication was fluctuating for no apparent reason.
Fig. 1 The gooey stuff!
I thoroughly checked the ALC board, suspecting a bad germanium diode - all checked out OK, as did all the transistors and trimpots on that board. Nothing there. I started suspecting some other part of the ALC feedback loop, but then my brain - in a rather rare epiphany - took in what my eyes had been seeing all along. Take a look at Fig. 1, click to enlarge.
Do you see the (kindly provided) red arrow? It points to a blob of some kind of gooey glue material intended to keep the directional coupler  toroid in place. I grabbed my ohm meter probes and placed them on two spots on the goo blob. Bingo! The goo was conductive! I read an indication of about 100 Kohm with the probes spaced a few millimeters. So the conductive goo was "leaking" RF current into the reflected power detector (D01), fooling the ALC circuit into believing that a serious SWR existed, so it promptly proceeded to reduce the output power (Yaesu calls this the APF function). Observe in the picture that the goo covers both the TX line (the bare thick wire through the toroid) and the winding ends on the right (where the enamel insulation had been removed).

Fig. 2 Off with the goo!
How to fix? Simple! Take a look at Fig. 2 (the "after" picture!). Using a thick needle, I scraped off as much of the brown goo as I could. I almost took it all off, it's hardened and breaks off in little chunks. I was careful not to scrape off the enamel insulation from the toroid windings.
After removing the goo, everything returned to normal. The ALC alignment went extra smoothly, and the power output is steady and OK on all bands. The high SWR protection now works perfectly.
This nasty goo has crossed my path for the second time: An old TS-440 that would impulsively unlock its PLL had plenty of the same goo on the VCO components. The resulting inter-component leakage was causing the PLL to unlock. It seems that with age this stuff oxidises and becomes conductive, upsetting sensitive circuits like VCOs and directional couplers. I wonder what brand name that goo was!!

15 February 2012

Summer, bicycle and ham radio - and "It's a cool, cool summer!"*...

Spring (and summer!) is ante portas, and sensitive guys like me just can't wait to hit the road to enjoy the season and pick daisies - getting on the air from my bicycle in the process! Nothing fancy - a garden variety 7 speed bicycle, the proper pannier with several big pockets, a rather modest collection of equipment, tools, mobile and wire antennas for HF/VHF/UHF, a battery, a solar panel and my trusty old FT-817, ready to give me me a lot of fun hours - and some much needed exercise, too! Also, I get to really observe many places I thought I knew - something you can never do cruising by in a car. The photo (from last year - click to enlarge) shows yours trully enjoying his coffee while operating on HF in front of an old, abandoned stone house in the vicinity of Agalas village, Zakynthos island. My antenna is a homemade whip for 20m, which doesn't show very clearly in this poor-quality cellphone picture. It's amazing, though, what you can do with such a simple setup on HF! More to come on the subject...
* I prefer cool summers to cruel ones!

13 February 2012

Noise for the boys

Fig. 1  Can you spot the missing part? (click to enlarge)
A few days ago, the digital TV broadcasts commenced (at long last) in my area, so I promptly installed a digital receiver and enjoyed perfect reception of those long awaited, crystal-clear TV signals. My initial joy vanished in a moment, though, when to my great dismay I found out that the digital receiver completely trashed my reception on HF with S9++ raucοus buzzing garbage all over the bands. I confirmed that the culprit was indeed the shiny new DVB receiver, by disconnecting it from the mains supply - then I proceeded to examine the entrails of that stinking rat. Not many surprises there - the usual, dirt-cheap switching power supply, where several corners have been brutally cut to keep the dreaded cost down. Take a look at the first picture (Fig. 1) - do you see anything missing? As the magnanimous host I am widely reputed to be, I have added a red arrow to help you figure out the answer - no frigging line filter at all, the one and only position for such a component had been jumpered in the most barbaric way. So the harmonics of the power oscillator that makes up the heart of the power supply are very efficiently radiated from the mains power lines - and I can assure you that those signals, although extra strong, don't carry any intelligence at all!

Fig. 2 The upgrade in place!
This is a very sad story indeed, causing serious grief to many unfortunate radio amateurs who suffer dreadfully degraded reception by the scores of big and small switching power supplies that infest the modern home (perhaps you have read my relevant QST 7/2004 article about "dirty", dirt-cheap computer switchers).
 Those power supplies are very cheap, but in this case cheap comes at a high price for radio enthusiasts who seek joy in the form of distant, low level signals. What's a young man (or lady) to do, then?
Other than replacing the offending switcher with a better quality one, which oftentimes isn't feasible at all, you could add the missing line filter elements yourself. Old computer power supplies will yield the necessary components for the tinkerer to lash-up a common mode line filter (Google it up and learn, don't expect everything on a platter from me!) - unless they're also rats themselves.
Take a look at Fig. 2 (click to enlarge), where the filter has been added to the rat, transforming it to something infinitely more tolerable. The toroid came from the line filter of a good, old computer switcher and worked fairly well, reducing in a single step the trash to about S2 - S3 worst case across the HF bands. (Someday I might get down to improving this even more.) The toroid has a bifilar winding, with each wire connected in series with each of the the mains conductors, just before they exit the case. 

Disclaimer: This mod requires working with dangerous mains voltages. Be cartain that you're up to it before tampering with a switching power supply, they definitely don't excuse ANY kind of mistake, and may immediately violently explode to show that fact!!