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.

23 July 2010

The perils of cheap adaptors II

The "T" adaptor in the photo was the reason a WACOM WP-639 duplexer cavity couldn't achieve a notch of more than about 12dB. As you may notice, there is a small helicoidal spring "joining" the two center conductor members in the adaptor. Being a small inductor, you may imagine what effect the presence of this spring has on VHF signals!
This is quite evident in the SA-TG screen photo. The response dip (notch depth) should have been about 35dB, but only 12 dB could be achieved, because of the impedance and loss of the spring, on the signal's way to the resonant cavity.
An "Amphenol" adaptor cleared the problem immediately and restored the notch depth to 35dB.

This unacceptable adaptor was the reason the duplexer had to be hauled down from the mountain and retuned. Even on HF, this kind of connection might create problems with its reactance - to say nothing of its deplorable reliability.

Do you have any such garbage in your VHF-UHF setup? 

If the captions in the photos all seem Greek to you, that's because they indeed ARE Greek! (Well, mostly!)

19 July 2010

The perils of cheap adaptors

Cheap RF connectors / adaptors may not only "damage" your signal, but also your rig! Take a look at a female type-N connector that suffered distortion of the center contact because of a cheap, unsuitable adaptor - the center contact is completely bent-up!
Fortunately, this specific connector could be brought back in shape, but I have seen others that had to be replaced because the contact "fingers" had broken or had bent in such a way that they couldn't be saved...

Remember, you only pay for a good adaptor or connector once!

It's getting hotter...

In a previous post I discussed the problem of imperfect contact between the power amplifier modules and the heat sink in an TM-D710. Last weekend I worked on three more D-710s, which had developed the infamous "withering filter" illness, and discovered that the imperfect contact matter may reach extreme proportions!

Take a look at the photos (click on them to enlarge). The power modules barely made contact with the heat sink, only near the affixing screws. Less than 20% of the surface was in contact! Not a very good scenario for the longevity of the modules!
The other two rigs showed about 25% and 10% loss of contact, and only at one of their two modules, respectively.  The photos are of the "worst case" rig. As an afterthought, I should have tried to see which is at fault, the modules or the heat sink, but I foolishly didn't do that. I will in the next transceiver that comes along, and let you know. My wild guess is that the die-cast heat sink machine finishing (leveling) at that spot is at fault; but  I am certainly not sure.

What to do if your rig suffers this way? A couple of drops of gear oil will probably slide your problems away and cool things off, see the older post.

12 July 2010

The Mysterious Case of the Withering Filters

The ceramic filters found in almost all  radio communications equipment are indeed extremely useful components. Small, cheap and efficient, those "little black boxes" have found widespread use by all the manufacturers. I used to think that they're almost indestructible, because I had never seen one of them fail - but this fact has changed.
I have recently come across several cases of VHF-UHF transceivers where the receiver suddenly went deaf, faintly hearing signals only above -60dBm. The culprit was the ceramic 2nd IF filter (450 or 455 kHz) in all those cases. All those filters I have examined showed the same symptoms: the output side showed a low (a few tens of ohms) resistance to ground for DC, where it should have been almost infinite resistance.
After a while (and having replaced several such filters in my friends' transceivers), I became curious and investigated the reasons why those fairly robust components had become bad.

First, let's talk a bit about their structure (see the diagram I made, click on it to enlarge). Most of the 6-pole ceramic filters used in amateur radio transceivers have the general structure shown on the left. There are six ceramic (barium titanate, if memory serves) resonator elements, three in series and three shunt, connected as shown. The series elements are thick, the shunt elements are thin, and both have their wide surfaces plated with a metal (I guess it's a silver alloy). The narrow edge surfaces are not plated.
There are metal inserts between the ceramic elements, making contact to the plated surfaces of the elements and providing the electrical connections to the outside world. The whole structure is housed in a small plastic case, which is hermetically sealed with epoxy resin at the bottom side, where the pins come out.

So, what was the problem? While waiting for a replacement filter for a rig, I decided to try to pry open the case of the failed filter. I did so with the edge of a very sharp X-acto cutter, and I carefully removed the black case. One of the ceramic resonators fell off, along with a (phosphor bronze?) tensioner spring plate, which keeps everything pressed together when the filter is in its case.
It was immediately evident that something was wrong, because the spring plate was visibly oxidised, and there were suspicious looking spots at the edges of the thin ceramic element (you may notice one of them just below the right corner). I measured with my ohmmeter and saw that that element was the bad one, because the resistance reading was 19 ohms, the same value I had previously measured between the output pin and ground. If you take a good look at the next macro photo, you will see that the corners of the other elements also have low-resistance deposited paths short-circuiting the elements in the same way (also, look at the lower part of the diagram). To my surprise, with a magnifying glass I observed tiny droplets of a clear liquid (water, I think) at the inside walls of the case! What was the story here?

Hello, electromigration! The "crime scene" had all the necessary elements required for electromigration to do its nasty stuff. But let's take things in order.
a) Ceramic filter manufacturers (ALL of them!) expressly warn against applying a DC voltage at the input and output pins of the filters. Why? The reason is electromigration!
b) Electromigration is a process where, under the influence of an electric field and in the slightest presence of moisture, metal (especially silver) starts migrating and forming conductive paths (called dendrites, from their tree-like appearance, δένδρον [dendron] in Greek) across insulating materials. This phenomenon is a major headache e.g. for IC manufacturers, significantly lowering the reliability of their products.
c) The final result in our case is that (especially across the thin ceramic element edges) conductive paths of metal (and oxides from the electrolytic process since moisture exists inside the filter) are formed, short-circuiting it. Good-bye, filter?

Don't fret, there is still hope! (if you're good at handling very small parts - that's the catch). I thought that if I could get the elements out of the structure one by one and clean their edges, thus eliminating the conductive path, perhaps the filter would work again. That's very easy, because they aren't soldered in place, they just get "clamped" between pairs of contacts when the filter case is in place. If you decide to do that, only get ONE element out at a time with a pair of needle-nose tweezers, clean all of its its narrow edges by wiping them lightly across very fine grit sandpaper a couple of times and then replace it exactly where and how it was - don't mix them up! Also, don't touch the resonator elements with your hands, finger oils will contaminate them and possibly change their resonant frequency! Carefully clean oxidation wherever you can spot it by scraping, always being careful not to spill the guts of the filter! If you do spill them, they can be put back in place IF you have taken notes and photos of the filter's structure. I cleaned and dried the interior of the case, too. Before putting each resonator back in its place, check with your ohm-meter, you should get an infinite resistance reading - anything else indicates you need to repeat the cleaning process - gently!
Finally, I put the filter back together, sealed it with a minute quantity of cyanoacrylate and soldered it back in the transceiver. Lo and behold, the receiver sprang back to life - and at full specified sensitivity, as my measurements showed. The pass-band response hasn't changed. I think that now, after my delicate sandpaper treatment, the filter is a lot less possible to again fall victim to the nasty electromigration, because all the edges are quite clean now, there isn't any metal there any more.Time will show!

The final word: Ceramic filter manufacturers are quite right in warning against applying DC voltages at the input / output pins of the filters. They specify the use of a DC blocking capacitor at the filter's input and especially the output. Application of DC voltage causes electromigration and corrosion to initiate (especially in humid environments where temperature variations eventually promote water vapor condensation inside the filter, which may have imperfect sealing), and after a period of time the filter fails in the way we discussed.

The funny thing is, most of the manufacturers of amateur radio (and commercial) transceivers amazingly and inexplicably DON'T use the blocking capacitors, instead they boldly apply DC potentials directly at the filter's pins. A survey of several schematic diagrams confirmed this, especially in transceivers where there are several ceramic filters switched in and out of the signal path with diodes  and DC bias (usually about 8V). Why they do so beats me, perhaps it is to save some cents for a pair of blocking capacitors for each filter, creating a huge reliability problem on the way...

The enterprising radio amateur can always add those capacitors in the circuit (0.1 μF, 50V, 0603 size SMD ceramic capacitors are great) and save her / his receiver from becoming deaf due to a ...withering filter! Admittedly, this is a bit difficult in most modern rigs, due mainly to the small dimensions of the components and layout... but it's certainly worth a try.

***ADDENDUM: For those who want to learn more about the phenomenon that I propose that causes DC-biased filter failure, please take a look here: http://www.ami.ac.uk/courses/topics/0158_emgr/index.html
There you can admire two great photos of the results of electromigration across tracks and solder resist on printed circuit boards, plus lots of interesting relevant information. Clearly, humidity and voltage gradients at small distances are a bad combination!!

Good ceramic filter reviving to all of you!! 

01 July 2010

Latest developments on the IC-7000 amplifier issue

After receiving an e-mail from Jan, DG3FDM (thanks, Jan!), who points out a mistake I made with the position of the anti-parasitic resistor on the PCB (it's only in the DC bias path at the point I connected it, since the RF signal comes from the other side of the PCB, after the PCB position I indicated - the schematic is OK though), I think it's time to present the latest information I have since gathered on the matter.

In just a few words, do ONLY the heat sink mod of the 23rd April 2010 post! The other mods don't hurt anything, but apparently they are not needed. (However, if you have already done them, just leave them in place!)

After observing the behaviour of the IC-7000s I have performed the mods on for some time, I think I have finally spotted the true reason for the malfunction, and apparently it is NOT the suspected parasitic oscillation of the DRIVER unit!

Let me explain: After performing the second part of the "stabilising" mods (16 March 2010), that IC-7000 disappointingly blew the driver again in about 15 days. Then, after again replacing the driver, I also installed the heat sinks on the pre-driver and pre-pre-driver transistors, which I noticed that became extremely hot after pressing the PTT for a few seconds (and then I posted the "last" 7000 mods, on April 23rd).

That particular IC-7000 has been in constant use on all bands since then, without any more problems. It had quickly blown the driver transistor three times before the installation of the heat sinks!

There is also more evidence coming from my FT-817, which also stopped blowing the (MOSFET) final amplifier only after I simultaneously installed similar improvised heat sinks on the (also terribly overheating) DRIVER  transistors, along with the anti-parasitic resistors!! I have concluded that the heat sinks solved the  problem, NOT the anti-parasitic resistors! (I had included the heat sink mod in the FT-817 paper in www.mods.dk, of course without recognizing its importance then, I just thought it would be nice to cool those poor flaming transistors).

So there is very strong evidence that the anti - parasitic resistors and other such measures are actually not needed, the overheated pre-driver transistors caused the problem in both the IC-7000 and the FT-817 (and perhaps in many more transceivers with overheating driver and pre-driver stages)!

With the pre-driver heat sinks, the problem seems to have immediately stopped for good in both transceivers (my 817 has been working OK for more than 18 months now, in mostly portable operation).

So, please install the heat sinks only.  The other anti-parasitic mods don't hurt the transceiver in any way, on the contrary, they may remain in place if already performed, but my current results show that the only mod that needs to be done is that of the heat sinks on the severely overheating pre-drivers.

21 June 2010

Not Flat Enough - or, Oiling a Transceiver

During an operation to install a new ceramic IF filter in a Kenwood TM-D710, I had to remove the rig's PCB from the diecast casing (heat sink), to gain access to the filter's pins (more on the filter issue to come soon in the "Mysterious Case of the Withering Filters"). The rig hadn't ever been serviced before. After unscrewing all of the screws, including those of the hybrid power modules, I lifted the PCB. I immediately noticed that the heat transfer compound spread indicated that the contact area of the heat sink (or that of the hybrid?) wasn't exactly flat, and a comparatively large spot hadn't been making contact at all (clicking on a photo enlarges it). The corresponding surface of the hybrid had been covered with the right thickness of thermal transfer compound, but it never made contact with the heat sink at all. This can be a serious situation, endangering the expensive hybrid amplifier module because of reduced heat transfer and the unequal thermal and mechanical stresses that develop as a consequence.
Using more heat transfer compound in such cases doesn't help much, because the compound itself is not a spectacular heat conductor, it just helps by filling up microscopic surface irregularities at the contact interface and replacing air, that would be an even worse heat conductor. The contact surfaces must be clean and flat and the heat transfer compound layer thickness must be exceedingly small for the compound to serve its purpose effectively, and this fact is emphasized in every power semiconductor manufacturer's application notes.
I thought of another heat transfer agent that is frequently used in other cases (for example, in oil-filled dummy loads). The right viscosity (heavy) oil would fill the gap nicely, stay there due to the forces of affinity and help transfer the heat to the heat sink at the problem spot. During reassembly, I used a couple of drops of SAE 80W-90 gear oil on the trouble area and gently pushed down the hybrids with a sliding motion, so all of the air trapped between the contact surfaces came out. The screws affixing the hybrids must be tight enough but not overtightened, as this could cause the ceramic substrate inside the module to crack. Just apply enough torque so that the split-ring on the screws closes, and then about an eighth of a turn more - but not much more!
This isn't a new trick for this dog - during my misspent youth, I used vaseline jelly in place of the (then) expensive and hard-to-obtain silicon grease. A couple of linear power supplies I built then (early eighties) are still in daily operation, with the same old 2N3055 and 2N3772 pass transistors "greased" this way.

23 April 2010

Some more mods for the IC-7000 amplifier chain

During the efforts to cure the self-destructive tendencies of the DRIVER unit of the IC-7000, I noticed that the pre-driver transistor, Q102 (RD01MUS1), which also works in class-A, gets too hot to touch (the dependable index finger test never fails - although my "probe" seems to suffer a bit with each test!). The copper surface around the transistor  serves as a heat sink, but it's obviously not enough. Elevated temperatures almost certainly lead to serious problems with semiconductors in the long run, so I decided to add an improvised heat sink to both the pre-driver and the pre-pre-driver Q101, 2SK2854. The photo tells the story (click to ENLARGE): A small heat sink is made with a piece of solid copper wire (of 1.5 mm diameter) and then soldered to the (grounded) source tabs of Q101 and Q102. Just make sure the heat sink doesn't touch anything when you put the PCB back in its place. The transistor operates without losing its cool now. One could even  slightly lower the idle current of the pre-driver Q102, by paralleling R112 (3.9 kΩ) with another resistor of suitable value (around 10 kΩ or so), but I haven't done that, as the transistor operates at a quite acceptable temperature with the new heat sink. The next photo shows how the new heat sink fits into the available space. Be careful! Accidents can cause much woe...

Good luck and take care (and your time)  in performing the mod!

15 April 2010

IC-775 DSP overheated voltage regulators

Recently I had the opportunity to service a good friend's  IC-775DSP (an excellent quality rig, by the way). After completing my work, I noticed that two SMD voltage regulators in the middle of the PLL unit (at the underside of the rig) were getting very hot, to the point of discoloring the PCB around them. The venerable and extremely accurate index finger test confirmed the situation, and I had to let out a muffled cry, as my bold "probe" suffered the dire effects of the alarmingly elevated temperature of the ICs. I measured their output voltage and saw that it had dropped a bit from the nominal value - a common symptom with chronically overheated three-terminal regulators. So, I proceeded to install an improvised heat sink (see the photo, click on it to enlarge).

The heat sink is made from solid copper wire with a diameter of 1.5 - 2 mm. The wire is bent to a shallow "Π" shape (that's the greek letter "pi") with the proper dimensions and soldered to the tabs of the regulators and the adjacent shield cans, which thus become part of the heat sinking arrangement (don't worry, there aren't any heat - sensitive circuits in them). The poor regulators work at a far lower temperature now, which is good for their health and longevity - as the IC-775 DSP even in our digital era is definitely a keeper, with a high spec receiver that's a pleasure to listen to and a lot of conveniences for the operator - including a very effective DSP system.

16 March 2010



After my first experiences with this problem, I had recently proposed a simple preventive measure, consisting of adding a series gate resistance to the driver unit, which apparently suffers from instability with destructive results in many cases.
This update is the result of further study of the problem. It attacks the problem in a more efficient way, taking more measures to eliminate the suspected instability but also taking measures to possibly prevent the destruction of the driver unit even if the instability or other deteriorating  phenomenon occurs.
The update consists of two parts, the first about improving the original simple modification, and the second about taking extra steps to ensure better stability and protection of the transistor in the driver unit. One can perform just the first part which is very simple. The second part is quite a bit more complicated and requires lifting the PA  PCB and performing more alterations and additions to the circuits - but it offers considerably more safety. Please understand that although all the mods aim to improve the situation, perhaps a better understanding of the transient underlying phenomena is needed for a full cure. For example, there is a possibility that the failure is caused by frequency-dependent secondary breakdown of the LDMOS device, which would potentially require extensive redesign of the amplifier chain to eliminate, and may not be practically feasible in our case.
Click on the pictures to see a large version.


The second part attacks the problem from different angles. It requires considerably more effort and skill. This part is for the more experienced technicians, so detailed baby-step instructions (e.g. "lift the PA PCB by desoldering... and then..." etc., will not be given here. Please be extremely careful.
You can choose to do only the parts of the mod that don't require lifting the PA Unit PCB, omitting the modification of the drain-gate feedback network that follows.

1) Modification of the drain - gate negative feedback network of the driver unit

Lift the PA UNIT PCB and remove the driver unit. The drain - gate negative feedback network flattens the gain vs frequency response of the broadband driver amplifier stage. It uses a capacitor (C101, 10nF) and a resistor (R102, 100 Ω). Change the capacitor to 0.1 μF, 50V (see the schematic at Part 1 above). This decreases the lower edge of  the range of frequencies for which the network provides negative feedback, stabilising the amplifier there also. Apply a thin film of silicone grease to the heat sink's surface and tighten the driver unit's  fastening screws well when reinstalling  the driver unit.

 2) Addition of an extra bypass capacitor

This step requires adding an extra tantalum bypass capacitor across C305 (470μF, 16V) (PA UNIT). Locate the capacitor's leads (see photo and schematic diagram) and solder a 47 μF / 35V tantalum capacitor across the electrolytic. Observe the polarity!!

3) Addition of a driver unit protection fuse


Adding a fuse to the 13.8V DC line feeding the driver transistor's drain improves the chances that even if instability occurs, the transistor will survive. (There won't be the usual fireworks and smoke, in any case!) Having lifted the PA UNIT, locate L302 on the PA UNIT (see the schematic diagram on this page and the next pages for diagrams and photos). Unsolder it and transfer it to the "bottom view" (the one you see after removing the rig's cover) of the PA UNIT (as shown in the service manual), soldering only the lead that connects it to the node with C305 and L301. Then solder an 1.5 A, fast-blow 20mm glass fuse (or equivalent) to the free lead of L302 in the way shown in the photo, soldering the other end of the fuse to the PCB trace that L302 used to connect to (it's the "b" line with 13.8V on it). Use a small piece of wire to solder the fuse to the trace. Don't locate the fuse elsewhere using long wires! Do it exactly as shown. Lay the fuse flat on the PCB. Use a small piece of thick paper or plastic sheet to insulate the end of the fuse soldered to the free lead of L302  from the PCB trace under it. After you have finished, cover the fuse with a piece of electric tape to prevent shorting the 13.8V line when replacing the rig's covers.

(Note: Originally, I thought about installing a current limiting circuit using two NPN transistors and some resistors in the DC line to the driver stage, instead of a fuse. This circuit would conceivably prevent a catastrophic secondary-breakdown scenario. Due to the severe lack of space and the relative complexity of this solution, I opted for the fuse. Anyway, I think this idea has merit, in  the future I may try it.)


4) Lowering the idle current Idq of the driver unit

In order to lower the gain of the class A driver stage, also reducing its thermal stress and the possibility of secondary breakdown of the LDMOS device, without seriously affecting its linearity at the RF drive level used, we can lower the idle current via the service menu.
The service manual procedure sets the idle current of the driver unit at 1A. Reducing it to 0.6 A produces no serious ill effects on linearity (as measured in a two-tone test in SSB).

Follow the procedure at page 4-3 of the service manual, "transmitter adjustment". Set the current at 0.6 A as per the instructions and exit the service routine.

This concludes the mods. Good luck! Enjoy using your IC7000!

1. John Pritiskutch - Brett Hanson, Understanding LDMOS Device Fundamentals, AN1226, SGS-Thompson  Microelectronics, 7/2000
2. John Pritiskutch - Brett Hanson, Relating LDMOS Device Parameters to RF performance, AN1228, SGS-Thompson  Microelectronics, 7/2000
3. S. Juhel - N. Hamelin, PowerSO-10RF: THE FIRST TRUE RF POWER SMD PACKAGE, AN1294, SGS-Thompson  Microelectronics, 2/2001
4. Norman Dye, Helge Granberg, Radio Frequency Transistors, 2nd edition, Newness 2001
5. Prasanth Perugupalli, Larry Leighton, Jan Johansson and Qiang Chen, LDMOS RF Power Transistors and Their Applications, Ch. 14, Ericsson Inc., Microelectronics Division
6. Various constructional etc. articles in QEX, The ARRL handbook
7. http://www.mwrf.com/Article/ArticleID/5899/5899.html

18 February 2010

IC-7000 driver amplifier self-oscillation problem


The IC-7000 is a very popular rig, and rightly so, since it combines many desirable features in a small, light transceiver. I was surprised when some of my friends reported that their rigs had literally gone up in smoke! Actually, one of them had died TWICE with the same symptoms (scorched driver unit). Later I found out that this phenomenon had also been reported by other users on the Internet. The whole thing reminded me of my experience with my FT-817 (which, by the way, still goes on strong after much the same treatment).
As I undertook to fix my friends' rigs, I decided to implement the same self-oscillation preventive measures to their IC-7000s, as the failure mode had a strong resemblance to that of my FT-817 final amplifier.
The problem probably lies with self-oscillation of the common driver stage of the TX amplifier chain. A series resistance of a few ohms is placed in series with the input (gate) of the FET amplifier, by cutting the appropriate PCB trace. The photos and schematic explain the modification. It is very simple to do, and can help prevent this "smoking" failure mode, without any side-effects and at negligible cost.

(click on the photo to enlarge)

THEORY OF OPERATION: This mod lowers the Q of the copper trace circuits feeding the driver amplifier's input, therefore quenching any tendency of the stage to self-oscillate at some VHF frequency, with destructive results.

IMPLEMENTATION: After removing the covers, locate the driver unit's input trace (see photo), cut it carefully and scrape the solder resistant enamel to expose the copper surface. Then, solder an 10 Ohm SMD resistor across the cut of the PCB. That's all!

IMPORTANT: After you have finished, carefully check and clean all of the springy ground clips on the PCB. Also, clean carefully and apply a thin protective film of grease or vaseline to the points on the covers where the grounding clips of the PCB make contact. The deterioration of this grounding scheme due to oxidation of the inside surface of the covers has been proposed as a possible reason for the driver unit self-oscillation and destruction.

RESULTS: I performed the mod in December 2009. Both the affected rigs have been heavily used after the mod, without any more problems.
If your IC-7000 hasn't suffered from this failure yet, this mod ensures that the possibility of destructive self-oscillation of the driver stage is significantly lowered.
This mod has also been posted at www.mods.dk.

Good luck and enjoy using your IC-7000 on the air!

 73, Tasos