LDG AT-1000 autotuner acting strangely on some bands?

LDG autotuners are famous products worldwide, and for good reason: they are very well designed and built, providing a convenient solution when needed. There are very few complaints about them, as a quick Internet survey showed me. Why did I search, you say?

The other day a friend sent me an LDG AT-1000 that was acting up on some bands, failing to provide a match or even maximising SWR instead of minimising it, but working fairly well on other bands. He had been told it's the microcontroller's fault, and LDG even sent him a replacement chip (thumbs up for that kind of customer service), but the problem remained exactly the same with the new chip.

A quick check showed he was quite right: on 80, 40 and 30 meters the tuner was acting as a mismatcher, rather than a matcher. On 20 meters and higher bands, it worked rather well, although it seemed a bit "sluggish" (how's that for a scientific term?).

After examining the circuit in and out, I was sure that all of the relays, inductors and capacitors of the matching network were OK. I connected a dummy load at the output of the unit and saw that the autotuner indicated significant reflected power on 40 m, although there should have been none (I had it in the "bypass" position). "The SWR bridge circuit must be unbalanced", I thought - but why was it unbalanced on a specific frequency range? A Bruene circuit, if misadjusted,  behaves progressively worse as the frequency is raised - here, it was just the opposite! Nevertheless, I proceeded to balance the SWR bridge. For this, I cut the wire connecting the SWR circuit to the input of the tuner PCB, and connected a precision dummy load right after the bridge PCB with a very short piece of coax. I set an RF generator on 30 MHz and nulled the reflected power reading using the trimmer capacitor. Curiously, it was off by just a little. Hmmmmm...

Fig. 1. The bridge PCB in its original state.

After reconnecting everything back, I checked again on 40 meters with the dummy load at the output of the tuner, which was in the "bypass" position. There was a significant reflected power reading on the tuner's meter again - but NOT actual reflected power, as another SWR meter between the generator and the tuner showed me! Furthermore, on 20 meters there wasn't any reflected power indication on the tuner's meter with the same conditions! So something was clearly amiss with the bridge - but all of its components had checked out OK! Curiouser and curiouser!

Fig. 2. Look ma, no screws!

Fig. 3. The hi-tech insulators.

 After a considerable period of head-scratching, the light went on. The fact that the bridge was balanced on 30 MHz but not balanced on 7 MHz showed that at the lower frequency the RF currents must have been taking a detour. The only way I could visualise that happening, was through the grounding posts and grounding wire connected to the SWR meter PCB (Fig. 1 - click on the photos to enlarge). So I removed the two screws affixing the PCB to the posts, also disconnecting the wire this way. I checked again on 80 and 40 meters - and bingo! No reflected power indication any more! The bridge nulling remained excellent across 1.8 - 30 MHz. When I touched the PCB to the posts, the reflected power reading jumped up again. 

So, I modified the bridge structure by insulating the PCB from the grounding posts, as shown in Figs 2 and 3. I used a little square piece of thick paper at each post, hot-glued to the PCB and posts to accomplish my goal. I completely removed the grounding wire. After re-nulling the bridge (just to be sure), the tuner worked perfectly on all bands, with no abnormal readings at all. It even produced a 1:1 match feeding my 20 m quarter-wave ground plane at 80 m!

So, if your AT-1000 is horsing around on some bands, this trick may well work for you too. LDG might want to have a look into this matter and modify the design. For me it was yet another good reminder that RF currents, given half a chance, rarely pass from exactly where we would like them to!

Addendum: I just purchased a very nice LDG Z-817H tuner for my bicycle HF hamming during the summer! It has a different structure at our point of interest (the Bruene directional coupler is located on the main board) and a series of tests with various types of load conditions showed no tendencies for whimsical behavior. It successfully matched whatever I threw at it and had a nice time doing it. The directional coupler stays balanced throughout the operating range. Well done, LDG!

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!

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!!

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? 

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

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.

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.

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!