LDG AT-200 Will Not Tune

 LDG Electronics are an American company that make antenna tuners for various power ranges from 100 Watts upwards.

The AT-200 is their 200 Watt series. They can be set to automaticallly tune an antenna to specific band or to interface with a radio and to tune manually at the press of a button.

This one came in with a fault after a power loss. The user reported that the tuner was hunting, trying to find the best combination of inductors and capacitors to match the antenna but was having no luck finding a match. As I have repaired various LDG tuners in the past, I know that a typical failure is the Bruene directional coupler used to sense the forward and reverse voltages on transmission lines. So the case was cracked open and investigation began.

These devices are beautifully laid out and very easy to understand. But the heart of any tuner is it's ability to accurately measure SWR. Without that, the microprocessors can tune forever and never find an adequate match. The main two components in the Bruene circuit used to measure SWR are the two Shockty diodes marked D2 and D3 in the picture below. These are low, forward voltage germanium diodes and are often prone to failure.

A quick test in circuit gave the following forward and reverse voltages:

• D2 - Vf=1.9v, Vr=0.19v
• D3 - Vf=0.18v, Vr=0.99v

Now, you cannot definitively test Diodes in circuit, but those results did not look right. So the board was pulled from the chassis to remove the diodes for definitive testing.

As the first diode was unsoldered it broke in half. Obviously it must have been cracked. The second diode was removed and it tested defect.

So I had a look in my spare parts for a replacement. Normally any BATxx diode is a good replacement.

As Brune voltages are low, two BAT43's were selected and soldered in place.

While the board was out, I decided to check the protection diode. These are designed to protect the ICs from reverse polarity. It too had blown.

So it was replaced with a new 1N4001 as well.

Finally, the board was insepected under the microscope. What I found was absolutely aweful soldering quality of the board from the factory. One pin was not connected at all, see video below:

The remaining pins were only partially soldered. Here are some examples:

So the board was re-flowed and all joints soldered as they should be.

The board was then inserted back into the housing and is good to go again.