Carl Blare

We are happy owners of two AMT3000 transmitters, and decided to make the modification described for using an external loading coil and pipe antenna, which includes changing C23 from 820p to 561p. This was done to both transmitters but then one of them worked and the other did not.

Putting the ailing transmitter on the bench was made extra convenient by use of the PhilB “Dummy Load”, a 30-ohm resistor/30pf capacitor in place of the antenna. A working transmitter can be received on a radio within a 2.5 foot radius.

It was quickly found that the audio path was absolutely fine.

Next, the two DC supplies were checked, 5V & 15V. Mighty fine.

The problem was in the RF section.

We found that ICs U3 and U7 were not generating the selected frequency, which should have been 1550kHz. We thought maybe an IC was blown.

Having a second transmitter made the next step easy, we swapped chips, and soon discovered that all of the ICs were in good condition.

At intervals we would repeat a visual inspection looking for solder points, but of course the transmitter had been good for several years, so solder didn’t seem like a likely problem.

This evening we returned to the bench and turned the AMT3000 on for further examination, and it worked perfectly. An hour has passed and it’s working fine. But do you see the problem? We have no clue why it didn’t work for several weeks, and now it’s running fine. It holds up to being shook and prodded with toothpicks (looking for loose contacts).

Now the question for you to answer… how can this situation be explained?