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?
Mystery Fix
Carl,
I presume the ICs are in sockets. I have seen many instances of failure due to poor connection between the IC pins and sockets to the point where I never use them for analog circuits. It could be that when you removed and replaced the ICs the wiping action on the pins cleaned them establishing contact.
Reasons for this type of problem with sockets vary but poor pin to socket contact can be caused by oxidation, lack of tension, and contaminants such as solder flux finding its way into the socket.
Of course, the old nemesis of kit builders can be at work namely a bad solder joint which is intermittent. If the problem returns you can try resoldering all connections. This seems like a lot of work but it usually saves time over trying to find a bad joint(s) by troubleshooting.
Neil
Worth Knowing
That is good information, radio8Z. The metal to metal contact of ICs and their sockets has everything against it, now that we’re focusing on it…
A lot of pins, very small contact area per pin, common metal not gold plated, and, as always, the world of contamination.
I cannot imagine a device or tool of the right size to clean down in IC sockets…. is their something that will work?
IC’s mysteriously “move” over
IC’s mysteriously “move” over time. I have routinely pressed on IC’s in sockets the feel them go in a bit farther. I agree with the other post that may have simply been the problem.
Mystery For Sure
That really is a mystery, ICs that move over time.
I remember countless routine “pressing” on ICs in their sockets to get that extra insertion. This has been true of computer boards as well as all other electronic equipment.
Are they attracted to the moon?
Yep, my money is on the IC
Yep, my money is on the IC sockets too. As convenient as they are, they have small drawbacks.
I remember back in the day when a 2MB computer board had lots and lots of socketed ICs. We had to reseat them every few weeks due to temperature cycles and fan vibrations.
SCRITCH…
I like that “scrunchy” sound they make when you press on them as they seat into the socket.
Scritch the Itch
That is sure true. You press the IC that extra nano-scritch into its socket and there is a feeling that everything will be fine now.
If anything fails after that point life isn’t worth living.
H2S
If you live anywhere near any kind of oil field or pump jacks or any industrial place (if there are any left in this country) that use chemicals, those chemicals and H2S oil field gasses wreak havoc on silver and copper. It leaves a very dark and nasty coating which dampens contact surfaces such as IC leads and their sockets.
Was not uncommon to have to pull chips and reseat them and have to resilver plate all the cavities in transmitters out in the field down in Tx. Almost every 4 to 6 months that corrosion has to be checked for and removed.
Or it could just be someone has not done some dusting around the house in awhile! :p
RFB