TRX2 test points

General discussion

TRX2 test points

Postby N3ZV » 22 Dec 2016 11:48

Greetings all,

I am assembling a TRX2A and just completed the DDS board. What are the test points? For that matter what are the test points for the rest of the build?

You folks at JUMA put together a very fine kit.

Thank you.


Whitney
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Re: TRX2 test points

Postby 5B4AIY » 27 Dec 2016 11:43

Hi, Whitney,

Yes, you are correct, there are no pads on any of the printed circuit boards that are specifically intended as test points. Nevertheless, by examining the various schematics and their printed circuit layouts you will find plenty of places where an oscilloscope probe or a multi-meter test pin can be connected for test purposes.

In general, most initial problems are caused by inadvertent shorts on adjacent pins of ICs - this is particularly true of the microprocessor and DDS ICs - as well as open soldered joints that have been missed during the construction phase - all too easy - as well as misplaced components. The incidence of faulty components has been very low.

Prior to applying power to any board it is important to conduct a 'sanity check' with a multi-meter to ensure that there are no shorts on any of the power lines. Again, the microprocessor is a common culprit. The pin spacing is very small, and there are 4 pairs of adjacent pins carrying the +5V power to the chip. The +5V is fed to pins 12, 32, 48, and 71, and the ground connections are on the adjacent pins, 11, 31, 51, and 70. There is also a +5V feed to pin 25 for the A-D convertor reference, and its ground connection is on pin 26. In my case there was an almost invisible whisker of solder shorting the +5V bus, and even back illuminating the PC board with a high intensity LED flashlight and carefully examining the board with a magnifying glass it was still extremely difficult to locate. I suggest gently stroking a sharp sewing needle between all the pins of both the microprocessor as well as the DDS chip to ensure there are no stray whiskers.

When you have completely assembled the transceiver, and are ready for the first 'smoke test', it is strongly suggested that you use a current limited bench power supply, and set the current limit to about 450mA to 500mA. Having assured yourself that there are no obvious shorts, with one hand on the power supply main switch, briefly press the transceiver PWR button. Note the intensity of the back-lighting! If it is very bright there may be a fault in the back-lighting circuitry. Immediately turn the power supply off, and investigate. It is very easy to destroy the back-light LEDs with too much current. Assuming the back-lighting is normal, and the display comes up with the normal prompts and then the normal transceiver display, I would suggest entering the configuration menu and checking that both the back-light contrast and intensity can be controlled, thus ensuring that this section of the transceiver is fully functional.

From there you can proceed to the receiver checks, and set the CW, TUNE, and SSB transmit levels. To verify that the receiver is functioning as a true single-signal SSB receiver, tune in to a steady carrier, preferably from a signal generator and with the mode set to LSB tune from a frequency higher than the carrier down verifying that the audio note decreases in frequency as you tune through it, and as you tune through zero-beat the audio output drops significantly. There should be around 45dB - 55dB of opposite sideband suppression. If this is OK, then switch modes to USB and tuning from the low frequency side of the carrier through zero-beat and verify that in this mode the opposite sideband is suppressed by a similar amount. This essentially verifies that both the DDS frequency is essentially correct, and that the mixer and polyphase networks or working correctly.

After adjusting the S-meter calibration, even with only a short piece of wire you should be able to hear plenty of signals.

I have written a fairly comprehensive set of construction and basic check-out notes but unfortunately the PDF is too large to upload to the website. If you give me your email address I can forward it to you. Unfortunately that will not be until the end of next week.

Even if you do encounter a problem, there is plenty of help available from both myself and others who have constructed this transceiver.

73, Adrian, 5B4AIY
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