Page
22
of
25
With the radio in ordinary RECEIVE mode, you can check the voltages at all the pins of IC3. I know, you cannot even
REACH IC3 because the LCD is in the way. You can temporarily un-plug the LCD while the radio is in receive mode. As
long as you don't press any buttons you can be sure that the radio will continue working, even without the LCD. The
processor only talks at the LCD, it does not look or wait for any response. The voltages on the IC3 pins MUST be as
follows, in receive. Remember that pin 1 is at the top left, when the chip is orientated with its dimple pointing upwards. For
convenience, this table is arranged consistently with the pin lay-out:
1: +5V 14: +5V
2: +5V 13: +5V
3: 0V 12: +5V
4: 0V 11: 0V
5: 0V 10: 0V
6: +5V 9: 0V
7: 0V 8: +5V
The voltages do not have to be EXACTLY 0V and 5V, but they should be very close. Measure the voltage at pin 14, this is
the 5V supply voltage from the voltage regulator IC11. It should be close to 5V. All the other voltages should also be close
to 5V, or close to 0V, as per the table.
When the radio is in TRANSMIT mode, the NAND gates are changed, so as to allow the signal generated by IC1
(Si5351A) Clk2 output to be gated through to the PA transistors. The 74ACT00 is also supposed to produce 5V peak-peak
at its output, so that the three BS170 MOSFETS Q1, Q2 and Q3 are saturated in the ON/OFF states, this is important for
achieving high efficiency Class-E operation. The voltages will now be as follows:
1: 5V squarewave 14: +5V
2: 5V squarewave 13: 0V
3: 5V squarewave 12: 0V
4: 3.3V squarewave 11: +5V
5: +5V
10: 3.3V squarewave
6: 5V squarewave 9: 0V
7: 0V
8: +5V
Again the 0V and 5V in this chart, should measure pretty close to 0V and 5V. 4.8V, 4.9... fine... 0.1V, fine... but if you see
1V somewhere, then that is a reason to worry.
This is an example of what you should see at IC3 pin 3, the 5V peak-peak squarewave. You can ignore the "ringing"
appearance, the imperfections on this trace - blame the 'scope probe, the ground clip, the oscilloscope's limited
bandwidth, whatever. That's another topic entirely. You can also usually ignore it when the pins which should be 0V and
+5V show a bit of RF on the oscilloscope trace - again probes and ground clips can pick up this sort of thing easily.
If you do NOT have an oscilloscope, and you just look at these chip pins with a DVM, then you may see that a 3.3V peak-
peak squarewave is measured as its average on a DVM, i.e. 1.65V (or thereabouts); and a 5V peak-peak squarewave
measures something around 2.5V.
