Page
21
of
25
Examples of receiver section faults I have found
1) There was no signal at R9. But there was a signal across the relevant Quadrature Sampling Detector capacitor C45. I
took a closer look visually, at the components nearby. It was easy to see that the wire of resistor R9 was broken (it looked
as though it had been cut!). It was easy to apply a blob of solder to fix it.
2) One time the signal just stopped abruptly. All the components looked fine, the soldering looked fine. I could not find any
short circuits. Eventually, using the DVM to check for continuity between component leads which were supposed to be
connected, I found one which wasn't. With the jeweler's loupe I eventually found a tiny track fault next to a component
pad. The track was disconnected from the pad, right at the pad. The break was so tiny it was hard to see even with a
jeweler's loupe. The only way to find it was by this signal tracing method, and the DVM for continuity. Once found, easily
fixed.
3) In one case, there was no audio output. When the radio was in mode "8.7 Peak BPF" the screen bar showed only 1 or
2 (top right hand corner). I used the above signal tracing method and I found that the signal stopped at IC9B, which had
no output. Visually checking the related components showed that the wire of R33 (36K resistor) had become detached,
right at the resistor body. Since it was the end of the resistor at the PCB, it was hard to see. I replaced the resistor and all
was well. Note that if you do not have the correct value component, you can make one up by series or parallel
combinations. For example, 36K is not a common value, but you could use a 33K and a 3.3K in series if you have them,
these are much more common values.
4) There was very low audio output, and when you turned up the volume, there was a loud squeal as the audio amplifiers
self-oscillated. Signal tracing revealed a strong (about 1V peak-peak) triangular-wave oscillation, at a frequency of around
240kHz (not stable), at IC6 outputs (both output pins 1 and 7). Inspection revealed no obvious flaws and I decided there
must be a problem with IC6, and I decided to replace it. It was VERY difficult to remove, because it is sandwiched right
between the multi-turn trimmer resistors R17 and R24. Eventually I did manage it (albeit with a one damaged trace and
one removed through-hole plating, which had to be repaired). I installed a new LM4562 op-amp... and... SAME
PROBLEM! Finally after some head-scratching and experiment, I realized the problem! R24 was right at the end of its
travel, with a resistance of zero ohms. The output of the op-amp IC6B was effectively connected straight to the 0.1uF
capacitor C12, and it did NOT like that - the result was the unstable oscillation. All that was necessary to resolve it was
just to turn R24 slightly so its resistance was no longer zero, and the op-amp started behaving! This is a great example of
me violating my own rule, about not replacing components until the last possible moment! Very often you find in the end,
that the component was NOT the problem.
TRANSMIT signal path
The transmit side is easier to trace, just because it is shorter. The components are more likely to be broken, because
there are high powers involved, which might have fried components.
Current consumption
The first clue to a problem with the transmit side, is often high current consumption. Refer to the current consumption
chart in the manual. For example at 12V supply, in receive you can expect a current consumption of about 120mA. If it is
significantly more than this, maybe you have a problem with the PA. If it is a lot more, then you probably can see smoke
rising from some of the components (transistors) in your transmit section at the top right area of the PCB. It is very useful
to use a power supply with current metering, and keep an eye on the current consumption to make sure it is in the
expected range.
IC3 the quad NAND gate
I start with IC3. This is a quad NAND logic gate, a 74ACT00. The 74ACT family was chosen because the logic "1"
threshold is 2.4 (old classic 7400-series TTL level) and therefore it is perfect for being driven by the 3.3V peak-peak
square-wave from the Si5351A Synthesizer IC1's Clk2 output.
