Re: Still digging... looking for a low audio issue #qcx #problem

Paul Blum

What DC voltages do you measure on pins 2&3 of IC9?  If they're close to expected (4.8v and 0.65v), then you have a wrong/bad part in that circuit or Q7 is leaky/shorted.  you should be able to measure the resistors in circuit (radio is off), and check the markings on the capacitors (222) C19 & 20 with a magnifying glass.  This stage is part of the audio bandpass filtering, and a wrong part will put the filter to something other than the desired 700 hz, giving low gain.

If you have a 'scope, you can rule out problems with Q7 by comparing IC9 pin 1 signal level with the volume set loud vs all the way down - it should stay the same as volume knob changes.  Hope this helps.

73 Paul K9ARF

On Saturday, September 22, 2018 2:55 PM, Alan de G1FXB via Groups.Io <g1fxb@...> wrote:

Hi Kurt,
before you warm up the Soldering Iron...
If the voltages around the op amp is correct, I would not condemn the IC as faulty.
It's a DC coupled device, It would be strange for it to have the correct DC conditions` but not pass AC (audio)

If troubleshooting with a wet finger from the earphone socket back to the gain control proves inconclusive.
One of your troubleshooting tools you said you had was an amplified speaker,
Have you tried tracing the audio from front to back in the other direction?
(Some times it's more revealing to trace a signal rather than injecting one.)
You can use the in-built RF generator to compare with your QCX's  for comparison.

If it indicates IC9 again, then fair comment.


On 22/09/2018 16:16, Kurt Zimmerman wrote:
Oddly voltages seem to compare about equal on both.


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