Hello again. I took some time this morning doing voltage checks etc. I again used the uBITX v6 to check into the Noon-Time-Net (NTN). No problem. Then, while listening to the NTN with headphones, the following measurements:

The input voltage is 12.8 volts from a battery, not a 13.8 volt supply. Thus my voltage readings are a bit lower than what you all mentioned. Still they are close except for one, the voltage drop across R34, is 1.06, rather than the 4.2 to 4.43 mentioned by Tomasz.

Touching either side of C51 produces a hum. There is no apparent difference in the hum on other side of C51.

Touching either side of C50 produces a hum. But touching the right side of C50, looking from the front of the radio, is much louder when touched.

With metal tweezers touching both sides of C50 and C51 (shorting them out), it kills the audio, I hear nothing. Removing the tweezers brings back the NTN audio.

Again, touching TP21 gives me a slight hum. Touching TP20 gives a louder hum.

Voltage drop across resistors:

R18 = 0.367
R38 = 0.305
R14 = 4.08
R34 = 1.06 (rather than the expected 4.2 to 4.43 per Tomasz)

Voltages at various transistors:

Q10. E= 1.64, B= 2.39, C= 7.94
Q11. E= 7.21, B= 7.93, C= 12.00
Q12. E= 6.46, B= 7.20. C= 12.00

Q30. E= 0.0, B= 0.28, C= 11.00
Q31. E= 10.34 B= 11.0 C= 12.06
Q32. E= 9.66 B= 10.35 C= 12.06

Q70. Base to Emitter = 0.65
Collector to Emitter = 1.33

At one point, note the radio has been running for about three hours, it crackled for an instant, loud audio appeared for a brief second, when went away again. Never to return.

So… what does all this tell you? I have yet to bring out the O’Scope.

73, and Thank you. Richard AG5M

On Tuesday, February 25, 2025 at 11:26:13 AM PST, tomekdm sp7etd via groups.io <tomekdm@...> wrote:

Hello,

Sorry for spamming -? trying to simplify these RX chain check and by the way - interested in this topic...

Quick check of Q10 and Q30 might be done by measuring voltage drop on R14 and R34 respectively.

Voltage drop on R14 and R34 is around 4.2V - 4.43V which gives 19mA-20mA current of Q10/Q30 transistors (with assumption of negligible base current of next amplifier, polarization branch etc).

Anyway, I am suspecting Q10 as this is first transistor of RX path - first to hit from antenna / TX path (in case of last relay failure ??!!). Of course might be wrong...

?

Regards

Tomasz