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uBITX v6 Low Audio - Back at it again

 

I appreciate everyone who were helping me troubleshoot my uBITX V6 with low audio before Christmas. I've since been gone spending time with the grand-kiddies. Now going to get back to this.

I compiled all the previous dialog and suggestions into one WORD document attached. I'll be going back going over all that was suggested. Checking voltages, resistances, etc.

I noticed in one suggestion - "Before you continue trying to align it, the first thing to do is to connect an external audio amplifier to TP20 and ground and see whether it makes a difference and continue using it to see whether the fault reappears. If it works and continues to work then the fault could be in the audio chain."

This morning I put my external audio amplifier at TP20. Nothing, no audio from the external amp. And as I mentioned earlier, I do hear the sidetone in the speaker, and it is LOUD! Really loud, cannot use headphones on CW, hurt these old ears.

Also this morning, I used the radio to check into the Noon-Time-Net with both CW and SSB. No problems, good reports. So we know TX is apparently working OK. Frequency is a bit off but I can get that later if and when the audio issue is solved.

So, in light of not having any audio from TP20 into the external amp, does that tell you something on where to go next? I'm about to drag out the O'Scope, signal tracer, RF generator, blow up the schematic, do some serious studying. Got to clean off this workbench. Thank you all. Take care...

73, Richard AG5M

uBITX V2 No Audio.doc
uBITX V2 No Audio.doc

 

Hi Richard,
?
Based on this new information, I would verify the Q70 DC operating point:
?
?
According to Raj's (VU2ZAP) sheet, the base should be 0.6 to 0.7 volts, and the collector should be 1.6 to 1.7 when receiving.
?
If Q70 is correctly biased, verify that C50 and C51 are not opened.? I do this test with a metal tweezers touching both sides of the cap with the radio on.? You should hear an AC hum.
?
73
Evan
AC9TU
?
?
?
 

I should have noted that the focus on Q70 assumes that the SSB transmission is working correctly.? The proper operation of SSB transmission and the audio sidetone leaves only Q70 as the link in between.
 

SSB operation is fine, I used it to check into the Noon-Time-Net this morning, both SSB and CW, no problem hearing the sidetone, that's for sure, it's really loud. Audio reports on the Net were FB on SSB. I'll check Q70. 73, Richard AG5M
On Monday, February 24, 2025 at 03:19:05 PM PST, Evan Hand via groups.io <elhandjr@...> wrote:


I should have noted that the focus on Q70 assumes that the SSB transmission is working correctly.? The proper operation of SSB transmission and the audio sidetone leaves only Q70 as the link in between.
 

Evan. What voltages would I expect to see at Q70? Will report back.
73, Richard AG5M
On Monday, February 24, 2025 at 03:15:48 PM PST, Evan Hand via groups.io <elhandjr@...> wrote:


Hi Richard,
?
Based on this new information, I would verify the Q70 DC operating point:
?
?
According to Raj's (VU2ZAP) sheet, the base should be 0.6 to 0.7 volts, and the collector should be 1.6 to 1.7 when receiving.
?
If Q70 is correctly biased, verify that C50 and C51 are not opened.? I do this test with a metal tweezers touching both sides of the cap with the radio on.? You should hear an AC hum.
?
73
Evan
AC9TU
?
?
?
 

See below - base to emitter should be 0.6 - 0.7 volts and collector to emitter should be 1.6 - 1.7 volts. The Base-emitter voltage is kinda critical, because you want the transistor to conduct the entire time during receive.

Lee KX4TT



On Monday, February 24, 2025 at 09:39:18 PM EST, Richard AG5M via groups.io <ag5m@...> wrote:


Evan. What voltages would I expect to see at Q70? Will report back.
73, Richard AG5M
On Monday, February 24, 2025 at 03:15:48 PM PST, Evan Hand via groups.io <elhandjr@...> wrote:


Hi Richard,
?
Based on this new information, I would verify the Q70 DC operating point:
?
?
According to Raj's (VU2ZAP) sheet, the base should be 0.6 to 0.7 volts, and the collector should be 1.6 to 1.7 when receiving.
?
If Q70 is correctly biased, verify that C50 and C51 are not opened.? I do this test with a metal tweezers touching both sides of the cap with the radio on.? You should hear an AC hum.
?
73
Evan
AC9TU
?
?
?
 

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Transmit and received have different signal paths.

When in RX check if there is 12V available on both sides R18 and R38 ?

The voltages of V6 are very close to V4 which I have noted in a file in the files section under my call VU2ZAP.

Good luck

Raj

On 25/02/2025 5:58 AM, Richard AG5M via groups.io wrote:

SSB operation is fine, I used it to check into the Noon-Time-Net this morning, both SSB and CW, no problem hearing the sidetone, that's for sure, it's really loud. Audio reports on the Net were FB on SSB. I'll check Q70. 73, Richard AG5M
On Monday, February 24, 2025 at 03:19:05 PM PST, Evan Hand via groups.io <elhandjr@...> wrote:


I should have noted that the focus on Q70 assumes that the SSB transmission is working correctly.? The proper operation of SSB transmission and the audio sidetone leaves only Q70 as the link in between.
 

Hello All,
1) Good points Raj - Richard check voltages during RX on R18 and R38. See the picture below. Supply voltage for two RX chains is coming "from the top". Voltage drop on R18 and R38 is around 0.4V. Please check this drop. This will give us some overview of current consumption by each RX chain:
?
2) Additionally You can check voltages on transistors of these RX chain in relation to GND (I have marked these transistors with red rectangles on above picture with pinout):
?
3) Search also for cold soldering, cracked capacitors, other irregularities in this area. Could You send us photo of this area?
?
My measurements were done at 7100kHz LSB with 50 ohm load on antenna connector.
?
Regards
Tomasz
 

Supplementary info:
R18 drop = 0.383V
R38 drop = 0.389V
 

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
?
?
 

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
?
?
 

Hi Topmaz,
?
Based on the lead voltages of Q30 and the fact that minimal current passes through R34, I suspect that Q30 is bad.? I would replace it and then recheck the voltage to see if Q31 and Q32 are OK.
?
Q70 is working, so it is not the problem I first thought it would be.
?
Note: Shorting out a blocking capacitor with the power on is not a good idea (both C50 and C51 are DC-blocking capacitors).? I had stated that I would touch each side of the capacitor, not both simultaneously.
?
Once the receiver is working, you can then do the calibration.
?
73
Evan
AC9TU
?
?
 

yes , Q30 2N3904 in smd package needs replacement. Thanks for measurements Richard. regards.
 

Richard,?
By the way, good way is to desolder Q30 with big drop of tin, even on one side - heat will transfer on the other side, and with small twizeers it can be picked up.
?
good luck
Tomasz
 

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Something is wrong with Q30 or components around it.

*Always* measure voltage from ground to a point unless otherwise state. Voltages across resistors dont mean much to many of us -

Unless mental math circus is done.

Raj

On 27/02/2025 6:01 AM, Richard AG5M via groups.io wrote:

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
 

Yes,actually something wrong can also be with C30 (low voltage on base of Q30). Richard, can You measure if there is low resistance of C30 while radio is OFF? Lower in the mean - with C10 comparison. If there will be something suspected you Can unsolder C30 and measure voltage on base of Q30??
 

Richard, so once more before soldering...sorry for spaming.
?
Try measure resistance of C30 while radio is OFF, and compare this value with C10 resistance. Cannot measure it by myself for reference as I'm at work :)
 

ok. got some measurements. While measuring resistance from base of Q30 to ground (top node of C30 to GND)? is around 836 ohm. While base of Q10 to GND (top node of C10) has 102 ohm (different transformer). Richard, if You will have much lower than 100 ohm - C30 (100nF) is also suspected to have short.
 

...less than 90ohms...
 

Before I stat removing components, I'll report the following based on your last suggestions:

Resistance from base of Q30 to GND = 50.8 ohms, no where close to 836 ohms.
Resistance from base of Q10 to GND = 0.82 ohms, no where close to 102 ohms.
Resistance across C30, in circuit, = 50.6 ohms.

I light of that, what's your opinions? Start pulling and ordering some parts?

73, Richard AG5M


On Thursday, February 27, 2025 at 07:15:47 AM PST, tomekdm sp7etd via groups.io <tomekdm@...> wrote:

ok. got some measurements. While measuring resistance from base of Q30 to ground (top node of C30 to GND) is around 836 ohm. While base of Q10 to GND (top node of C10) has 102 ohm (different transformer). Richard, if You will have much lower than 100 ohm - C30 (100nF) is also suspected to have short.... less than 90 ohms.

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