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Thank you for letting me join the group.

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I just became the proud owner of a uBITX v6 assembled by another Ham who sadly went SK.? When I turned it on I had plenty of audio.? And I already knew it needed alignment.? But otherwise it was working as expected.? I used it to check into the Noon-time-Net, no problem.? I can get to alignment later.

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Then suddenly, the audio dropped to such a low level I could barely hear the station with volume control wide open.? Same for speaker and headphones.? After some time passed, perhaps 15 minutes or so, the volume popped back to normal.? About blew me out of my chair.? However, the next time time I turned it on, VERY low volume again.? And it has stayed that way since.? Barely audible.

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Not being familiar with this radio, can someone give me some ideas how to trouble shoot the missing volume?? Where to look.? I do have the schematic.? Will be much appreciated.

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73 Richard AG5M

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Hi Richard,

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I would start with the antenna relays. ?When the sound is low, try clicking the transmitter or tapping on K1 and K3. ?

Are there any mods made? ?A cold solder joint on a BCI filter or an AGC mod.

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73

Evan

AC9TU

No, no mods done. I have tapped the relays to no avail. Tapped all around the PCB to no avail. Re-seated connections. Have not removed the PCB yet to give it a good look underneath. Will do that around Thanksgiving. Since nothing happens when I tap on things I do not suspect a cold solder joint at this point.
73, Richard AG5M

If the issue is constant now (no longer intermittent), then I suggest starting at the input of the LM386 and verifying that there is an increase in noise when a length of wire is touched to pin 3 of the LM386, U1, then TP21, followed by TP20.? If there is an increase in the hum/noise out of the speaker, then the issue is not in the audio sections.? I have seen failures of the Si5351 on the Raduino cause receiver sensitivity issues.? That is hardened to test without a scope or frequency counter.

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73

Evan

AC9TU

Thanks Evan
Issue is constant now. I'm going to be pretty busy this coming week, will check that out later. I do have frequency counter and oscilloscope. BTW, just in case, I did reseat the LM386, was not sure how long it sat at my friends place unused. No change.
73, Richard AG5M

Evan

This morning I fired up the radio again. Very low volume, but after half a minute, the volume came blasting back. That lasted for some 10 seconds, then went low again. Stayed low.

I have touched TP1, TP2, and Pin 3 of the LM386. I hear a faint hum at TP1, slightly louder at TP2, and a very nice loud hum at Pin 3. So, it's not in the audio chain - would that be right? If right, where to look next? Time to bring out the freq counter and Oscope?

73, Richard AG5M

I would check the earphone jack. Plug in a pair of headphones and see how much audio you get. Then remove the headphone as see if the audio came back.

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Barry

I guess I should have mentioned, low volume from speaker or headphones. No difference.
73, Richard AG5M

On Tue, Nov 19, 2024, at 01:10 PM, Richard AG5M wrote:

Evan

This morning I fired up the radio again. Very low volume, but after half a minute, the volume came blasting back. That lasted for some 10 seconds, then went low again. Stayed low.

I have touched TP1, TP2, and Pin 3 of the LM386. I hear a faint hum at TP1, slightly louder at TP2, and a very nice loud hum at Pin 3. So, it's not in the audio chain - would that be right? If right, where to look next? Time to bring out the freq counter and Oscope?

73, Richard AG5M

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Was that TP1 and TP2, or TP21 and TP20?? All are on the board.? TP1 tests the antenna input, TP20 tests the audio after the demodulator, and TP21 is the input to the audio amp with muting.

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With the volume control fully clockwise, the AC hum should be the same when touching pin 3 of the LM386 as TP21.? If not, then there may be an issue with the volume control.? Try turning it back and forth to see if there is a broken connection.

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Another test is to verify that the mute MOSFET (Q74) is switching correctly.? Measure the gate voltage of Q74.? It should be zero when receiving and 5 volts when transmitting.? Also, you can measure the resistance from TP20 to ground with the radio on.? It should be higher than 5kOhms while receiving and less than 200 Ohms when transmitting.? If not, then Q74 may be bad.

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Another problem I have seen with low signal strength is related to the Raduino board.? The Si5351 chip failed to lock at the right frequency, which would appear as a jittery signal on any of the three clocks to the mixers and demodulator.? In particular, the BFO seems to drop in and out of sync.? This could be the case for any of the three clock signals.? To test the Raduino, I removed it from the radio, powered it from a USB connection to my PC, and measured the outputs on the mainboard connection.

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73

Evan

AC9TU

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