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I have a new sBitxV3 board only. Everything is working except the Mic input. I started experimenting with some microphone I had and the recorded transmission has lots of noise and very low audio. I measure 2.1v on mic input tip. I think it should be 5v. Can anyone guide me as to what the issue may be and where I should look. Here are my measurements/ calculations.?
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Yeah, tell us everything about the microphone!!!
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On Mar 9, 2025, at 12:02, WF4RC Carroll via groups.io <WF4RC@...> wrote:
? I have a new sBitxV3 board only. Everything is working except the Mic input. I started experimenting with some microphone I had and the recorded transmission has lots of noise and very low audio. I measure 2.1v on mic input tip. I think it should be 5v. Can anyone guide me as to what the issue may be and where I should look. Here are my measurements/ calculations.?
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Bad mic. ?Is drawing the bias provided by RD21 down from 11.9v to 2.1V.?
Dr.?William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch – K9ZC Staunton, Illinois ? Owner – Operator Villa Grand Piton - J68HZ Soufriere, St. Lucia W.I. Rent it: www.VillaGrandPiton.com
email:??bill@... ?
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On Mar 9, 2025, at 9:03?AM, WF4RC Carroll <WF4RC@...> wrote:
? I have a new sBitxV3 board only. Everything is working except the Mic input. I started experimenting with some microphone I had and the recorded transmission has lots of noise and very low audio. I measure 2.1v on mic input tip. I think it should be 5v. Can anyone guide me as to what the issue may be and where I should look. Here are my measurements/ calculations.?
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It is possible he’s trying a dynamic microphone
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On Mar 9, 2025, at 12:23, K9HZ via groups.io <bill@...> wrote:
? Bad mic. ?Is drawing the bias provided by RD21 down from 11.9v to 2.1V.?
Dr.?William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch – K9ZC Staunton, Illinois ? Owner – Operator Villa Grand Piton - J68HZ Soufriere, St. Lucia W.I. Rent it: www.VillaGrandPiton.com
email:??bill@... ? On Mar 9, 2025, at 9:03?AM, WF4RC Carroll <WF4RC@...> wrote:
? I have a new sBitxV3 board only. Everything is working except the Mic input. I started experimenting with some microphone I had and the recorded transmission has lots of noise and very low audio. I measure 2.1v on mic input tip. I think it should be 5v. Can anyone guide me as to what the issue may be and where I should look. Here are my measurements/ calculations.?
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Exactly. ?
Dr.?William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch – K9ZC Staunton, Illinois ? Owner – Operator Villa Grand Piton - J68HZ Soufriere, St. Lucia W.I. Rent it: www.VillaGrandPiton.com
email:??bill@... ?
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On Mar 9, 2025, at 9:31?AM, Gordon Gibby KX4Z via groups.io <docvacuumtubes@...> wrote:
? It is possible he’s trying a dynamic microphone
On Mar 9, 2025, at 12:23, K9HZ via groups.io <bill@...> wrote:
? Bad mic. ?Is drawing the bias provided by RD21 down from 11.9v to 2.1V.?
Dr.?William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ ? Owner - Operator Big Signal Ranch – K9ZC Staunton, Illinois ? Owner – Operator Villa Grand Piton - J68HZ Soufriere, St. Lucia W.I. Rent it: www.VillaGrandPiton.com
email:??bill@... ? On Mar 9, 2025, at 9:03?AM, WF4RC Carroll <WF4RC@...> wrote:
? I have a new sBitxV3 board only. Everything is working except the Mic input. I started experimenting with some microphone I had and the recorded transmission has lots of noise and very low audio. I measure 2.1v on mic input tip. I think it should be 5v. Can anyone guide me as to what the issue may be and where I should look. Here are my measurements/ calculations.?
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Even with a dynamic mic, QD1's operating point is incorrect. The emitter and base should have a difference of .6 to .7 volts. The listed voltages are 3.9 and 2.3 volts or 1.6 volts. The transistor is not in the linear mode. I would replace QD1 and remeasure.
?
73
Evan
AC9TU
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Thanks for the input and replies,
There was no mic connected when I made the measurements. Can anyone verify/ confirm that the Tip of the TRS MIC plug should be at 5v? I suspect QD1 as well .? The mic I was using was electret and needed a 3.3-5v Vin. That was how I found out the Tip was only at 2.1.
Any ideas are wlecome. Thanks everyone.?
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73.
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CD29 could be leaking.? Without a mic connected, the voltage on the tip should be approximately 9.9/10th of the voltage supplied to the sBitx.? That assumes a 10meg input resistance on your meter.? There should be no current draw through RD21, though I suspect you derive the current reading from the voltage drop across it.? The voltage drops when the mic is connected.
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Try disconnecting CD29 and measuring the voltage again.
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Reducing RD21 can increase the bias voltage to the mic.
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73
Evan
AC9TU
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the base-emittter voltage of QD1 schould be about 0,7 V but is
3,9 V -2.3 V = 1.6 V, my guess: defect transistor or wrong
measurement.
Am 09.03.2025 um 17:02 schrieb WF4RC
Carroll:
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I have a new sBitxV3 board only. Everything is working except
the Mic input. I started experimenting with some microphone I
had and the recorded transmission has lots of noise and very low
audio. I measure 2.1v on mic input tip. I think it should be 5v.
Can anyone guide me as to what the issue may be and where I
should look. Here are my measurements/ calculations.?
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the mystery continues.?
removed :
CD28
LD6
CD29
CD27
QD1
?
Still have 2.1v at junction of LD6 / CD29 [both removed].? With power off measuring about 2K to ground at that point. Going to tke a break and start over tomorrow... Welcome any thoughts.
?
?
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There appear to be one or more measurement errors here. ? The bulk of the measurement suggest that the transistor is indeed within its linear range despite the unexplained base emitter voltage. ?So I strongly suspect some measurement errors or a faulty measurement device.
If there still is DC voltage present on the other side of the one micro F capacitor ?then the capacitor is defective. But again, I suspect measurement error is what’s really going on and if this were me, I would just very very carefully reconfirm every device and every measurement before I try to figure much more out.
It’s very strange to have a base emitter voltage anything greater than 0.6. ?And the collector measure measurements demonstrate the device is not saturated. ?
So I suspect the reason this makes ?no sense is that something isn’t correct. ? Voltmeter lugging circuits down or reading incorrectly, some form of RF input to the voltmeter who knows.
Waiting to hear more clues.
Gordon kx4z?
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On Mar 9, 2025, at 17:33, WF4RC Carroll via groups.io <WF4RC@...> wrote:
? the mystery continues.?
removed :
CD28
LD6
CD29
CD27
QD1
?
Still have 2.1v at junction of LD6 / CD29 [both removed].? With power off measuring about 2K to ground at that point. Going to tke a break and start over tomorrow... Welcome any thoughts.
?
?
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QD1 is toast, the base to emitter voltage is too high!
Recheck and if you find Voltage between base-emitter is much more
that say 0.7 volts then replace the transistor and you audio will
be back.
Raj, vu2zap
On 09/03/2025 9:32 PM, WF4RC Carroll
via groups.io wrote:
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I have a new sBitxV3 board only. Everything is working except
the Mic input. I started experimenting with some microphone I
had and the recorded transmission has lots of noise and very low
audio. I measure 2.1v on mic input tip. I think it should be 5v.
Can anyone guide me as to what the issue may be and where I
should look. Here are my measurements/ calculations.?
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Thanks everyone for the discussion, much appreciated.
I did find a typo in my measurements documentation. The base is 2.9 v not 3.9. That resolves the concern with QD1.
Using a fluke 8062 to measure. It has been reliable for years and I checked it against my Simpson 260.?
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I have removed from circuit.?
CD28
LD6
CD29
CD27
RD15
QD1
There should be 11.9 volts at? open end of RD21, and I'm still only getting 2.1 v.?
I have removed RD 21 and verified 10K Ohms and reinstalled. I'm now looking for a solder bridge, lifted pad or some other PCB issue.
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May just bypass this mic preamp and use an external one that I know works. Spent too much time on this.... especially since I will probably use the sBitx for FT8/POTA most of the time.
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Will update you with next findings.....
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73,
?
WF4RC-Rob
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Rob, is this with mic or mic unplugged/disconnected ?
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On 10/03/2025 6:01 PM, WF4RC Carroll via groups.io wrote: There should be 11.9 volts at? open end of RD21, and I'm still only getting 2.1 v.
I have removed RD 21 and verified 10K Ohms and reinstalled. I'm now looking for a solder bridge, lifted pad or some other PCB issue.
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Disconnected
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WF4RC, Rob Carroll
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Rob, is this with mic or mic unplugged/disconnected ?
On 10/03/2025 6:01 PM, WF4RC Carroll via groups.io wrote:
> There should be 11.9 volts at? open end of RD21, and I'm still only
> getting 2.1 v.
> I have removed RD 21 and verified 10K Ohms and reinstalled. I'm now
> looking for a solder bridge, lifted pad or some other PCB issue.
>
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Hi Rob,
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I wonder if there is a 2KOhm or so resistor that is not on the schematic?? That would explain the voltage on RD21.? With LD6 removed, the mic is out of the picture.
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Can you send a picture of the board area?
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73
Evan
AC9TU
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RD2 ? I do not see this resistor on the and appears to be in parallel with C29. That would make your answer correct.
It would be great if another sBitx V3 owner could measure the Mic +ve on the tip of the mic TRS connector with mic unplugged.
?
?
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Well, there ya go!? ?change that resistor to get the bias voltage you prefer and you should?be in business!! I didn't have that (to my knowledge) in any radio I have worked on, and I added resistors to get the voltage under 10 V? (a lot of electrets want it below 10V)
In order to get adequate?volume, I had to use very high output mics -- around -25 dB or -30 dB in their way of measuring it.? ?? Wish you good luck!? ?there is always something to learn -- and in this case comparing what IS to what is SUPPOSED TO BE is the trick!
Congratulations!!
Gordon KX4Z
RD2 ? I do not see this resistor on the and appears to be in parallel with C29. That would make your answer correct.
It would be great if another sBitx V3 owner could measure the Mic +ve on the tip of the mic TRS connector with mic unplugged.
?
?
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Hi Rob,
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I measured my v3 mic input at 11.6 volts with a 12-volt supply, and no mic was connected.
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Here is the picture of the preamp area:
?
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RD2 is not there.
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If you want a different bias value, adjust RD2 to get whatever value you need.
?
73
Evan
AC9TU
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On Mon, Mar 10, 2025 at 02:09 PM, Evan Hand wrote:
I measured my v3 mic input at 11.6 volts with a 12-volt supply, and no mic was connected.
Exactly what I would expect. Interesting that RD2 is not on the schematic. I'm befuddled as to what is happening with my PCB. I should have 11.xx v on either side of LD6. Even more so with the list of components I have removed. I might try to reinstall all and see what happens.?
?
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K9HZ
Raj vu2zap