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uBitx just started transmitting a loud hum #ubitx-help

James
 

Hi all,

I have had a few issues with my ubitx since it arrived a month ago.

Initially it worked well although I had the loud pop on SSB transmit, and I could also hear my own voice through headphones on transmit (which I assumed was normal for the radio).
After about an hour, of receive and a few short transmissions on 40m, the TDA2822 caught fire (yes actual flames). I blew out the fire, and replaced it with an lm 386 which was working well. Suprisingly R75 was also damaged so I replaced it. I wonder if R75 damage was coincidental or related to the mode of failure of the TDA2822. I didn't notice any damage to R75 prior to the failure of the TDA2822.

After another few hours of good use without anyobvious problems (could still hear my voice through headphones on transmit) I encountered a problem on transmit. Mid transmission, I could hear a loud tone/hum/interference though my headphones, which was also being transmitted according to the ham I was in QSO with.
It is still receiving fine, but every time I key the mic, loud tone/noise both through the headphones and over the air.

Hooked up to dummy load:
I am still making 10w output power on 40m.
Same sound and tone when trying to transmit on 20m.
I have changed back to a replacement tda2822 - loud sound still present.

Tx/Rx power is switching correctly at K1.
K3 appears to be working correctly - M1-M2 correctly shows infinite ohms on transmit when probed with multimeter.

Probing with an oscilloscope, the collector of Q6 shows a saw tooth wave form with freq around 50hz which appears to match what I can hear.
Power supply has small amount of 50hz ripple at 11.8V.

I'm really scratching my head about where this noise could possibly be coming from, and I'm running out if ideas on how to troubleshoot any further!
Any thoughts would be greatly appreciated as I'm sure it must be something simple, since the radio was working well prior to developing the fault on transmit.
Receive is still great.

 

Check your grounds! Could be a loose contact to ground. Tighten everything.

Raj

At 08-04-18, you wrote:
Hi all,

I have had a few issues with my ubitx since it arrived a month ago.

Initially it worked well although I had the loud pop on SSB transmit, and I could also hear my own voice through headphones on transmit (which I assumed was normal for the radio).
After about an hour, of receive and a few short transmissions on 40m, the TDA2822 caught fire (yes actual flames). I blew out the fire, and replaced it with an lm 386 which was working well. Suprisingly R75 was also damaged so I replaced it. I wonder if R75 damage was coincidental or related to the mode of failure of the TDA2822. I didn't notice any damage to R75 prior to the failure of the TDA2822.

After another few hours of good use without anyobvious problems (could still hear my voice through headphones on transmit) I encountered a problem on transmit. Mid transmission, I could hear a loud tone/hum/interference though my headphones, which was also being transmitted according to the ham I was in QSO with.
It is still receiving fine, but every time I key the mic, loud tone/noise both through the headphones and over the air.

Hooked up to dummy load:
I am still making 10w output power on 40m.
Same sound and tone when trying to transmit on 20m.
I have changed back to a replacement tda2822 - loud sound still present.

Tx/Rx power is switching correctly at K1.
K3 appears to be working correctly - M1-M2 correctly shows infinite ohms on transmit when probed with multimeter.

Probing with an oscilloscope, the collector of Q6 shows a saw tooth wave form with freq around 50hz which appears to match what I can hear.
Power supply has small amount of 50hz ripple at 11.8V.

I'm really scratching my head about where this noise could possibly be coming from, and I'm running out if ideas on how to troubleshoot any further!
Any thoughts would be greatly appreciated as I'm sure it must be something simple, since the radio was working well prior to developing the fault on transmit.

Tim Gorman
 

If you are hearing yourself in your headphones then the most likely
culprit is crosstalk between the mic amp and the receive audio circuit
while the audio circuit is still energized.

Check to see if either the tda2822 and/or Q70 still has power on
transmit. If they do then you need to start looking at where the power
leakage is occurring.

You might also unplug your headphones while transmitting to another ham
and see if that removes the hum and noise. If it does its another
indicator of problems in the audio circuit.

tim ab0wr

On Sat, 07 Apr 2018 23:05:00 -0700
"James" <james.manion@...> wrote:

Hi all,

I have had a few issues with my ubitx since it arrived a month ago.

Initially it worked well although I had the loud pop on SSB transmit,
and I could also hear my own voice through headphones on transmit
(which I assumed was normal for the radio). After about an hour, of
receive and a few short transmissions on 40m, the TDA2822 caught fire
(yes actual flames). I blew out the fire, and replaced it with an lm
386 which was working well. Suprisingly R75 was also damaged so I
replaced it. I wonder if R75 damage was coincidental or related to
the mode of failure of the TDA2822. I didn't notice any damage to R75
prior to the failure of the TDA2822.

After another few hours of good use without anyobvious problems
(could still hear my voice through headphones on transmit) I
encountered a problem on transmit. Mid transmission, I could hear a
loud tone/hum/interference though my headphones, which was also being
transmitted according to the ham I was in QSO with. It is still
receiving fine, but every time I key the mic, loud tone/noise both
through the headphones and over the air.

Hooked up to dummy load:
I am still making 10w output power on 40m.
Same sound and tone when trying to transmit on 20m.
I have changed back to a replacement tda2822 - loud sound still
present.

Tx/Rx power is switching correctly at K1.
K3 appears to be working correctly - M1-M2 correctly shows infinite
ohms on transmit when probed with multimeter.

Probing with an oscilloscope, the collector of Q6 shows a saw tooth
wave form with freq around 50hz which appears to match what I can
hear. Power supply has small amount of 50hz ripple at 11.8V.

I'm really scratching my head about where this noise could possibly
be coming from, and I'm running out if ideas on how to troubleshoot
any further! Any thoughts would be greatly appreciated as I'm sure it
must be something simple, since the radio was working well prior to
developing the fault on transmit. Receive is still great.

 

Tim,

In uBitx the TDA2822 is always powered.

Raj

At 08-04-18, you wrote:

Check to see if either the tda2822 and/or Q70 still has power on
transmit. If they do then you need to start looking at where the power
leakage is occurring.

Tim Gorman
 

Yep, that's true. Thanks for noting that. The preamp isn't however.

That needs to be checked.

tim ab0wr

On Sun, 08 Apr 2018 17:03:26 +0530
"Raj vu2zap" <@Raj> wrote:

Tim,

In uBitx the TDA2822 is always powered.

Raj

At 08-04-18, you wrote:

Check to see if either the tda2822 and/or Q70 still has power on
transmit. If they do then you need to start looking at where the
power leakage is occurring.


Jerry Gaffke
 

Was your blown TDA2822 a WX brand part?
No reports here yet of other than the WX clones blowing on a uBitx.
And they blow for pretty much no reason when powered from 12v.
The WX might be safe enough if run with the 5v regulator fix.

R75 has smoked before:
    https://groups.io/g/BITX20/message/42693
I'm imagining that die turning into molten silicon, shorting 12v into R75.

Hum is usually power supply noise, this thing has some high gain amps.
Try a 12v battery, perhaps take it outside and pop a car hood somewhere.

As you observed, the M1-M2 relay contacts should mean no audio into that TDA2822.
Try shorting M2 to ground, does that kill it?
If not, try shorting VOL-M to ground near that TDA2822, that should certainly kill it.
My best guess is that the wires out to the volume pot are picking something up.
And/or as Raj suggested, a bad ground somewhere.

Jerry, KE7ER


On Sat, Apr 7, 2018 at 11:05 pm, James wrote:
Hi all,

I have had a few issues with my ubitx since it arrived a month ago.

Initially it worked well although I had the loud pop on SSB transmit, and I could also hear my own voice through headphones on transmit (which I assumed was normal for the radio).
After about an hour, of receive and a few short transmissions on 40m, the TDA2822 caught fire (yes actual flames). I blew out the fire, and replaced it with an lm 386 which was working well. Suprisingly R75 was also damaged so I replaced it. I wonder if R75 damage was coincidental or related to the mode of failure of the TDA2822. I didn't notice any damage to R75 prior to the failure of the TDA2822.

After another few hours of good use without anyobvious problems (could still hear my voice through headphones on transmit) I encountered a problem on transmit. Mid transmission, I could hear a loud tone/hum/interference though my headphones, which was also being transmitted according to the ham I was in QSO with.
It is still receiving fine, but every time I key the mic, loud tone/noise both through the headphones and over the air.

Hooked up to dummy load:
I am still making 10w output power on 40m.
Same sound and tone when trying to transmit on 20m.
I have changed back to a replacement tda2822 - loud sound still present.

Tx/Rx power is switching correctly at K1.
K3 appears to be working correctly - M1-M2 correctly shows infinite ohms on transmit when probed with multimeter.

Probing with an oscilloscope, the collector of Q6 shows a saw tooth wave form with freq around 50hz which appears to match what I can hear.
Power supply has small amount of 50hz ripple at 11.8V.

I'm really scratching my head about where this noise could possibly be coming from, and I'm running out if ideas on how to troubleshoot any further!
Any thoughts would be greatly appreciated as I'm sure it must be something simple, since the radio was working well prior to developing the fault on transmit.
Receive is still great.

twowindsbear@...
 

I'm thinking power supply issues.  Have you tried using a 12v battery?

Good luck!  And post how you resolved your issues.

James
 

Thanks for the great replies,

I haven't been through all of the grounds exhaustively, but all jacks/pots/rotary encoders and switches I have soldered are properly grounded to the ubitx main board (0 ohms on multimeter). I have also confirmed the emitter of q6 is grounded via r63 and r64. Emitter of q70 is grounded directly.
The main board isn't earthed (it's inside a plastic chassis) and is only grounded via power supply -ve terminal.
Haven't had a chance to try it with a 12v battery, but will do in the next day or 2. No RF earth at present.

Q70 is showing 0 volts on Tx, but there is a visible fine wave form on the oscilloscope when probing during tx. I haven't got a capture of it to measure frequency but I suspect this is related to the noise I am hearing.

Will continue to work through the suggestions re audio circuit tests and report back.

Also, yes it was a WX branded TDA2822 that failed.

Cheers,
James

 

An audio recording of the hum will be nice.

Raj

At 09-04-18, you wrote:
Thanks for the great replies,

I haven't been through all of the grounds exhaustively, but all jacks/pots/rotary encoders and switches I have soldered are properly grounded to the ubitx main board (0 ohms on multimeter). I have also confirmed the emitter of q6 is grounded via r63 and r64. Emitter of q70 is grounded directly.
The main board isn't earthed (it's inside a plastic chassis) and is only grounded via power supply -ve terminal.
Haven't had a chance to try it with a 12v battery, but will do in the next day or 2. No RF earth at present.

Q70 is showing 0 volts on Tx, but there is a visible fine wave form on the oscilloscope when probing during tx. I haven't got a capture of it to measure frequency but I suspect this is related to the noise I am hearing.

Will continue to work through the suggestions re audio circuit tests and report back.

Also, yes it was a WX branded TDA2822 that failed.

Cheers,
James

Paul Galburt - K2AYZ
 

Or a scope trace capture?

ajparent1 <kb1gmx@...>
 

The symptoms and the fact that the hum is 50hz and the local power seems to be 50HZ says your power supply may have developed
problems when the TDA2822 failed and the power source is the source of the hum.

Test on battery and see if it goes away.

Test the power supply for hum while on transmit at power (CW mode).  Likely if the power
supply has developed an issue hum (power line ripple) could be the source.
IF the power supply is supposed to be 12V why is it sitting at 11.8V with some noise?

Another problem can be that something in your antenna system has changes such to allow excess
RF to get into the radio circuits.  A common problem is high SWR on the feed line and microphone
cable picking up RF.

Allison/kB1GMX

James
 

Thanks for the help - will test with 12v battery soon, but the power supply is working fine with my 100w rig on transmit - nil issues - so I'm not convinced the power supply has any major faults. Also the 3A fuse didn't blow when the tda2822 failed.

I've done some more probing with the oscilloscope and have some photos to add, both from the audio jack and from the collector of q70.


Audio jack - frequency measured by markers is 7.1mhz, approx 100mV amplitude.

FFT analysis of probe on audio jack shows major peaks at 546khz, 7.265mhz (largest), 21.6mhz, 36.0 and 43.1mhz.


PROBE on collector of Q7:




FFT shows peaks at 312khz and 2.9mhz at collector of Q70

I'm not sure if this makes it any clearer.
I couldn't reproduce the 50hz that I initially found - I think maybe it was just the ripple from the power supply being amplified in the audio amp from the vol pot wires (which I had moved whilst taking the measurements tonight).

James
 

Also re antenna - same noise present into antenna initially and into 50 ohm dummy load (which is what I have been using for all testing) with 1:1 swr into dummy load.
It is also present if headphones are unplugged. Still present with TDA2822 removed from socket.

PSU is adjustable voltage (I have set it to 11.8 in order to avoid frying the next TDA2822) and there is no sag on transmit.

James
 

Further investigations, and I'm pretty sure I've cracked it.
Probed Q6 collector - showing 4v swing at 50hz... No other 50hz anywhere else in power supply on scope.



I traced this to the mic output - which also shows 50hz - very low voltage however.
Re-examined the electret microphone and found low and behold a bit of the trace had lifted where soldered to my mic cable.

Having removed the electret element, there is no more loud obnoxious noise on Tx (albeit no noise at all).

I'll replace the electret with a new one and hopefully be back in business!

Once again, a great thanks to all the advice and help with potential sources of the noise!!!!

Satish Chandorkar
 

I have seen failure of electret microphone in my BITX (Not uBITX) 2 3 times. In my opinion the biasing ( Supply ) resistance in case of uBITX 4K7 ohms is passing
high voltage spike ( Resulting instantaneous high current ) causing failure of the mic unit. May be even stray RF generated is causing these spikes and they are the
the cause of the mic failures

On Tue, Apr 10, 2018 at 5:41 PM, James <james.manion@...> wrote:
Further investigations, and I'm pretty sure I've cracked it.
Probed Q6 collector - showing 4v swing at 50hz... No other 50hz anywhere else in power supply on scope.



I traced this to the mic output - which also shows 50hz - very low voltage however.
Re-examined the electret microphone and found low and behold a bit of the trace had lifted where soldered to my mic cable.

Having removed the electret element, there is no more loud obnoxious noise on Tx (albeit no noise at all).

I'll replace the electret with a new one and hopefully be back in business!

Once again, a great thanks to all the advice and help with potential sources of the noise!!!!


 

Just put a 470uF or more across C76 and see. There is a possibility that C76 failed, usually you would expect a bulge on the top but may
not happen.

All the higher frequencies you see on the scope will not be heard as a hum. The audio chip wont amp the higher frequencies other than audio.

OTOH somewhere I suspect a ground is open. That would explain the pics that you sent.

Raj

At 10-04-18, you wrote:
Thanks for the help - will test with 12v battery soon, but the power supply is working fine with my 100w rig on transmit - nil issues - so I'm not convinced the power supply has any major faults. Also the 3A fuse didn't blow when the tda2822 failed.I've done some more probing

 

Thats a big hum level at the mic out! Make sure the collector has no DC voltage.

Raj

At 10-04-18, you wrote:
Further investigations, and I'm pretty sure I've cracked it.
Probed Q6 collector - showing 4v swing at 50hz... No other 50hz anywhere else in power supply on scope.

Michael Shreeve
 

Any measurements of current ? Did you have a fuse ? I'm thinking the audio circuit had an extreme amount of audio, oscilation or something like that. . You didn't say you could hear anything, so perhaps you also had a shorted output, perhaps at the jack ? If the jack is set up incorrectly you will short out the amplifier. Remove  the jack from ground .. Combination of problems ? 

Michael Shreeve
 

My mistake, you did say you heard a loud hum. So, current measurement would be a good start. 

James
 

Raj, just to clarify, that scope was on the collector of q6 with the 4v wave form. The base had only shown much less voltage. The collector had DC voltage as well (somewhere around 10v but I forgot to measure it exactly last night) which is about what I would expect.

The mic input to the base of q6 does have DC of around 1 or 2V again I didn't measure it exactly as it looked about right for the voltage divider bias made up of r61 and r62.

As pointed out, there may well be a grounding issue elsewhere, which would/may explain hearing audio on transmit at the beginning, but at least the culprit for the hum was related to the electret mic.

Just to clarify, haven't measured current of rig yet on tx but it is less than 3A as my fuse is intact.