Topics

Help troubleshoot no TX

Lance Conry
 

Hi Group,

My BitX has never really been able to show much of a jump in current when performing the loud HELLO test.  With my fiddling, I can now not get any current increase when talking, so may have damaged something.  

I will describe my testing methodology here, in the hope that the community can guide me to the next steps of troubleshooting.

Glossary
  • RX - The radio is receiving.
  • TX - The radio is transmitting.
  • TXM - The radio is transmitting with a modulating signal.
  • OSC1 - Oscilloscope CH1
  • OSC2 - Oscilloscope CH2
  • mV - Unless otherwise specified (e.g. across), all measurements are taken between the test point and GND.
Test setup
  1. I have my computer generating a 400 Hz sine wave with a 4 mV swing.  (I came to these input settings as they match the readings I got at the mic input (MIC1) when offering a loud "HELLO" into the mic).
  2. I have one multimeter measuring current, connected in-line with the positive power line between my 12V power supply and the BitX's power jack.
  3. I have OSC1 connected to the mic input, to monitor the input source.
  4. I have OSC2 connected that I'm using to probe.
  5. I have another multimeter which I use for precise readings.
  6. The antenna is connected to a 50 Ohm dummy load.

Measurements
  1. At RX, the BitX draws 115 mA.
  2. At TX, the BitX draws 330 mA.
  3. At TXM, the BitX continues to draw a steady 330 mA.  No change from TX without modulation.
  4. Across MIC1 shows 4 mV in both TX and RX.
  5. Before R126 shows 0 mV in TX.
  6. Before R126 shows 11 mV in TXM.
  7. Across R126 shows 11 mV in TXM.
  8. Between before R126 and after D5 shows 0.5 mV in TX.
  9. Between before R126 and after D5 shows 11 mV in TXM.
  10. Across each of R43, R44, R53, R54 shows 0.5 mV in TXM.

Discussion

  • I believe measurement 7 is the audio signal that gets added to the balanced modulator (D5).
  • I would have thought that I should see an amplified signal across both R43 and R54, but am seeing nothing.
At this point I'm stuck, and don't know where else to go.  I can see that audio is getting through the audio amp and to the balanced modulator, but the schematic is unclear to me where I should next measure.  Any assistance would be appreciated.

My time zone is GMT +10, and happy to arrange an EchoLink sched if that will be an easier way to diagnose.

Kind regards,
  Lance

P.S.  If anyone knows where there is a text searchable copy of the schematic and board layout, that would greatly help finding components to diagnose.


Jerry Gaffke
 

Signal into the microphone input should be more like 50 to 100 mv.
You might look over this post, especially the first link:  https://groups.io/g/BITX20/message/23640

Jerry, KE7ER


On Fri, Jul 28, 2017 at 10:27 pm, Lance Conry wrote:
I have my computer generating a 400 Hz sine wave with a 4 mV swing.  (I came to these input settings as they match the readings I got at the mic input (MIC1) when offering a loud "HELLO" into the mic).