Topics

RFI from uBitx TX after AGC and other mods installed

RowlandA
 

I just finished installing:
1) ND6T AGC mod using Kees' kit. 
2) AGC off/med/long switch as described in ND6T's instructions
3) T/R LED (Sunil's approach)
4) An RF gain control with an on/off switch so I have front panel on/off using Sunil's enclosure.
5) A wire from J1 to the purple wire on the Digital connector for an S-meter reading

The good news is - no smoke and everything seems to be working.  But now when I transmit (so far just tried SSB), I get very loud RFI (my own garbled voice) in my headphones or speaker, so something I added is getting RF into the system.

I installed the AGC board right at the RF in/out point using the approach outlined by Kees.  I ran RG-174 to the RF gain control, grounded at the RF gain control but not grounded (per instructions) at the AGC board.

I used twisted pair for the runs from the AGC board to the audio gain pot and the +5/ground which I picked up from the green Raduino wire, tapped at the point where the green wire connects to the resistor at the Key jack.  The ground lead twisted to the +5V lead is connected to the ground "bus wire" that connects the front-panel PTT, Phones and Key jacks together.

The wires to select AGC hang time from the front panel switch are not twisted together.

Everything is connected to the AGC board using header pins and plug-in wires so I can disconnect easily from the AGC board, but not easily from the other end of the wire(s).  

Is there an obvious place to start troubleshooting this and/or since I'm already using coax and twisted pairs as recommended in ND6T's instructions, thoughts on a fix?

73,

Rowland K4XD

Don, ND6T
 

Rowland,
Check to see if the ground at the center of your new receive RF point is good when the board is unplugged. If it is, then unplug the AGC board and jumper the two outer pins of that RF tap. It sounds like it might not be grounded well. Of course it could be many more things but that is a start. Try to disconnect or bypass one thing at a time to isolate the cause. Since you are hearing what sounds like SSB then I doubt that the problem is improper gating of the audio. Unless you are using an amplified microphone and you are just hearing peaks that are over-powering it. In which case, unplug the PA power and see if it went away. Are you transmitting into a dummy load or into a radiating antenna? The more information, the better. Please keep us posted. 73, Don

alans77@...
 

Don: I have a similar problem only I have not installed the RF gain control. On SSB phone transmit my voice comes out of the speaker loud and clear. I find it strange the audio amp / speaker are not muted when transmitting.

73 ,
Alan

Don, ND6T
 

Alan,
How did you mount yours audio gating board? Where did you ground it? And are we talking about the K5BCQ kit on yours, too? Since it is not distorted I would almost bet that Q3 isn't turning off. Do you have an oscilloscope? -Don

alans77@...
 

Hi Don: I mounted my K5BCQkit  board as shown in Kees photos. AGC appears to work. I do not have an O scope.

Alan

Mark M
 

I have exactly the same situation with mine. I hear my voice loud & clear from the speaker, it's like a PA system.

It's Kees' kit installed like the photos...a three pin header, center to ground. The ground pin shows 0 ohm to ground with the board removed. I don't have the RF pot installed, just the jumper. When I remove the board and jumper the outer pins the radio works normally, no audio from the speaker on xmit. With the board in place, disconnecting the 5v stops the speaker audio.

I'm not sure the S meter driver is working either, I get less than 2 v even on a really strong BCB signal.

I'm afraid I might have reverse powered the board the first time I powered it up...the 5V marking was a little confusing, I think there was a thread about that here a while ago. It was not installed at that time, just hanging in mid air with the power & audio & s meter connections...I wanted to see if the S meter worked. I don't imagine that would have done it any good.

I have a borrowed scope so I can do some checking with that.

I also have a second kit that I haven't assembled yet. I might try it & see if it's any different (being careful with the power polarity).

Any advice will be appreciated.

Mark... AA7TA

On 6/24/18 1:18 PM, alans77 via Groups.Io wrote:
Don: I have a similar problem only I have not installed the RF gain control. On SSB phone transmit my voice comes out of the speaker loud and clear. I find it strange the audio amp / speaker are not muted when transmitting.
73 ,
Alan

Don, ND6T
 

I am a bit overwhelmed, perhaps. Alan, are you referring to the AGC board, not the "Click eliminator"? I'm not finding where Kees posted any photos of mounting the click eliminator. That would be the board that I would expect to cause the problem (if the receive audio gate was always on). The AGC board (the one that plugs into the receive RF trace) would have the devil's own time passing microphone audio into the speaker.
Mark, your observations may really help Alan and I understand what's going on. I doubt that you caused any problem on the board by running the +5 volts to ground. The Raduino regulator would have been stressed but seem to remember the 7805 has a  thermal foldback feature for short duration faults. You may, however, try removing that +5 ground lead, I don't know where you ran that from but it shouldn't be necessary and inserts another unknown variable into the mix for me.
The volume control connection, however, is essential. The high side should NOT be connected to the wiper. Aside from swapped leads, about the only thing that I can think of is improper grounding. If anyone is using a plastic panel or case they need to go to extra lengths to provide short and solid ground returns. This is especially important where large signals or currents are present. The antenna RF connector needs to have its own additional ground back to the BITX board connection, not relying upon the panel or chassis. Same goes with the speaker and headphone jacks. The AGC and RF manual gain control should have the best grounding possible with connections to the BITX ground plane.
Aside from that, I'm out of ideas for now. What did you find Mark? -Don

RowlandA
 

Hi Don - thanks for your speedy assistance.

To be clear - what I am hearing when I transmit SSB is RFI - loud garbled sounds from the speaker or headphones, not a clear rendition of my voice.

The offender seems to be the wiring to the volume control (audio).  If I unplug the audio pot from the audio pins on the board, the RFI goes away.  I am using a twisted pair from the pot to header pins on the AGC board.

With the pot connected, the RFI is heard when I transmit into a dummy load or an antenna, no difference.

Independent of the RFI, I noticed my use of J1 to drive the S-meter was giving a reading of zero.  I checked the voltage on J1 (with everything hooked up to the AGC board) and it never deviates from zero, so I must have a problem with the AGC board.  This happens even if I disconnect my AGC off/med/long switch and just check the voltage at J1 - it's zero regardless of signal level.  

So I think I have (at least) two problems - RF being picked up by the cable going to the volume control pot, and something wrong with my assembly of the AGC board so there is no voltage across C4.

I'll see if I can re-route the audio cable - I was trying to keep it on the short side so it is routed across the board, not along the case as you recommend.  I'll try that.

I do have a second AGC kit so I can try assembling that and see if my "no voltage across C4" issue goes away.

73,
Rowland K4XD

Don, ND6T
 

Rowland,
With the AGC board mounted so far from the volume control (and right near the high RF area) it might be best to run shielded cable instead of just twisted pair. Some RG174 or similar small cable (impedance unimportant) might be the answer. I think that you may have found it! Or at least one of 'em.
The two others with problems were also using that spot! That is something that I had not considered. My builds are all right on the front panel, just a couple of inches for the audio run. -Don

alans77@...
 

Don: I am referring to the AGC. My next step is to run shielded cable instead of a twisted pair between the volume control terminals and the the AGC board.

Alan

Don, ND6T
 

FB Alan! Got my fingers crossed. -Don

RowlandA
 

Thanks Don, I'll try some RG-174 then.  I might not have time until Wednesday, flying out tomorrow for a two day trip, but will report back when I try it.

I assume one coax with shield to ground at both ends - unlike the RF gain control coax, which is two coax cables, grounded at the pot end but shields not connected at the AGC board.  Let me know if that's not right.

73,
Rowland K4XD

MVS Sarma
 

IMHO, small interconnections, you need both sides ground.
Earlier in telecom we used balance twin wire cable with shield.
 There we used to ground at one  end and  if some one grounds at both ends, the shielding was lost and some longitudinal current were said to be flowing through shield . In such cases, only one side Gnd was recommended.
  But here the shield is a return conductor also.


Regards
MVS Sarma
 

On Mon, Jun 25, 2018 at 6:49 AM, RowlandA <rarcher@...> wrote:
Thanks Don, I'll try some RG-174 then.  I might not have time until Wednesday, flying out tomorrow for a two day trip, but will report back when I try it.

I assume one coax with shield to ground at both ends - unlike the RF gain control coax, which is two coax cables, grounded at the pot end but shields not connected at the AGC board.  Let me know if that's not right.

73,
Rowland K4XD


Don, ND6T
 

Yes, you are both quite correct. Ground both ends of this connection. -Don

RowlandA
 

Hi Don - I tried a small RG174 cable and unfortunately it didn't solve the RFI problem.  Still getting a very strong garbled RFI when I transmit SSB and the audio cable is connected.  Grounded at both the pot end and the AGC board, with the center of the coax going to the hot side of the pot (not the wiper).  

Same problem with TX into a dummy load or antenna.  

Not sure what to try next unless I move the AGC board towards the front of the enclosure and redo the cabling.  I guess it's hard to know where the RF is coming from so I know the best way to route the cables to avoid it?

Any other thoughts?  Is anyone successfully using the AGC kit mounted at the tap point of the RF trace and Sunil's enclosure?  Maybe a photo showing cabling for a successful install would help.

73,
Rowland K4XD

alans77@...
 

Based on Rowland's results it appears that we are left with trying ferrite beads on the volume control to AGC wires or muting the speaker. Since my ICOM wired hand Mic PTT switch is SPDT with the common terminal grounded I can wire the Mic to disconnect the speaker ground on transmit.

Alan

Don, ND6T
 

Rowland, Does it go away when you unplug the power to the PA? -Don

alans77@...
 

Can anyone specify (photo of location needed) a relay contact that is open on transmit and grounded on receive?

73,
Alan N4AYE

Don, ND6T
 

No photo but that would be K1, the relay that sits over at the left by itself. Underneath you will see one of the contacts is grounded. That's the normally open contact. The next contact down will be the normally closed contact that goes to the bandpass filter. The next contact in that line is the common one that goes to the receive line. Sorry, but my version 3 board is in a difficult case to extract from.
-Don

RowlandA
 

I don't have a separate power source for the PA - I assume to test this I'll have to cut one of the three wires going to the plug into the PCB?  I'll try it Wednesday if it will help with troubleshooting.

Thanks,
Rowland