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Strong carrier on transmit. #bitx40help

 

I set up my bitx40 for transmit and on key up I heard a strong carrier from the bitx40. The carrier was 20 over on monitor receiver.? No it wasn't feed back from the mic. I looked at the balanced modulator circuit on the board and discovered that D15 and D16 and the 100 ohm trim pot we're not in the circuit. But a dual diode D20 was there. Could someone explain this modification and if anyone else has had this problem. Will see if any DC is getting back into modulator.? Thank you Bob WB2TFG
Vince Vielhaber
 

It was a change HF Signals made about a year ago and has worked fine. The dual diode is better balanced than you would get via the pot.

Did you do any of the mods to the board, like perhaps the one that involves a 10K resistor to the area of the balanced modulator for CW?

Vince.

On 02/10/2018 08:54 PM, radioant@... wrote:
*I set up my bitx40 for transmit and on key up I heard a strong carrier
from the bitx40. The carrier was 20 over on monitor receiver. No it
wasn't feed back from the mic. I looked at the balanced modulator
circuit on the board and discovered that D15 and D16 and the 100 ohm
trim pot we're not in the circuit. But a dual diode D20 was there. Could
someone explain this modification and if anyone else has had this
problem. Will see if any DC is getting back into modulator. Thank you
Bob WB2TFG*
--
Michigan VHF Corp.

Dale Brooks KG7SSB
 

Hi Bob, the diode package you are talking about was substituted for the 2 diodes and pot with the thought that the packaged diodes were a very good match and the pot was not necessary. My BITX40 came the same way and I haven't had any issues with a strong carrier. I have built this circuit on a home brew rig and used two individual diodes and pot with better results than the BITX without the pot adjustment. But, you shouldn't be getting a strong signal on the output without audio. If you have a scope check the mic audio amp circuit and make sure that there is no oscillations with the mic disconnected. You can also check the input to the diode package legs(D20) and see that you are getting equal signal on both diodes. It's possible that one of the wires on the transformer is not making a good contact on the board. It will be a 12mhz signal from the carrier oscillator input.?
I'm sure others will have more ideas and tests you can perform to isolate this problem. Good Luck and let us know what's happening. Dale kg7ssb

On Sat, Feb 10, 2018 at 6:54 PM, <radioant@...> wrote:
I set up my bitx40 for transmit and on key up I heard a strong carrier from the bitx40. The carrier was 20 over on monitor receiver.? No it wasn't feed back from the mic. I looked at the balanced modulator circuit on the board and discovered that D15 and D16 and the 100 ohm trim pot we're not in the circuit. But a dual diode D20 was there. Could someone explain this modification and if anyone else has had this problem. Will see if any DC is getting back into modulator.? Thank you Bob WB2TFG


 

Hi Dale, I disconnected D20 from the input of the attenuation pad going into the tx amp and the carrier was reduced but still there. I was wondering if the 12 MHz oscillator output could be leaking into the tx amp. I also get a large burst of of RF when the ptt is pressed. I read in earlier posts that they never found a solution to the ptt RF burst problem. I also read that with the diodes and the pot that it was normal to get a 1vpp output at the antenna with no audio input. Would you mind you mind discussing this problem on the land line.? Thank You Bob WB2TFG ex
 

I have described how to reduce or eliminate PTT carrier burst in BITX40. Please search!

Raj

At 26/02/2018, you wrote:
Hi Dale, I disconnected D20 from the input of the attenuation pad going into the tx amp and the carrier was reduced but still there. I was wondering if the 12 MHz oscillator output could be leaking into the tx amp. I also get a large burst of of RF when the ptt is pressed. I read in earlier posts that they never found a solution to the ptt RF burst problem. I also read that with the diodes and the pot that it was normal to get a 1vpp output at the antenna with no audio input. Would you mind you mind discussing this problem on the land line. Thank You Bob WB2TFG ex
Dale Brooks KG7SSB
 

Hi Bob,
We can certainly talk on the phone. Go to QRZ.com and look up my call sign KG7SSB and my email address is listed. Send me your phone number and I will call you when I get a chance to be in the shop where I can follow along with your troubleshooting. I will tell you; there will be a small amount of carrier that will leak through but if at normal levels it should be fine. On a spectrum analyzer(looking at the output) with no modulation you will see a reduced carrier at the transmit frequency. When you modulate the carrier frequency, the carrier level will go down and the lower sideband signal will go up. So with a good receiver in the room and no modulation there will be a relatively strong carrier heard on the receiver. Now if the balanced modulator circuit is faulty then there will be a constant carrier at higher levels but still partially attenuated because it has traveled through the crystal filter network. Talk to Raj about getting rid of the rf burst when keying up. I just live with it because I don't use a headset and the noise doesn't bother me that much. On my new scratch built rig I had trouble with the VFO leaking through but I haven't seen much in the way of the carrier. By the time the signal travels through the BPF stage most of these signals should be down more than 40 dbm except for the 7mhz band. A one volt or less signal on the antenna could just be noise getting picked up in the different parts of the radio and getting through to the final where it is amplified.?
Send me your information and i will try to help you by phone to analyze the different parts or circuits that should be looked at. By the way my BITX receiver audio is very week and I need to start troubleshooting it also. Remember this is just part of the fun! 73's Bob, Dale

On Sun, Feb 25, 2018 at 8:20 PM, Raj vu2zap <rajendrakumargg@...> wrote:
I have described how to reduce or eliminate PTT carrier burst in BITX40. Please search!

Raj

At 26/02/2018, you wrote:
>Hi Dale, I disconnected D20 from the input of the attenuation pad going into the tx amp and the carrier was reduced but still there. I was wondering if the 12 MHz oscillator output could be leaking into the tx amp. I also get a large burst of of RF when the ptt is pressed. I read in earlier posts that they never found a solution to the ptt RF burst problem. I also read that with the diodes and the pot that it was normal to get a 1vpp output at the antenna with no audio input. Would you mind you mind discussing this problem on the land line.? Thank You Bob WB2TFG ex
 

Bob,

The carrier burst occurs because the c122 is charging up on TX through D15, causing the mixer to unbalance momentarily.

Solution is simple:

Cut the track from D15 and D16 common that goes to R38 and 39 and scrape the green paint off the cut track ends and solder a
0.1uF capacitor. Additionally add a 1K across C107 to provide a DC path.

Raj
 

Alternatively, install the latest Raduino sketch which eliminates?the RF-burst?by software:


73 Allard PE1NWL

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