At5555n ii Biasing
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Radiomadman
- Registered New User
- Posts: 1
- Joined: 31 Mar 2024, 07:00
- Call Sign: G4JXX
At5555n ii Biasing
Hi all. Bit of a newbe here and hoping someone can help with a problem with my Anytone At5555n ii. I am sure that the basing of the driver and PA stage is wrong as my received audio reports are bad. I suspect that both are not linear, a clear indication is the quiescent current on tx seems too low. I am an electronics engineer by trade with all necessary test gear but no circuit diagram or service manual. Any help would be much appreciated.
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lbcomms
- Radio Addict
- Posts: 509
- Joined: 04 Oct 2015, 08:10
- Location: Sydney, Australia
Re: At5555n ii Biasing
First up, is the transmitted audio OK on AM and FM? This will clear the microphone element and the audio stages of the radio as the culprit.
Set up a two tone audio source - one directly connected to the mic input works best for this as it won't pick up any room noise - and look at the SSB transmitted waveform on an oscilloscope, you should see a pattern of repeating envelopes. Reduce the audio drive and increase the vertical gain on the scope to match, until it drops into the noise. If it starts to get flatlining between the envelopes then the biasing is the trouble, but if it smoothly disappears into the noise then your issue is elsewhere.
Set up a two tone audio source - one directly connected to the mic input works best for this as it won't pick up any room noise - and look at the SSB transmitted waveform on an oscilloscope, you should see a pattern of repeating envelopes. Reduce the audio drive and increase the vertical gain on the scope to match, until it drops into the noise. If it starts to get flatlining between the envelopes then the biasing is the trouble, but if it smoothly disappears into the noise then your issue is elsewhere.