Theory & Poor Receive. What should I expect around U6 / U9?

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TangoBravoMike
 

Posts: 11
Joined: 20 Apr 2022, 20:34

Theory & Poor Receive. What should I expect around U6 / U9?

Post by TangoBravoMike »

I’m not hearing any CW or USB / LSB activity on any bands so I’m trying to debug my boards. I do hear static; the noise floor. I also hear occasional strong tones as I tune around, but I think these are just hash from local electronics.

Antenna is a magloop. My FT817nd hears reasonable numbers of people transmitting, CW and USB / LSB, with the magloop appropriately tuned.

I’m suspecting problems in the U6 multiplexer or U9 op amps area. I will have an oscilloscope soon to look at what’s going on.

My plan is to inject a fairly weak signal, sine wave at perhaps -80 dBm, and snoop through the signal path. This should simulate a continuous CW signal, with no additional modulation. I can do this with the tinySA spectrum analyzer (see https://www.youtube.com/watch?v=RvYwAn32Dl4 starting at about 2 minutes).

My understanding of the schematic is this: SMA connector feeds the 1-turn primary of T1. Then signal goes through the selected band’s relay and bandpass filter. From there it goes through the pin marked ‘RF’ of the J3 inter-pcb connector to the main board. On the main board, signal passes to Q1, the Tx/Rx switch. During receive, signal goes to pin 9 of the U6 SN74CBT3253 multiplexer/demultiplexer. In there, the Tayloe detector magic happens.

Here’s a YouTube video with what looks like a very similar Tayloe detector circuit:
https://www.youtube.com/watch?v=iwBk3jIxszI

If I understand correctly, the output of the U6 multiplexer plus the capacitors, should be the audio signal expressed as a quadrature (two waveforms 90 degrees out of phase).

For a CW signal there isn’t any audio modulation. What should I expect to see on the outputs of the demultiplexer? What should I expect if I look at the outputs of the U9 op amps on the ‘scope?

Then I intend to do it all again, but injecting a 1kHz amplitude-modulated signal on the same carrier frequency, again using the tinySA device. With a 1kHz AM signal, what should I expect on the outputs of the U6 mux/demux, and at the outputs of the U9 op amps?

If I sound like I know what I’m doing, well, I’m faking it. I’ve never owned an oscilloscope, though I know approximately how to use one. And the tinySA is new to me as well.

Thanks everybody! This is a fun project.
Last edited by TangoBravoMike on 25 Apr 2022, 23:04, edited 1 time in total.
Faraaz VK4JJ
 

Posts: 215
Joined: 30 Dec 2021, 21:56

Re: Theory & Poor Receive. What should I expect around U6 / U9?

Post by Faraaz VK4JJ »

Dead receive usually indicates poor soldering. Check U6 and U9 soldering on the legs. Positively check, with magnification and tweezers wiggling each pin. Ensure there are no solder bridges between pins.
AC9YY
 

Posts: 133
Joined: 21 Mar 2022, 00:04
Location: Illinois, USA

Re: Theory & Poor Receive. What should I expect around U6 / U9?

Post by AC9YY »

If none of your relays are energizing you will get no receive. You could check the relay for the last band you were on and see if it is switched in circuit.
DL3MHT
 

Posts: 21
Joined: 14 Mar 2022, 09:46

Re: Theory & Poor Receive. What should I expect around U6 / U9?

Post by DL3MHT »

I would start with a simple test:

Test your Main Board standalone. Connect a few meters of wire to the RF pin on the interface connector.
Make sure you are not creating any shorts! Then just go to 40m or 20m FT8 frequency. If your mainboard is fine, you should hear the FT8 transmissions already. It does not take much to receive FT8.
That will already tell you if you should focus on RF board or Main board for further debugging.

-Tobias

P.S.: I for myself had somewhere a bad solder connection around the OpAmp - Not me soldering on the OpAmp, but somewhere around the peripheral components from the stuff populated by the factory. Can't tell you which one, because at some point i got desperate and resoldered every capacitor and resistor.
Ohwenzelph
 

Posts: 207
Joined: 01 Jan 2022, 03:47

Re: Theory & Poor Receive. What should I expect around U6 / U9?

Post by Ohwenzelph »

Usually if there is poor reception everywhere, most often I have a bad spot where the COAX meets the connector. When that seems ok, I look out the window to see if the antenna is on the ground..
dougjeff
 

Posts: 3
Joined: 26 Nov 2022, 03:16

Re: Theory & Poor Receive. What should I expect around U6 / U9?

Post by dougjeff »

@TangoBravoMike - Did you ever resolve the issues that you were having? I have a kit that I have just assembled and seem to be on the same path as you were planning (Ordered a TinySA and have a scope to troubleshoot). I hear noise, but no clear Rx. I even tried down in the AM band for a local radio station at 980KHz. I can hear something, but it's not clear enough to understand.

I intend to follow Manuel's troubleshooting strategy (viewtopic.php?p=3244#p3244) but was really hoping to hear the outcomes of your investigation.

73
Doug
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