QCX 40


Paul Harrison
 

Hi,

I have a strange situation, after removing 5 turns from the large T1 Toriod to optimise the BPF output, I can now adjust the BPF to maximum without the Variable C being fully open. However when trying to adjust the 3 variable resistors nothing happens (cannot obtain minimums). No change whatsoever.

However, the RX seems perfectly sensitive and just the same as when I first constructed the kit. I am able to merrily make contacts no problems and having great fun.

Are the 3 variable resistors basically superfluous? I am loath to desolder the SMD
to try another IC.

Thanks in advance for any suggestions.

Paul DJ0CU.

 


Mark G6PMN
 

Hi Paul,

You may want to check you got the enamel off the T1 winding as this was the culprit for me - the three variable resistors did little to nothing until I resoldered my T1, then they worked fine.

Cheers

73 Mark G6PMN


Alan G4ZFQ
 

However when trying to adjust the 3 variable
resistors nothing happens (cannot obtain minimums). No change whatsoever.
Paul,

These set the image rejection.
The receiver will seem to work particularly well as you may be receiving the images as well as the wanted signal.

This is often caused by a missing signal from one of the T1 secondaries.
Maybe the transformer wound wrongly or not properly soldered. Or maybe a fault in one of the signal paths to IC5A or B.

73 Alan G4ZFQ


However, the RX seems perfectly sensitive and just the same as when I first
constructed the kit. I am able to merrily make contacts no problems and
having great fun.


Al Holt
 

Just a suggestion, but you might want to try getting all three back to about the center of each of their ranges and go through the procedure again. Since the resistors are multi-turn without real 'stops' it can be a little tricky. They have a subtle click to them when they are at the end of their range. I'm not sure if they are 10, 15 or 20 turn types. 

R27 is the I-Q balance control and should have the greatest effect on the null. The other two resistors help balance out high and low sides of the audio bandpass. 

I hope that helps!

--Al
WD4AH


Paul Harrison
 

Thanks Mark, I'll double check that, thanks also to everybody for the other suggestions, much appreciated.

Paul DJ0CU

On Thu, Sep 6, 2018, 12:23 Mark G6PMN via Groups.Io <g6pmn=icloud.com@groups.io> wrote:
Hi Paul,

You may want to check you got the enamel off the T1 winding as this was the culprit for me - the three variable resistors did little to nothing until I resoldered my T1, then they worked fine.

Cheers

73 Mark G6PMN


Hans Summers
 

Hi Al, all

FYI the presets are 24-turn types.

Another thing: make sure gain control is not at minimum. The audio for the unwanted sideband adjustment is taken AFTER the gain control. So if you have gain at minimum this will also show the same symptoms (trimmers have no effect).

73 Hans G0UPL 

On Thu, Sep 6, 2018, 12:25 Al Holt <grovekid2@...> wrote:
Just a suggestion, but you might want to try getting all three back to about the center of each of their ranges and go through the procedure again. Since the resistors are multi-turn without real 'stops' it can be a little tricky. They have a subtle click to them when they are at the end of their range. I'm not sure if they are 10, 15 or 20 turn types. 

R27 is the I-Q balance control and should have the greatest effect on the null. The other two resistors help balance out high and low sides of the audio bandpass. 

I hope that helps!

--Al
WD4AH


Paul Harrison
 

Thanks Al and Hans for the tips, I did Increase the Audio to about midway but it had no effect, I was most surprised that R27 did nothing as it balances the output. I will attack the problem later this evening. I also wonder if the problem is related to the fact that WSPR transmissions are not being heard.

Paul. DJ0CU.


James Daldry W4JED
 

Hi, Paul

Your radio has 2 detectors working "in parallel", either one of which will do a good job of receiving signals. There are 2 detectors because by adding the detected signals 90 degrees out of phase one sideband is canceled out - signals are all on one side of zero beat. Like a "real" radio. The 3 controls are to adjust the phase of the audio signals to exactly 90 degrees. The fact that they do nothing suggests that one of the signals is missing. Since T1 has been removed the probable cause is that one of the "little" coils has a bad connection on it. IC4 should show the same voltage on pins 7 and 9, and should show equal resistance to ground with an ohmmeter.

Jim W4JED


On 09/06/2018 06:06 AM, Paul Harrison wrote:
Hi,

I have a strange situation, after removing 5 turns from the large T1 Toriod to optimise the BPF output, I can now adjust the BPF to maximum without the Variable C being fully open. However when trying to adjust the 3 variable resistors nothing happens (cannot obtain minimums). No change whatsoever.

However, the RX seems perfectly sensitive and just the same as when I first constructed the kit. I am able to merrily make contacts no problems and having great fun.

Are the 3 variable resistors basically superfluous? I am loath to desolder the SMD
to try another IC.

Thanks in advance for any suggestions.

Paul DJ0CU.

 


Paul Harrison
 

Thanks for explaining that Jim, it's very helpful. Everything seems to point to T1 and a dodgy solder joint. I will report my results. I was not able to complete the fault finding tonight due to other obligations, tomorrow hopefully i will have more time 

Paul DJ0CU.

On Thu, Sep 6, 2018, 22:24 James Daldry W4JED <jim@...> wrote:

Hi, Paul

Your radio has 2 detectors working "in parallel", either one of which will do a good job of receiving signals. There are 2 detectors because by adding the detected signals 90 degrees out of phase one sideband is canceled out - signals are all on one side of zero beat. Like a "real" radio. The 3 controls are to adjust the phase of the audio signals to exactly 90 degrees. The fact that they do nothing suggests that one of the signals is missing. Since T1 has been removed the probable cause is that one of the "little" coils has a bad connection on it. IC4 should show the same voltage on pins 7 and 9, and should show equal resistance to ground with an ohmmeter.

Jim W4JED


On 09/06/2018 06:06 AM, Paul Harrison wrote:
Hi,

I have a strange situation, after removing 5 turns from the large T1 Toriod to optimise the BPF output, I can now adjust the BPF to maximum without the Variable C being fully open. However when trying to adjust the 3 variable resistors nothing happens (cannot obtain minimums). No change whatsoever.

However, the RX seems perfectly sensitive and just the same as when I first constructed the kit. I am able to merrily make contacts no problems and having great fun.

Are the 3 variable resistors basically superfluous? I am loath to desolder the SMD
to try another IC.

Thanks in advance for any suggestions.

Paul DJ0CU.