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Old 28th Sep 2018, 10:01 am   #31
crackle
Rest in Peace
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 4,100
Default Re: Building the 'Minimod' AM modulator-questions!

Hi Ian
Yes I do have a radio with a BFO control. I have tried it but the Minimod does not seem to sound any different to any other commercial or BBC station, it is possible to turn the BFO to more or less null out any whistles so the audio is pretty clear.
For some strange reason the effect of a null in the centre of the operational frequency seems less noticeable now, and good quality audio can be obtained with the radio tuned to the centre of the frequency..

I tried a 260pf polystyrene cap in the VFO and the frequency instability was worse. I could watch the frequency counter go up 0.1kHz every 10 seconds.

However one thing I have found out today, is the drift in frequency slowly upwards may not necessarily be a result of the Minimod being powered on. I have noticed that if I disconnect the power for a minute or so and reconnect it, the frequency displayed has still risen slightly.
I have ruled out an issue with the frequency counter as the radio being used also has to be adjusted to keep on tune. Testing the frequency counter on my Leader Signal generator seems to give a solid reading.

My suspicions now are maybe there is an issue in the little inductor used in the VFO. As the frequency adjustment is so fine, I suspect that that after setting the frequency the ferrite core settles in the thread and so the frequency moves slightly.
On a normal radio which can be tuned to any frequency this drift may not be so noticeable, but on one of the radios I was testing with, which can only tune to multiples of 9kHz, it is very noticeable.

I think the next option is to swap the 2 coils over to see if that makes any difference.
I double checked and the tap on L1 is the connection with the lowest resistance to ground.

This is a photo of my Minimod, the yellow round thing near the top left is a small trimmer for matching the level of the input device..
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The display on the frequency counter after about an hour of the Minimod being on. I initially set the Minimod to be at 1071kHz.
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Mike
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