Les,
If you can get at least 200 mV peak to peak out of the Pre-Mixer
bandpass, the VFO level is OK.
The VFO and band module xtal oscillator are mixed in U7 and the output
of that mixer (after the bandpass filter) is applied to U1 pin 6.
If you find you need to check for loading of the U1 output, lift one
lead of C22 to isolate it.
73,
Don W3FPR
On 8/18/2014 6:08 PM, Les Garwood wrote:
Hi Don,
Extremely helpful and encouraging; thanks. Actually, I was able to get
the trimmers set pretty well, i think, using a strong signal source. But I
will make the measurements you recommended.
Right now, I suspect either low VFO output or something is pulling down the
gain of U1.
Les
On Aug 18, 2014 3:00 PM, "Don Wilhelm" <[email protected]> wrote:
Les,
Yes, the NE612 should show plenty of gain. When it does not, the usual
reason is little or no oscillator signal to pin 6.
The Pre-mixer Bandpass filter must be aligned before you will get much of
a signal at U1 pin 6.
Since you have a 'scope (hopefully with a 10X probe), I will give you an
alternate method of tuning the Pre-Mixer Bandpass filter.
Since you have the 4 band board, you must follow the proper band order -
adjust 15 (or 17) meters before adjusting 20 meters, and likewise adjust 30
meters before 40 meters.
Make a check before starting - align all the slots in the trimmer
capacitors parallel with the long edge of the band board. Then check the
activity of the crystal for each band - put your probe on band board P1 pin
4 and not the amplitude of the signal as you switch through the bands - you
should have greater than 200 mV peak to peak. The position of the trimmer
slots when tuned should be no more than 30 degrees from the center position
if your toroids are correct.
Then switch to 15 (or 17) meters, and with your probe on P1 pin 6, adjust
the Pre-Mixer trimmer caps for 15 meters for the maximum amplitude. Switch
to 30 meters and do the same for that bands trimmers. Do the same for 20
meters and 40 meters. You should be able to get 200 mV peak to peak there
on all bands.
Remember that your 'scope probe will load the Pre-Mix filter a bit, so the
trimmers to the right of the filter will not be at their optimum setting,
but it will be enough for receive - you will set the optimum position when
you adjust for transmit.
You should check to see that the Pre-Mix signal is getting to U1 pin 6 -
if it is not, suspect the coax link on the bottom of the board between J9
and J10.
Once you have a good signal at U1 pin 6, you should see that the output at
pin 4 is much greater than the input at pin 1.
73,
Don W3FPR
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