Magnum Deltaforce PLL alignment question

[T-CON]

Possibly overlooking something here but can't work this out... Set the 37.66MHz reference oscillator frequencies perfectly on a Deltaforce I have while I first ensured that I had the clarifier in the centre. I then accurately set the three IF system 10.695 MHz frequencies, again without apparent issues. What I later noticed after the alignment was that on air, S-signals at about S3 to S6 increased in strength and clarity as I rotated the clarifier away from the 12 o'clock position towards 3 o'clock (12 o'clock is just on the edge of clarity and full strength) My clarifier is locked so I used RV108 to match two transmit and receive 37.66MHz oscillator reference frequencies together. What I now find it that the transmitted Tx frequency is 100% accurate but after keying up II find have to rotate the clarifier to about 3 o'clock for best FM reception and signal strength, I therefore assume that the receive centre voltage is at about 3 o'clock when it should be with the Tx centre voltage at the centre. Does anyone understand why my receive is apparently off and what I can do about it please?

Cheers!

Terry

[lbcomms]

Does anyone understand why my receive is apparently off and what I can do about it

The bandpass of the IF filter is probably out of whack, if the centre of the filter is off then you'll find offsetting the local oscillator a bit (i.e. turning the clarifier!) to suit the filter error will give you better reception.

Do you have a way of sweeping the IF filter? Even something basic like a NanoVNA (less than $100 for a clone) is more than good enough for this.

Once you've confirmed this, either replace the filter, or redo the alignment taking the required offset into account.

[solarstorm]

I've run into this exact headache on the Deltaforce before, and it usually boils down to the DC offset on the varactor line rather than the crystal itself. Even if your 37.66MHz reference is spot on for TX, the radio relies on a specific "center" voltage to keep RX aligned with the physical 12 o’clock detent. Check the voltage at the wiper of your clarifier pot; if it’s not hitting the service manual's spec (usually around 4.5V or 5V depending on the board revision) right at the center notch, your RX passband will be shifted. Since you've already tweaked RV108, you might need to back in and recalibrate the RX frequency offset specifically, or check if a previous owner did a "super-slide" mod that's pulling the range unevenly. It sounds like a classic case of the electrical center not matching the mechanical center.