[David G4EBT]

The design concept behind both the Mini-Mod and the FM/AM Converter was to attain an acceptable level of performance whilst minimising complexity.

In that regard, both projects tick all the boxes, have been popular, and deservedly so. However, the mini-mod is less of a compromise, but has a higher component count - 8 resistors, 13 caps, two tuned circuits using adjustable inductors, an IC and two transistors. The modulator section of the FM/AM converter on the other hand, uses only 3 resistors, 7 capacitors, 1 trimmer and one transistor, and two fixed 'resistor style' chokes, originally built on a piece of Veroboard 1" wide x 3.5" long. Despite that, it performs remarkably well and in using the modulator part of the FM/AM converter, I've tried to retain the same concept - 'keep it simple'.

With that in mind, maybe I shouldn't have gone poking around with my 'scope! In thinking more about this, it dawned on me that the original FM/AM converter used only a 3V supply, and by feeding this 'clone' from a 9V 'wallwart' to serve the digital FM module, with R4 to drop the Voltage to the modulator, there was still 7V after R4, and I'd wondered if that was perhaps too high, and causing the kink in the negative going side of the sine wave.

Firstly, as to the supply Voltage, the Digital FM Module has an on-board 5V regulator so will be happy with a 7V supply rather than 9V. Hence, I fed it with 7V, temporarily substituted R4 for a preset pot which I adjusted to drop the Voltage to 3V This indicated that R4 needed to be 2K7, so I fitted that. I also changed the transistor just in case that had a bearing on the waveform 'kink', and on checking the unmodulated output of the modulator, though the kink is still present, I think it's less so.

Below are the 'before' and 'after' traces.

When I've fathomed out how to get the FM module to scan and store stations, I'll give it a go and report back any progress.

Thanks for the interest, advice and assistance.