Unilab KL70 10m FM Conversion

The Unilab KL series were rugged FM commercial repeaters very similar, if not electrically identical,  to the Kyodo radio's. Converting KL150's to 2m and KL450's to 70cm is relatively straightforward, giving a 50W near indestructible radio. After successfully converting a KL70 to 6m FM operation I wondered if a 10m FM conversion was possible.

The KL70 was made to work between 66 and 88MHz. So a number of challenges arose or were foreseen:

1. The receiving band pass filters can't simply be re-tuned,
2. The local oscillators need a substantial amount of re-work,
3. The PLL loop filter may need modification, I'm yet to confirm this, and
4. The transmit harmonics will require filtering.

While still relatively straight forward, I've not found time to complete my conversion. However, writing about it is a sure way to elevate the project to a "finished" status so here goes.

In this post I will cover the receiver band pass filters. If you looked at my previous posts you will have noticed I'm a big fan of the Iowa Hills RF Filter Design software. Since the original bandpass filters on the receiver will not tune down to 29MHz I was forced to rebuild them.

Scouring the junkbox I found some nominal 160nH adjustable inductors. A few minutes work with the software and I had the following filter design:

My rejection of the image frequency of 72MHz is likely to be no better than 80dB for the two filters.At this point in time I didn't know about the series tuned filter transformation.  The same inductors used with series LC tanks would yield around 80dB for just one 3 element filter. If you knew of local transmissions on 72MHz then I'd use one filter like that shown, and one with series LC tanks. Let me know if you want more details on this alternative filter.

After etching a board and building two filters with the same pinout as the original filter I removed the filters from the radio, disassembled them, and put my new filter into the filter housings. Prior to putting the filter in the housing they looked like this:

 

With these two re-assembled filters soldered back into the receiver I turned my attention to the other parts of the project. More on that next time.

73's
Richard VK6TT

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