TMN1 VCOs to 23 cm ham band

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These modifications of ancient Mobira radios are meant for licensed radio amateur use.
In addition to this modification you also need the ham software for the radio.
If you don't know where to find it, tough luck...

These modifications are given as an example of a modification.
Your rig may have different components, their tolerances may be somewhat different and so on.
Please do not blame me if this doesn't work in your radio.

TMN1 RX VCO modification

TMN1's RX VCO modified to 23 cm the stub length is about 8 mm.
The capasitor connected to the stub is changed to 1 pF (later I found out that 1.5 pF gives more stable oscillation)

On the next three pictures you'll see what happened.
The oscilloscope is showing the Lock Detect signal (pin 7) from the MB87006.

After the modification there is a perfect PLL lock at 1.241000 GHz

And perfect lock at 1.299000 GHz

The lock range is about 1230-1315 MHz in the room temperature.

TMN1 TX VCO modification

As instructed by Ramesh OH3MWY the modification of the TX VCO goes like this:

First change the following chip capacitors to these values.

(1) C662 : 1.5
(2) C670 : 1.5
(3) C680 : 2.2
(4) C651 : 4.7
(5) C656 : 1.5
(6) C655 : 3.9
(7) C687 : remove

Then I removed R682  - below (3) - and shotrtened the coaxial stub to 8 mm.

Then after a few rounds of iteration of capasitors
(8) C652     10+1.8 pF
(9) C653     2.2 pF
(10) C654    1.5 Pf
I got the tuning range to 1237 - 1312 MHz

It looks that the capasitor values are critical, so your mileage may vary.

(c) 2000 Vesa OH3NWQ

PS.  The capasitor (7) is on the other side of the circuit board, byt there's really no need of removing it.

PPS. If you break it, it's your problem - not mine.

See also the TMF1 and HSN2 modification pages

© 2000 OH3NWQ (both the photos and the text)