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)
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'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.
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 OH3NWQ
PS. The capasitor (7) is on the other side of the circuit board, but there's really no need to remove it.
PPS. If you break it, it's your problem - not mine.
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© 2000 OH3NWQ (both the photos and the text)
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