Hello, recently I was playing with some cheap video
transmitters I had around to spot the working ones.
I needed a fast way to see if there was RF out of the TX,
and to check the channel frequency, so I remembered that
I had this:
it's a radioshack 22-306, a friend of mine gave it to me
because it was not workig, the reason is easy to spot,
it sat unused for long and the batteries made a mess inside:
fortunately the corrosion was not that bad:
here is the front side of the pcb, there is the micro, that
is marked TANDY TCR8001, I havent found any info about
it, probably it's an asic but if anyone knows more it welcome.
There is the main cristal that feeds the micro and also is used
as timebase, the trimmer capacitor is used to trim the frequency.
On the top left of the image there is the prescaler, an MB506 from
fujitsu.
This is the other side of the pcb, nothing interesting here, only the
LCD driver IC:
The unit was repaired before, here is a mod made to the backlight
on the side of the LCD:
Seem that the 50 Ohm input was overloaded, the termination resistors
were badly burned:
The corrosion is starting to eat under the LCD driver:
I desoldered the switches and the lcd driver, after washed the
pcb with water and degreaser, and with TCE to eliminate glue
and flux residues. The backlight is two leds in series connected
to 5V, so I modded the connection to have the leds connected
in parallel and mounted white smd ones.
Soldered everything back together and it is working again,
fortunately the problem was only battery leackage.
First try with a RC transmitter, FM 35,2 MHz nominal:
This is the result with a video transmitter:
Now some accuracy test, feeding with Rb oscillator:
The Rb ampitude is too low to test counter's low
frequency range, so I used the 1992's OCXO output,
the unit seem still under original specs of 1ppm:
I touched a little the trim cap to read the correct frequency,
naturally the quartz used is a normal unit that is
very temperature sensitive, so the last digits will float
around with small temperature variations:
The counter is useful for spotting RF on the field, its
reasonably sensitive, is able to show up to 1.3GHz
maximum, it would be very useful it it was able to go to 2.5Ghz.
The prescaled datasheet tells that the MB506 is able
to go up to 2.4GHz and prescaler's pin 2 is tied to Vcc,
so it should be possible to double the dividing factor.
Another option would be to insert an external x10 divider
on MB506 output to make easier spotting the frequency.
I tried to check the crystal temperature stability:
heated it with the hot air gun set to 100°C while reading
the 10Mz reference. The reading at first raised up
from 10,000,000 to 10,000,037 and then started
going down rapidly under 9,999,980.
So the temperature dependency is not bad.
I didnt measure precisely the temp but it seem that
somewhere in 30 to 80°C interval there is a flat tempco
zone, so it would be possible to try to stabilize the
quartz temperature around 50-60°C. But this will
kill the batteries, so I dont know if it will be useful
Fabio.
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