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These pagers were available in the former POCSAG net by Deutsche Telekom AG and later by e*message. The HF part of the receiver is identical to Skypers. The modification for 439.9875 MHz is thus the same. The shortcome is that the display is much smaller and there is no built-in function for setting time and date autmatically. In addition the date only allows years up to (20)13 starting from (19)94 to be set. This has to be done by hand and is lost on longer absence of battery power.
The original design of the pager is designed for usage on 466.075MHz. Same as for Skypers these can be modified for working in UHF ham band range and the German central frequency 439.9875MHz. Basic modification is to exchange the crystal of the Lo and removing the front end filter. It may be required to add two extra capacitors to make the LO work (not needed in my case however). After modification the pager has to be re-aligned with a signal generator.
Like the modification for the Skyper pagers it is necessary to remove the SAW filter which is desgined for 460MHz range. On this PCB you have to take care about the adjacent SMD parts at the outer pins of the SAW. Remove solder with a pump or solder wick on the in- and output. The middle two pins are GND and should be heatede at once to remove the filter afterwards.
When the filter is removed put in a little piece of silver wire to reconnect in- and output. A good source is the shortened ands of the crystal which is also exchanged. Of course this is just an easy hack and obviously reduces sensitivity. When re-adding the piece of wire be sure that it is not shortened to GND. It is recommended to add some heat shring tube to isolate.
Next is to exchange the crystal to get the LO to the correct frequency. You will need a crystal with 69.764583MHz to get the Primo working on the (German) centralized DAPNET frequency of 439.9875 MHz. To remove the old crystal you can remove solder with a desoldering pump.
The new crystal (from funk24.net for example) should be soldered to the two pads.
To improve the LO you can add two capacitors. In my case I tried the pager without those and it worked out of the box. So this is skipped here for now.
After the modification the pager needs to be realigned. First step is to realign the LO with the new crystal. You need a transmitter on the new working frequency (439.9875 MHz) transmitting a 600Hz tone to trigger the Skyper. An easy way is to use the test function of UniPager (connected to a RasPager board).
Different to Skypers you cannot power the pager from battery because this would need the case to be closed. So you may power the pager my an external 1.5V power supply to the pads on the PCB that are normally connected to the battery compartment.
This is an overview of pins and trimmers used for alignment:
Then you have to dig onto pin 16 of the receiver IC with a scope. You should see a more or less symmetrical signal. The goal is to align it to be symmetrical calibrating it with trimmer CT2. It should look like this when finished:
When this is finished you can improve reception by tuning the antenna circuit to maximum signal strength. Therefore conncect your scope to pin 12 and tune to maximum signal by turning trimmer CT1 carefully.
If all is done your Promo should receive messages from DAPNET network. You can test that using the „Send Message“ function of UniPager which circumvents the DAPNET connection and sends out POCSAG calls directly.
The Primo pagers obviously share some similarities with Skypers. There is a connector for external programming which probably is the same as found on Skypers. This needs further investigation.