Virtual Downconversion for the 64/2041 
(Or is it image reception? Who knows.) 
A fellow PRO-64 user has discovered a modification for the PRO-64 which allows reception of bands normally used for cell phones. The mod is interesting, and is presented here for educational purposes. Please check with local authorities for rules before listening to any transmissions. 
The text below was taken from a post on rec.radio.scanner. I do not know the originator of this mod. 
There are two mods which follow, "temporary" and "switchable". I have not tried either version, but a reliable friend has tried the temporary and it DOES work. Please note, you can have EITHER cellular or the 800 band, but not both at once. 
The description is for modifying a PRO-64. The PRO-2041 is an electronically identical base unit. Modification requires cutting and soldering on the 2041's circuit board. If you're up to that, there is a drawing at the bottom of this page that shows where to cut and solder. 
There are also a couple of pictures of the pertinent innards of a PRO-64. 
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Quotes from the modification donor follow: 
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Note: The disadvantage of this mod is that it will pick up only the output frequencies of the CELL "A" (non-wireline) service, due to the limitations of this scanner. 
TEMPORARY MOD: 
Disassemble the PRO-64, and look at the circuit board on the BACK of the unit. You will see a multi-pin connector(CN101). It has 7 pins, with colors BRN(1), Red, Orange,Yellow,Black,Blue and Violet(7). Pull the connector out from its socket. 
Use a small screwdriver or needle to release the RED and BLACK pins from their socket, and interchange them (place the red pin where the black pin was, and vica versa). 
Place the 7 pin plug back into its socket. Reassemble the unit. 
Scan between 48 and 54 Mhz. (I also found some at 29-30.) 
You can calculate the actual cell frequency as follows (using the 48-54 MHz segment): 
Actual cell frequency=772.5 + (2 X [Display frequency]) 
To return the scanner to normal operation, you must reverse the above procedure. 
SWITCHABLE MOD: 
The switch is a submini DPDT switch. I was able to install it just below and to the right of "MAX" of the squelch control. I removed the upper screw from the rear circuit board, drilled a hole in the case, and mounted the switch. It JUST fits! 
As to the wiring: 
Here are the terminals of the DPDT switch. 
1 2 3 
4 5 6 
1) connect 1&6 together 
2) connect 3&4 together 
3) cut the RED and BLACK wires going to the connector we discussed earlier. 
4) connect the RED wire of the connector to 2 
5) connect the BLACK wire of the connector to 5 
6) connect the other RED wire (from the lower circuit board) to 1 
7) connect the other BLACK wire (from the lower circuit board) to 4 
As you can see, placing the switch in one of the positions electrically connects RED to RED and BLACK to BLACK. Placing the switch in the other position connects RED to BLACK and BLACK to RED. 
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More notes from the anonymous donor: 
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As far as the "CELL A", there are two systems, Cell A and Cell B. One is "A" non-wireline (Cellular ONE) and one is "B" wireline (ALLTEL). You will have two competing cell carriers in your market as well. Cell "A" outputs are between 869.04-879.0 or so, Cell "B" between 880.4-890. 890-894 are extended frequencies, the lower half for Cell A, the upper for Cell B. 
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VD is a scanner term for "Virtual Downconverter". There have been numerous write-ups in Monitoring Times on this technology, and there was a company that called themselves "The Cellular Security Group" that advertised this method. They have since gone out (driven out) of business. Since 1994, many scanners have the actual cell frequencies blocked from their EEPROM, and it is impossible to tune those frequencies on those scanners. Cell images were everywhere, though, and you could tune cell images to listen to cell conversations. I used to look for those frequencies by doing the following calculation: 
Desired Cell frequency + (2 X 1st IF frequency) = Display frequency 
or 
Desired Cell frequency - (2 X 1st IF frequency) = Display frequency 
one of the two would usually work. For example, some dual conversion scanners have an IF of 10.7: 
870 Mhz + (2 X 10.7) = display frequency 
870 Mhz + 21.4 = 891.4 Mhz 
894 Mhz + 21.4 = 915.4 Mhz 
So, to hear cell, you would hear the image from 891.4 to 915.4 Mhz, and you would scan those ranges! Since the scanners are blocked up to 894, you would miss the lower few MHz of the cell band. Virtual downconversion "fools" the radio by swapping frequency bands. For example, one scanner I have will receive cell from 157.435-161.600 MHz. It is somewhat complicated, but it has to do with a VCO (variable carrier oscillator) and different frequencies that are used to "beat" against that VCO signal. 
The way the mod on the PRO-64 works (I think) is that 
1) cell images are heard in the VHF-Lo (48-54 Mhz) band. 
2)The PRO 64 has bandpass filters for each frequency band, VHF-Lo/Air/VHF-Hi/UHF/800. 
3)Bandpass filters, when activated, will selectively pass/amplify only the band they are intended to pass. 
The mod ACTIVATES the 800 Mhz Bandpass filter when one is seeing VHF-Lo on the display. This selectively passes/amplifies the 800 Mhz images. Since the VHF-Lo bandpass filter is off in this situation, one hears ONLY the 800 Mhz images. 
In the purest sense, I do NOT believe that this a "TRUE" virtual downconverter circuit. The effect is the same, though. 
BTW, I think this mod blindsided Radio Shack. I really don't think they or the "powers that be" had a clue that this could be easily done! 
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Pictures of the Innards of the 64 and 2041 
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Inside the PRO-64: CN101 is prominent 
A switch installled; see the wires spliced to CN101  
Drawing of PRO-2041 board