Sunday 10th June, 2012 22:30


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Motorola Spectra (Range 1) UHF VCO
Modification for operation to 440 Mhz
The following schematic diagram details the simple mod
Disclaimer



This modification is offered as a all care, no responsibility mod. People wishing to modify their Spectra do so at their own risk.

This modification is not complex, however it does require a certain level of technical competence.   If you are not experienced in repairing or servicing complex radio equipment, then you will need to enlist the assistance of someone who is, in order to perform this upgrade.

The Motorola Spectra uses extremely small surface mounted components, some of which will be prone to damage in high static fields.   It is important to follow safe ESD practices and ensure that you are working in a fully static safe environment.   It is also difficult to access some of the solder points and unless you are confident in your soldering ability, I would recommend that you do not attempt this on your own.

Whilst these modifications have been performed successfully without incident,  I cannot be held responsible if you choose to go ahead with the modification and it does not work out for you, or you damage your radio in the process.    This information is provided as-is.   If you choose to use this information, then the risk is entirely yours.


Good luck, hope to hear you on APCO P25 soon.

73ís and regards

Ben
VK3KBC






Update  (15/6/2012)


I've been advised by different sources that the ex-South Australian Police Motorola Spectra's have a number of different variants of the VCO Printed Circuit Boards and that the components may vary in value and the board layout may be different in appearance.  

The method described above should still be applicable to the different versions of VCO Board however you may need to experiment with the resistor value to compensate if your board differs from the one I had access to.

If you find the values for your board are different and want the information posted here, email me with the details and I'll update this page.

I have received confirmation that the modification works on 'A' Series boards.




P.S.  In accordance with the A.C.M.A. Regulations, when operating APCO P25 on the Ham Bands, please ensure that your APCO P25 Unit ID is set to your Amateur Radio License Number.   By complying, you are properly identifying yourself when operating in APCO P25 digital mode in Australia.
VCO Steering Voltage Modification


The Motorola Spectra VCO is controlled via a negative fixed rail DC voltage along with a positive rail voltage which varies with the frequency of operation.   Both of these lines exhibits a high impedance of over 1 Megaohm.

Further investigations revealed that the negative VCO control rail varied from between -2.5V TO -4.5V on different radios and this caused some radios VCO's to drop out earlier than others.   Ensuring that the negative VCO control line remains below -4V will permit the VCO to operate to at least 440 Mhz.

In order to modify the Spectra to operate to 440 Mhz,  there is no requirement to remove or modify or trim the VCO tuning stub as suggested on the Net in some mods posted for the 800 Mhz versions of the Spectra.

All that is required is the addition of one 560 KOhm 1/4 Watt (or smaller) resistor to Pins 4 & 6 on the header of the VCO PCB!   The new additional resistor is encapsulated in a short length of heatshrink tubing in order to insulate it from the surrounding circuitry.


The following pictures show the location of the modification.
Background


A large number of Motorola Spectra UHF transceivers have recently become available to Australian Radio Amateurs after the South Australian Police Department disposed of their fleet via Public Auction.

Most of these radios were fully functional post decomissioning and only needed to be reprogrammed for Amateur use.   The Spectra radios are capable of both Analog and APCO P25 operation on the 70cm band, making them ideal for digital experimentation.  

In the United States, many hams consider APCO P25 radios a digital mode alternative to DSTAR and following in the same trend, a number of P25 repeaters are now operational in VK also.

Whilst most ex-SA Police Range-1 (403-433 Mhz) radios operate perfectly to 440 Mhz, some radio VCO's unfortunately cut out at around 435 to 437 Mhz, making them useless for Amateur Repeater operation on the 70cm band in VK.

After hearing that a few amateurs had attempted different methods to extend the VCO range with varying success, a group of local amateurs including VK3CSS, VK3DPM and VK3KBC examined the radios and found that a simple modification to the VCO steering lines would remedy the problem, thereby allowing any radios with limited VCO range to be quickly and easily modified to perform satisfactorily to 440 Mhz +.

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