Spectra VCO Modifications

You have come to this page in order to learn how to modify the VCO in your Spectra. The reason you would want to do this? Well, in order to change range of frequencies over which the VCO will stay in lock, thereby changing the operating frequency range of your radio (slightly).

In general, most of the modifications people want to do involve stretching the operating frequency of the radio down from the bottom factory limit. This is good, since it makes the modification easier (you will see why soon).

This procedure is written for Low/Mid power radios. The same theory should be applicable for high power radios, but the pictures may not be accurate.

In order to perform this modification, you will need the following equipment (at least):

The first thing you will want to do is program up your radio with a range of conventional modes that span the current range of the radio. You will want some modes with frequencies at the top end, bottom end, and middle of the current bandsplit. Program receive (RX), transmit (TX), as well as talkaround (TA) frequencies into the codeplug. This is so you can take some measurements before you start changing things. These modes that you program will be referred to as the "Test Modes".

Flip your radio over and open the bottom cover to expose the RF section of the radio:

Take note of what you see (image of a UHF Spectra board). You should see the VCO (covered) on your left, and the main RF board on the right.

Hook up your radio (including dummy load) and power it up.

Make a list on a piece of paper of all your RX/TX/TA frequencies that you programmed into you radio. Locate Pin 3 on P601 (it should be the third from the top if the heatsink of the radio is away from you).

Step through all your Test Modes (RX is easiest to do first, followed by a short TX and then a short TA) and record (on your sheet of paper beside the frequency) the voltage on Pin 3 of P601.

Pin 3 of P602 is the positive Steering Line (SL+), and controls the frequency of the oscillator. We want to establish, before we start, that the VCO and the SL+ are operating over the correct range.

The range of operation (to maintain lock) for the SL+ is as follows:

If the SL+ exceeds this range by too much, the VCO will unlock, and you will get a FAIL 001.

You should now have a table that looks something like this (modification done on a 900MHz Spectra):

RX Vsl TX Vsl TA Vsl
931.0000 0.41 895.0000 1.76 931.0000 0.78
941.0000 4.27 895.0000 1.76 941.0000 4.43
935.0000 0.85 892.0000 0.65 931.0000 0.78
935.0000 1.99 902.0000 4.30 n/a n/a
935.0000 1.99 895.0000 1.76 931.0000 0.78

You will note that some of the voltages are outside of the theoretical limit of 1.5 VDC, and the VCO is still locked. The 1.5VDC is just that, theory, and is a guaranteed voltage at which the VCO will remain locked.

It is now time to get on with modifying the VCO.

Remove the screws holding the VCO and the cover in place. Gently pop the VCO connector up and off. Continue gently pulling up until you feel the coax connector on the underside of the VCO board release. You will now need to gently pull back (towards you) on the VCO board and get the other coax connector (that goes into the PA housing) to release. A pair of small needlenose pliers to hold the coax/connector may help here.

You should now have a VCO board in your hand. Flip it over and you should see something like this:

This is a picture of a UHF VCO board, but others are similar.

Notice the slot cut into the shield, if you look inside, you should see a PC Board trace with some fine cuts burned into it. This is the microstrip line that tunes the VCO's operating range. It is tuned at the factory using a laser to cut the trace until the desired tuning is complete. You will probably notice large cuts off the end and smaller ones the trace progresses. This is caused by the initial coarse tuning cuts by the laser, and smaller ones as the measurements fall into spec.

The way this tuning circuit works is that the longer the microstrip, the lower the operating frequency range of the VCO. This is good for us, because it means that in order to bring the range of the radio down, we just need to make the strip longer again. This is where your silver conductive ink pen comes in.

Put a small dab of conductive ink on the end of the tuning strip to at least cover over the fine tuning cuts from the laser. You have now effectively restored that much length to the tuning microstrip. If you get too much ink in there, you can usually get it off by using a "Q-tip" and some Isopropyl Alcohol.

That's it... sort of. Now the fun begins. Your first challenge is to get the VCO board BACK into the radio. :)

The procedure to re-install the VCO board that seems to work the best goes like:

Fun, isn't it.

Okay, at this point you should have a modified VCO board in your radio, the logic side is closed up again, the RF side is still open, with a couple of screws holding the VCO board in place.

Give yourself a break from the hardware modifications for a while and go hack the bandlimits in your RSS, go here for the page on that fun.

Now, using your newly modified RSS, create a codeplug that has Test Modes that cover the original range of the radio, as well as some test modes that cover the new (slightly lower) range you want to move to.

Program up your radio and get it ready to do some more voltage measurements (don't forget the dummy load).

Using the same procedure we started with, step through all your Test Modes and check and record the voltage of SL+. You should at some point also notice where the VCO unlocks and you get a Fail 001.

You should get something that looks like this:

Frequency RX TA Frequency TX
940.0000 7.40 7.46 910.0000 FAIL 001
939.0000 7.00 7.08 909.0000 FAIL 001
938.0000 6.61 6.70 908.0000 FAIL 001
937.0000 6.22 6.32 907.0000 FAIL 001
936.0000 5.83 5.96 906.0000 FAIL 001
935.0000 5.44 5.59 905.0000 FAIL 001
934.0000 5.06 5.23 904.0000 8.72
933.0000 4.68 4.86 903.0000 8.29
932.0000 4.29 4.50 902.0000 7.89
931.0000 3.90 4.14 901.0000 7.49
930.0000 3.52 3.78 900.0000 7.11
929.0000 3.12 3.41 899.0000 6.73
928.0000 2.73 3.04 898.0000 6.35
927.0000 2.33 2.67 897.0000 5.98
926.0000 1.93 2.30 896.0000 5.61
925.0000 1.53 1.93 895.0000 5.24
924.0000 1.11 1.55 894.0000 4.87
923.0000 0.71 1.17 893.0000 4.50
922.0000 0.30 0.80 892.0000 4.12
921.0000 FAIL 001 0.42

By doing this you can get a better idea of how stable your VCO will be at certain frequencies. Things will change with temperature and other factors, so if you are close to being unlocked at a particular frequency you are interested in, you can tell this from your measurements.

If you radio stay's locked over the range you want, great, button it all back up and you are done. If not, remove the VCO board once again, add/remove some ink, and take some more measurements. Repeat as necessary.

There are a couple caveats in this modification that you should be aware of. By adding some length to the microstrip tuning line, you can quite easily push the frequency range of the radio down a bit. However, this will be at the expense of the upper part of the band. So, if you use frequencies near the top end of the stock band, you risk losing them (VCO will unlock) by pushing the range down.

As well, and this affects 900 MHz radios in particular, while you can nicely push the receive and talkaround ranges down from 931 to 923 (for example), you at the same time are also pushing down the transmit side, and may in fact push it too far to be usable. For instance, you can get the RX/TA side to lock nicely at 915 if you really want, but you TX side may now only lock at 899 (outside the ham band). This is where it becomes a balancing act. It may take some trial and error to find how much ink is enough to do the job you want.

Lastly, just because you moved the VCO range, does not mean that the radio will work all that well where you are trying. The transmitter and PA may not be too happy if you go too far outside the stock specifications. As well, the receiver will start to loose sensitivity as you move outside of the curve of the front end filtering.

That's it. This modification is applicable on both conversions to 900 as well as UHF radios to get them to operate in the ham band. There is no reason why this should not also work on the other bandsplits, should it be required.