BC780XLT: Uniden BC780XLT modifications

[Ubbe]

BC780 are a great scanner but there are a couple of modifications that can be done to it.

Narrow filter for AM mode.

Uniden's specification says:

For AM aircraft the maximum modulation will be between 6KHz and 9KHz wide so it's bit strange that Uniden chose to use a 20KHz wide filter. For CB radio and mil air that use voice I can't see why a wide filter are needed. But it's very easy to instead make AM use the more narrow 12KHz filter, not optimum but still better than 20KHz.
The AM detector module gets powered only when the mode are set to AM for a frequency. Squelch functions are always done from the FM detector as it is more reliable to use for a noise squelch.

Luckily the underside of the circuit board have two solder points that can be used and only needs a diode to be soldered to them. Any general purpose diode can be used, like a 1N914 or 1N4148 as it is only a DC voltage and a 5mA current it needs to handle. The diode takes the AM module voltage to also activate NFM mode but stops it from working in the opposite direction. The more narrow a filter can be the better the reception will be but for AM a too narrow filter will cut off the higher frequencies in the audio and makes it sound more muffled and harder to hear. But 12KHz are still much wider than needed and reception still improves from this simple modification.

/Ubbe

 

[Ubbe]

To improve reception even further the NFM filter can be replaced by a super narrow FM filter.

Replace NFM filter with a SFM.

I wanted to try a really narrow filter +/-3KHz type LTM450HW. On the circuit board there are two light grey plastic cubes that are the FM and NFM filters and the closest one to the front are the NFM that are replaced by a more narrow one.

The old filter are soldered by three points at the front of it and one at the back. Use a solder wick to remove all solder and heat with the solder pen at the solder points while prying slightly at the filter to make it come lose and alternate the solder pen between all solder points while doing this.

The same type of filter from Murata have been obsolete for some time so the new filter I got are totally different in appearance.
There are three pins at one side that all needs to be soldered to grounded and opposite to those are two pins that are in and out of the filter. Impedance are the same as the old filter so no other modifications are needed. I solder the thre grounding pins together and also attach a wire to them and also solder a wire to one of the other pin, that can be seen in the picture.

I solder one of the two in/out pins directly to the left solder point of the three on the circuit board and then the grounding wire to the middle point and the wire from the out pin to the right solder point. The signal level at the filter are very strong so it doesn't need to be shielded wires and are uncritical as the impedance are around 1kohm and low enough to not pick up any stray signals in the scanner.

In my location I have never heard any FM signals clipping while using the NFM setting so for those who want to improve normal FM reception, I guess most analog systems have switched to using NFM modulation, the FM filter can also be unsoldered and removed and the unused NFM filter can instead be soldered to the FM filters position.

By using a different filter the noise level can shift and the squelch could need an adjustment. Program a channel WFM and the next to FM and the next after that to NFM. Disconnect antenna and select the WFM channel and check where the squelch knob open and close squelch. Select the FM channel and adjust the trim pot on the circuit board labeled RT 7 so that the squelch knob close squelch at the same position as in WFM. Then chose the NFM channel and adjust RT 3 to get the squelch to close at the same position as in the other modes.

/Ubbe

 

[Ubbe]

The AM reception in 780 have the audio die away at lower signal strengths, that's usually how AM modulation works. But by increasing the gain the audio can be more like it works in FM, voice level are more constant even at lower signal strengths and noise increase in level. I prefere that as it gets easier to hear what are said at weaker signals.

Improved AM gain at lower signal strength.

There are two amplifier transistors in 780 right after the bandpass filters and the second one can be modified without sacrificing any noticeable RF performance loss in sensitivity or NF noise level or even strong signal handling, it might even be improved. The operation level of the transistor are set at the lowest NF noise which is 1.1dB and at the higher amplification that the modification use it is 1.2dB, so nothing that should be noticed but the higher current thru the transistor probably makes it handle strong signals even better.

At its antenna connector are a metal screening box over the bandpass filter components and below that to the front of the scanner are another screening box and between the two boxes are the amplifier transistors. It's a 820 ohm SMD resistor that needs to have a 100 ohm resistor soldered on top of it. It's probably easier to use another SMD type than using a resistor with legs. My SMD resistor, the 101 one, where of a larger kind so I had to make sure it made contact by solder a blob to a capacitor going to the same point.

In the picture I have removed my screening box over the bandpass filter for another upcoming modification.

When doing this increased amplification mod the audio level at lower signal levels starts to increase somewhere at -90dBm -80dBm and stays constant over this RF level in my scanner and to correct this bump in audio I had to change how the AGC functions operate to make it work more aggressively by solder a jumper over a resistor that normally limits the AGC action. Maybe the bump in the AM audio level isn't noticed in normal use but if you have a thin enough solder tip it will be easy enough to do the mod. I used a blank wire soldered over the 103 resistor.

/Ubbe

 

[nessnet]

Interesting....
Thanks for the 'R&D".....

 

[wtp]

mods.dk - Articles for Uniden 'BC-780XLT'

 

[SigmaDelta]

I inserted these mods some time ago, with very good results. Thanks Ubbe for the clear descriptions.

For those who find the down facing internal speaker a bit "muffled" there is a very easy mod to increase high frequencies only for the internal speaker. The ext speaker output is not effected.

Place a capacitor across R413, this is the large resistor going from internal speaker minus to ground.

Placing 4,7uF (I used MKP/audio type) across R413 gives the internal speaker a nice treble boost.
A very easy 5 seconds mod to try out.

 

[SigmaDelta]

Another very simple and entertaining mod is tapping the 10.7MHz IF for pan adapter analysis.
Note: the 10.7 IF is only used for WFM mode, so inactive for NFM/AM.
You have to do viewing and demodulation on any external RX, receiving 10.7MHz and having a spectrum/waterfall.
The 780 has basically become your variable downconverter.

The upside is: you can simply tap the signal from the first ceramic filter's (FT3) input, no buffer is needed.
I never use the recording audio output, so I made a connection with a capacitor from the first 10.7 ceramic filter (FT3) input to the recording jack output.

If no cable is connected, WFM of the 780 is still perfectly okay. If you connect an external panadapting RX you will put load on and attenuate the 10,7 IF for the 780 demodulator, so work needs to be taken over by the external RX.

I dedicated some memories (programmed in WFM) for interesting frequencies I want to view and demodulate externally.

Here you see MIL SAT data on an external Reuter RX:


This is the simple mod tapping the 10,7 IF to the recording jack:
(leads are so short that without a jack connected, there is no adverse effect on the 10.7 IF strip)