Blown front end or just not sensitive? BC780XLT

[AI6VX]

Received a pair of used 780XLTs today. Was looking for a desktop scanner to have at work to complement the BCD325P2 portable I bought a few weeks ago. Local services are all still analog.

Powered them up, plugged in a few local PD frequencies and find their sensitivity is poor. Even just searching the WX channels, the BCD325P2 and even the Baofeng UV5R have no issues giving a good signal but both these 780XLTs have a very weak signal in comparison with the same portable antenna (pretty much all static)

Another weird problem - BCD325P2 has a very clear audio of the local PD on 155.7450Mhz but the 780xlt might unsquelch but no audio is heard.

Could the front end of these 780xlt's be blown or is the sensitivity not good?
(I don't remember my old Pro-2004/BC200xlt/pro-2026 having these issues)

 

[Ubbe]

780's have a varicap tracking filter for the VHF band and protection diods at the antenna input and all band filters sits between antenna and the first amplifer.

If the scanners came from the same user he might have transmitted high power from his amateur rig too close to the 780's antennas. If that is the case it probably are only the protection diods that have blown and now semi shortcircuit the antenna signal. There are to much protection and filters that will make it hard to blow any amplifier transistors.

Do you notice any changes when you use the attenuate?

There are diagram and schematics in the web. I know there are in the 780 yahoogroups file sections.

/Ubbe

 

[a29zuk]

I agree with Ubbe. The 780 was one of the most sensitive scanners I ever owned. I have also read about the protection diodes in the past but never had any trouble with mine.

Good Luck
Jim

 

[Ubbe]

Another weird problem - BCD325P2 has a very clear audio of the local PD on 155.7450Mhz but the 780xlt might unsquelch but no audio is heard.

Try that PD frequency again but in different modes, WFM FM, NFM and AM. It almost sounds as if it receives in AM. If you open the squelch and listen to an empty frequency, do you hear loud clean noise in both WFM, FM and NFM?

/Ubbe

 

[AI6VX]

I agree with Ubbe. The 780 was one of the most sensitive scanners I ever owned. I have also read about the protection diodes in the past but never had any trouble with mine.

Good Luck
Jim

I did find this today when I was trying to see if what I'm experiencing is normal or not. I can't find one for the BCD325PD.
https://forums.radioreference.com/g...2-sensitivity-shootout-20-various-radios.html

Brought the scanner to work today and it seemed to be sensitive enough for what I wanted to hear - local airport and university. University has an analog trunked system which I don't think I can follow but a few frequencies I plugged in work.

Try that PD frequency again but in different modes, WFM FM, NFM and AM. It almost sounds as if it receives in AM. If you open the squelch and listen to an empty frequency, do you hear loud clean noise in both WFM, FM and NFM?

/Ubbe

No difference when changing modes. If I leave the squelch open, the hiss does get a little quieter when the PD is transmitting, but still nothing intelligible is heard on the 780xlt vs clear audio on the BCD325P2.

 

[AI6VX]

Here's some videos.

That police frequency
https://youtu.be/E6vyRvpvWeA

Weather channel
https://youtu.be/C2njWtQfKOk

Antennas swapped
https://youtu.be/9cE4-Sl2VM4

 

[Ubbe]

That signal strenght indicator always show full signal? Then it's not a front end problem.

To me it looks as if the demodulator are set to WFM when receiving FM signals. Then you get a lot of noise with the signal and very weak audio level. If the setting is FM with the weather channels then it must be a problem with the proper selection of the IF filters.

Enter that weather frequency manually and check that the display indicate FM. Change to NFM mode. You need to program that frequency to a memory channel to be able to change mode.

And that second 780 have the same problem? Have you tried a full reset, hold in 2-9-man buttons while powering on?

/Ubbe

 

[AI6VX]

That signal strenght indicator always show full signal? Then it's not a front end problem.

To me it looks as if the demodulator are set to WFM when receiving FM signals. Then you get a lot of noise with the signal and very weak audio level. If the setting is FM with the weather channels then it must be a problem with the proper selection of the IF filters.

Enter that weather frequency manually and check that the display indicate FM. Change to NFM mode. You need to program that frequency to a memory channel to be able to change mode.

And that second 780 have the same problem? Have you tried a full reset, hold in 2-9-man buttons while powering on?

/Ubbe

Full reset done when I first got them.

I'll try a different mode again but I think I already did without any difference for that frequency. There are some other PD freqs that come in and sound okay but they're 'hiss-ier' than the BCD325p2. Changing the modes on those channels does change the audio quality so I think the mode switching is fine.

Yes, the S meter is always fully on whenever I'm stopped on a channel regardless if the squelch is open or not. I figured it was a worthless S meter and not really indicative of received signal strength. Not true?

When both scanners were here they both seemed to work/not work the same. I'll be taking the other one home today and can play with them this weekend.

 

[jonwienke]

Yes, the S meter is always fully on whenever I'm stopped on a channel regardless if the squelch is open or not. I figured it was a worthless S meter and not really indicative of received signal strength. Not true?

Something is wrong with that. It should reflect actual signal strength, not be on/off Baofeng style. And even a Baofeng only shows a signal indicator when squelch is open.

 

[AI6VX]

Something is wrong with that. It should reflect actual signal strength, not be on/off Baofeng style. And even a Baofeng only shows a signal indicator when squelch is open.

I'm at work now and playing with the other 780XLT. The S meter on that works similarly as the first unit - when in manual mode the S meter is full scale regardless of whether there is any signal or not. However I did see that it did change value when on a valid signal and I changed the mode from FM to WFM (it reduced to 3-4 bars) and seemed to track signal strength.

This unit I can get the WX channel but I also am in a very different location and likely it's much stronger here vs at home.

edit: regarding the S Meter, looks like what I'm seeing is normal.
https://forums.radioreference.com/u...780xlt-signal-strength-meter-explanation.html

 

[AI6VX]

Have both units at the house now - they act the same. Sensitivity isn't so great, WX reception is very bad vs great on the BCD325p2, same with the local PD on 155.7450. S meter working the same too.

At work it seems better though it's a completely different county so I cannot compare freqs 1:1. Wonder if I have RF noise in the house causing grief to the 780xlt.

FWIW, I opened up one of the units and found that Uniden potted the input preselector parts with a hard black epoxy. Similarly on a group of components near the center of the board.

 

[jonwienke]

RF interference could be overloading the receiver and causing the signal strength meter to max out. but it would have to be something pretty spectacular like a pager tower next door. When you're at work do the scanners still have the signal strength meters maxed out?

 

[Ubbe]

Is this all happening on VHF and you haven't tried UHF? What about receiving FM broadcast? That use seperate IF filters, discriminator and signal strenght circuits only dedicated to broacast FM.

/Ubbe

 

[AI6VX]

Is this all happening on VHF and you haven't tried UHF? What about receiving FM broadcast? That use seperate IF filters, discriminator and signal strenght circuits only dedicated to broacast FM.

/Ubbe

UHF for the local college near work came in okay.
VHF AM from the local airport came in okay.
VHF FM near that frequency for other PD's work okay.
VHF FM for a weather channel near work is okay.

It's just that one freq and generally less sensitive than the BCD325pd (probably my pro-2004 too - haven't dug that out yet)

Just tried an fm broadcast station - local stations are fine.

 

[sparklehorse]

UHF for the local college near work came in okay.
VHF AM from the local airport came in okay.
VHF FM near that frequency for other PD's work okay.
VHF FM for a weather channel near work is okay.

It's just that one freq and generally less sensitive than the BCD325pd (probably my pro-2004 too - haven't dug that out yet)

Just tried an fm broadcast station - local stations are fine.

So it's only 155.475 that has the problem? And it's the same problem on both 780's? And it only occurs at your house? On other frequencies both 780's perform as you'd expect, including the S-meter? If all that is true I'd say there is something in or near your house that is generating interference on 155.475. Maybe your 325 uses a different intermediate frequency. That could explain why it's better behaved on that one frequency. The BC780XLT was one of the best analog scanners ever made. Even today, a properly working 780 will hold its own against all modern scanners on analog signals in terms of sensitivity and selectivity. In fact it likely has a better front end than the 325, so it would be surprising to see a 780 overloaded by something that is not overloading a 325. But again, a different IF might explain that.

.

 

[Ubbe]

I just now come to think about the adjustment pots in the 780. There are potentiometers that can be adjusted to tune the signalstrengt meter, audio levels, frequencies and sensitivity on the 108-137 and 137-174MHz ranges.

Maybe some of the pots have gone bad, due to a smoking person or some other bad enviroment.
The pot to adjust for max sensitivity in the 137-174MHz range are the RT-9.
Try to locate that on the circuit board and turn it back and forth a couple of times and set it back to its original position.

If you listen to a weak weather channel you can tweak that RT-9 pot and listen to what is happening to the reception, it should change, and then tune to max reception.

If that worked then try to adjust the signal meter. RT-3 are for NFM channels and RT-4 for FM and set those without any antenna connected to a 1 bar indication, or with antenna and to full signal if you have a very strong transmitter nearby you can use as a reference.

/Ubbe

 

[AI6VX]

Almost decided to send in the 780XLT to Uniden for repair ($69.95) but was afraid it would have been returned with a note of 'no trouble found'.

Ended up buying the BCT15X. Wow! Back to the sensitivity I've been used to with my other scanners.