BC125AT: BC125 off frequency

[Falcon9h]

Just got a BC125 (previous owner) and it works great, except for one thing.
Transmissions sound raspy, like off frequency raspy.
I found a PCB layout on (I think) radioscanner.ru and I see one trimmer next to the only crystal in the radio. I assume it could be tweaked.
Question is, has anyone here done this? Accessible from the back side without having to remove boards, etc.?
I'd like to get some input here before I bother. I have no qualms at all about opening the radio since I was a tech, good at the small stuff.

Thanks-would love some feedback!

 

[Bob132]

Are you sure the speaker isn't shot?

 

[Falcon9h]

Are you sure the speaker isn't shot?

No-definitely not.

 

[Bob132]

If it's an older model might rebanding have affected it? Check and see if you can select AM/FM/NFM and try toggleing between them, unless you're listening to citizens band or aircraft just toggle between FM/NFM and see if that makes a difference. You might also be able to change the spacing of the bands between the frequencies. You should be able to download the user manual at my.uniden .com if you don't already have one. Good luck

 

[Falcon9h]

If it's an older model might rebanding have affected it? Check and see if you can select AM/FM/NFM and try toggleing between them, unless you're listening to citizens band or aircraft just toggle between FM/NFM and see if that makes a difference. You might also be able to change the spacing of the bands. Good luck.

It's practically brand new with the latest firmware so that's ruled out. No, I know off frequency when I hear it, and this is. Other scanners I've had I just went in and tweaked the reference oscillator next to the crystal and it would sound good.
What I really wanted to know is if anyone has actually disassembled the radio and could tell if the trimmer is accessible.

 

[Bob132]

It's practically brand new with the latest firmware so that's ruled out. No, I know off frequency when I hear it, and this is. Other scanners I've had I just went in and tweaked the reference oscillator next to the crystal and it would sound good.
What I really wanted to know is if anyone has actually disassembled the radio and could tell if the trimmer is accessible.

Sorry, hopefully someone more knowledgeable than me will chime in soon.

 

[Scan125]

No-definitely not.

Have you plugged headphones or high impedance external speaker in to check the audio etc.?

 

[tvengr]

If the windings of the speaker voice coil are rubbing on the magnet, it can give you distorted audio similar to being off frequency. I would check the audio with an earpiece or headphones. I am not familiar with the circuitry of the BC125AT. If you are unable to eliminate the distortion by warping the frequency of the reference oscillator by adjusting the trimmer, it is possible that the crystal is too far off frequency to be corrected. It is also possible that there could be a second or third oscillator crystal off frequency or a crystal filter. Crystals normally decrease in frequency while aging.

 

[G7RUX]

Just got a BC125 (previous owner) and it works great, except for one thing.
Transmissions sound raspy, like off frequency raspy.
I found a PCB layout on (I think) radioscanner.ru and I see one trimmer next to the only crystal in the radio. I assume it could be tweaked.
Question is, has anyone here done this? Accessible from the back side without having to remove boards, etc.?
I'd like to get some input here before I bother. I have no qualms at all about opening the radio since I was a tech, good at the small stuff.

Thanks-would love some feedback!

The trimmer is not easily accessible and requires significant disassembly to get at it; you will then need to power it and trim the frequency against a reference which is not easy with the unit disassembled.

I would say it can be done (but I'm an RF engineer with all the necessary test kit and experience) but just "tweaking it" to sound better might not be the best approach since you don't really know just what is misaligned, if anything.

 

[Falcon9h]

The trimmer is not easily accessible and requires significant disassembly to get at it; you will then need to power it and trim the frequency against a reference which is not easy with the unit disassembled.

I would say it can be done (but I'm an RF engineer with all the necessary test kit and experience) but just "tweaking it" to sound better might not be the best approach since you don't really know just what is misaligned, if anything.

Ah. That's the answer I was looking for-in the two way job I had they called me the microsurgeon-I'm good with small stuff, but I can see juggling circuit boards with flex ribbons and having something tear or short together.
I've done it before to various scanners and receivers-I just tuned it to a weak NOAA station and tweaked the trimmer = win. But I'm gonna pass on this one, and I use it mostly for air band, where it works great.

Thanks all!

 

[Scan125]

But I'm gonna pass on this one, and I use it mostly for air band, where it works great.
Thanks all!

Probably a good idea to skip. Assuming it is not the speaker (headphones/external feed test) then out of curiosity you could see if you can find someone local to you with a calibrated RF signal generator to see how fat off your scanner is in your critical use bands.

 

[tvengr]

I just tuned it to a weak NOAA station and tweaked the trimmer

It's better to use an 800 MHz frequency for the adjustment. Frequency error increases at higher frequencies. Also, weather frequencies are wideband (FM), making the frequency even more critical with 800 MHz trunked systems using a narrow bandwidth (NFM). If you are off 2 KHz with the wideband WX frequencies (which would not be noticeable), you would be off more than 10 KHz with the 800 MHz narrowband frequencies which would be very distorted.

 

[G7RUX]

Ah. That's the answer I was looking for-in the two way job I had they called me the microsurgeon-I'm good with small stuff, but I can see juggling circuit boards with flex ribbons and having something tear or short together.
I've done it before to various scanners and receivers-I just tuned it to a weak NOAA station and tweaked the trimmer = win. But I'm gonna pass on this one, and I use it mostly for air band, where it works great.

Thanks all!

That’s an interesting point; if it works fine on AM then I would suggest it’s probably not off-frequency so I would suggest looking elsewhere for the distortion. Personally I would be looking at whether there’s a problem with the discriminator or the audio amplifier, or perhaps the receiver is being overloaded by something nearby (FM uses very high IF gain to drive the discriminator into limiting, removing any amplitude modulation/variation components before the FM demod takes place.)

 

[tvengr]

It's better to use an 800 MHz frequency for the adjustment.

My error! The BC125AT only covers up to 512 MHz. A 2 KHz error on the weather frequencies would still be off a little less than 6 KHz on the narrowband (NFM) UHF frequencies which could still cause distortion. It would be better to adjust the frequency using a UHF frequency.

 

[Ubbe]

If the frequency are off with a Frequency Modulated signal it sounds much worse than a Amplitude Modulated signal, where it's hardly noticeable. The scanner has only FM and not NFM so not possible to switch between them to try and find out if it gets worse in NFM, I.E. master oscillator are off in freq.

/Ubbe

 

[Falcon9h]

If the frequency are off with a Frequency Modulated signal it sounds much worse than a Amplitude Modulated signal, where it's hardly noticeable. The scanner has only FM and not NFM so not possible to switch between them to try and find out if it gets worse in NFM, I.E. master oscillator are off in freq.

/Ubbe

Sure it does-it's one of the selectable modes and about the rest, you hit it. I'm still gonna go with master oscillator because what you describe is right on. Trouble is-insanity-the frequency steps are 5 khz. If it was 2.5 I could check easier. So I'm not gonna worry about this one now.

 

[Ubbe]

I though the BC125AT were the same as UBC125XLT that only has FM mode, but BC125 also has NFM. If you choose FM and listen to NOAA with open squelch you should be able to tell if the plus or minus 5KHz setting sounds clearer.

The UBC version seems to be a simpler one and there's no trim what I can see in the schematic for the 12,8MHz reference oscillator that sits in the big RF can. But the UBC has two IF filters and then has IFX and a 800MHz band that the BC do not. So its not only the low-VHF band that usually differs between US and EU models.

The 12,8MHz reference crystal are specified to drift max 1ppm per year. That's 500Hz at 500MHz each year.

/Ubbe

 

[G7RUX]

That said, if you’re near a suitable VHF or UHF beacon then you can check the tuning without opening it up. If you’re stuck and you’re in the London/SE area then I’m happy to pop it on the bench and check the performance.

 

[G7RUX]

If the frequency are off with a Frequency Modulated signal it sounds much worse than a Amplitude Modulated signal, where it's hardly noticeable. The scanner has only FM and not NFM so not possible to switch between them to try and find out if it gets worse in NFM, I.E. master oscillator are off in freq.

/Ubbe

It would have to be grossly off-frequency or the receiver isn’t fully quietening; FM detector capture would hide much of it. Still, I’m happy to pop it on the bench if the OP is interested.

 

[Ubbe]

It would have to be grossly off-frequency or the receiver isn’t fully quietening

It depends of the level of modulation. If it's only a 1Khz deviation signal at a FM wide filter then you can be 3KHz off without any problem. But having a 5KHz full deviation from a datasignal only takes 1 or 1,5Khz off in frequency if its not fully quieting to hear a distorsion.

When I test receivers I overmodulate 25% and then it's easy to first check the last IF stage to see that it is tuned properly and then check the other IF's and the RF receive frequency as those oscillators often comes from the same reference oscillator.

I wouldn't think that FM capture effect has anything to do with it when it sounds raspy and off frequency.

/Ubbe