Uniden bc xlt mod ?

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SCPD

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Looks like someone modified this section of my BC8500XLT. Can anyone identify this mod ? Def not factory.uploadfromtaptalk1406520731286.jpguploadfromtaptalk1406520747930.jpg

From my understanding this scanner is very similar to the 9500xlt and 895xlt and radio shacks pro-XXXX(x= fill in blank).

While I'm at it, does anyone know if I can add a cable to the board to decode APCO-25 via PC with this scanner, or add a rs232 for programming ?
 

gewecke

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Good question ... Way too much hardware for a cell mod, maybe additional frequency steps judging by the cap and resistors ... but not the right IC either.

73,
n9zas
 

Boatanchor

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Simple audio filter

Looks like someone modified this section of my BC8500XLT. Can anyone identify this mod ? Def not factory.View attachment 46799View attachment 46800

From my understanding this scanner is very similar to the 9500xlt and 895xlt and radio shacks pro-XXXX(x= fill in blank).

While I'm at it, does anyone know if I can add a cable to the board to decode APCO-25 via PC with this scanner, or add a rs232 for programming ?
The MC3361 IC is the FM discriminator (demodulator) chip. It has nothing to do with frequencies covered (i.e cell band coverage etc).

Looking at the component placement, it appears that a previous owner has added some rudimentary audio filtering between pins 9 (recovered audio output) and ground (pin 15).

I would hazard a guess that this simple RC network was installed to shunt high audio frequencies to ground. It is basically a very crude low pass filter. It was probably done to reduce high frequencies / hiss from FM audio.

I wouldn't worry too much about the mod. The good thing about the 'old school', through board components is that they are easy to work on, unlike the tiny SMD components that are commonplace nowadays.

If you want to see what effect the filter has, simply unsolder the capacitor from pin 15 and pull it away from the IC pin slightly to break the contact (but leave the capacitor it in place). Then while receiving an FM signal, carefully push the capacitor leg down so it makes contact again. You may hear a slight difference in audio tone through the speaker.
 

Boatanchor

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It should be relatively simple to add a discriminator output to this scanner for external P25/DMR decoding :)
 

Mike_G_D

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Boatanchor

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As far as I can tell, it's modifying the discriminator IC (MC3361 - really common) noise filter that dictates the proper squelch action. But I'm not sure where the one right most resistor is going.

Stuff that might help you work it out:

MC3351 data sheet: http://www.discriminator.nl/ic/mc3361.pdf.

Squelch description in Wikipedia: Squelch - Wikipedia, the free encyclopedia.

Resistor color code: Electronic color code - Wikipedia, the free encyclopedia.

-Mike
Thinking about it further, Mike is probably correct.
Since the resistors are fitted directly between the discriminator output and the squelch noise amp input, the mod most likely impacts on squelch operation rather than audio.

If it ain't broke....
 

FeedForward

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The 3361 is actually a complete 10.7 mhz IF in a package, including quadrature discriminator. The blue plastic thing seems to be the IF bandpass filter. Not as compact as the Mu_Rata SMD device in later radios is it? Spec sheet indicates that the 2 IC pins with resistors is a modification of the squelch noise filter. Those circuits can get complex if done like the examples in that sheet. Unless the original circuit was obviously incorrect it would take considerable effort to redesign it, or "make it better". Before these integrated solutions became available, you had to all of the noise filtering and squelch with some discreet transistors and lots of external parts.
 

SCPD

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Thinking about it further, Mike is probably correct.
Since the resistors are fitted directly between the discriminator output and the squelch noise amp input, the mod most likely impacts on squelch operation rather than audio.

If it ain't broke....
What would I notice if I unsolder one leg on the cap and lift it away from the IC ? I have noticed the scanner makes a loud KERCHUNK sound every time someone in keys on a repeater or simplex.

Sent from my SCH-I605
 

FeedForward

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What would I notice if I unsolder one leg on the cap and lift it away from the IC ? I have noticed the scanner makes a loud KERCHUNK sound every time someone in keys on a repeater or simplex.
That supports my theory that the squelch circuit is a bit more complex than the average tinkerer realizes. The fact that you a hear a nasty kerchunk may be the reason someone sold the radio.

If possible, I would try to return the circuit to its original state. Probably some parts were removed, but it should be obvious if you examine the board. If no parts seem missing, just remove the hacks and see if things work a little better.

FF
 

FeedForward

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I don't know if any schematics exist, but if not I'm sure another forum member with the same scanner could take some closeups of the IC. That way, you could see if anything is missing.

FF
 

SCPD

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How hard would it be to get the sensitivity back up to specs on this scanner ?

Sent from my SCH-I605
 

SCPD

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Mod cleans up white noise. When I remove it there is more hiss in transmissions, where as with it intact there is less noise to ground ratio. Pretty cool mod...never heard of it nor is it a listed MOD on mods.dk

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Mike_G_D

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When you write "noise to ground ratio" I assume you mean signal-to-noise ratio? Anyway, as another poster mentioned, there are parts missing from the pcb in the picture. From the picture you can spot three vacant surface mount components. There may be more not shown in the picture. Also, I don't know if they were not placed in the original manufactured unit (due to design changes, different models using same pcb with different features, etc.) or were removed after sale by the owner. Some appear to have some slight dis-colorization possibly from solder flux residue which may indicate the later case. If so, it is possible that a previous owner made some modification that involved removing some components from the discriminator circuit area and then lost the original surface mount components. Then, when deciding to sell the unit that owner replaced what was removed with equivalent through-hole parts in lieu of the original surface mount parts. I can't tell completely from the pictures if this is the case and don't have an original schematic but it might account for the "mod".

If that is the case then you would notice better performance from the "mod" being in place because it's not really a "mod" per-se so much as a replacement of original parts with same value but different form factor parts.

-Mike
 

SCPD

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When you write "noise to ground ratio" I assume you mean signal-to-noise ratio? Anyway, as another poster mentioned, there are parts missing from the pcb in the picture. From the picture you can spot three vacant surface mount components. There may be more not shown in the picture. Also, I don't know if they were not placed in the original manufactured unit (due to design changes, different models using same pcb with different features, etc.) or were removed after sale by the owner. Some appear to have some slight dis-colorization possibly from solder flux residue which may indicate the later case. If so, it is possible that a previous owner made some modification that involved removing some components from the discriminator circuit area and then lost the original surface mount components. Then, when deciding to sell the unit that owner replaced what was removed with equivalent through-hole parts in lieu of the original surface mount parts. I can't tell completely from the pictures if this is the case and don't have an original schematic but it might account for the "mod".

If that is the case then you would notice better performance from the "mod" being in place because it's not really a "mod" per-se so much as a replacement of original parts with same value but different form factor parts.

-Mike
I c I wonder why the parts were removed in the first place ??? So due to the small size parts and or original owner missppacing then older large components were re-added.

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Mike_G_D

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I c I wonder why the parts were removed in the first place ??? So due to the small size parts and or original owner missppacing then older large components were re-added.

Sent from my SCH-I605
It's a possibility. It may have been for a mod involving a specific specialized type of discriminator tap or modification. Some folks monitor signals that require certain types of bandwidth modifications to the discriminator output for better performance. I think LTR and some paging signals decode better with certain RC filters added to the line.

-Mike
 

herseja

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I'd be interested in the values of the two missing smd resistors as well. I picked up the exact same model at a hamfest earlier this summer with exactly the same modification thinking I would be able to source a service manual and get it back to factory spec. So far have only been able to find a user manual and a discriminator mod which doesn’t show an image with enough detail to identify the resistor values. With this mod in place I haven't been able to hear much in VHF bands. I am going to remove the mod and try a few resistor values from the examples in the mc3361 data sheet. If anyone is willing to look inside their unmodded device, the resistors in question are (looking top down, towards the rear of the unit) directly above pins 9 & 11 of the mc3361.

Thx
 

pro92b

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I'd be interested in the values of the two missing smd resistors as well. I picked up the exact same model at a hamfest earlier this summer with exactly the same modification thinking I would be able to source a service manual and get it back to factory spec. So far have only been able to find a user manual and a discriminator mod which doesn’t show an image with enough detail to identify the resistor values. With this mod in place I haven't been able to hear much in VHF bands. I am going to remove the mod and try a few resistor values from the examples in the mc3361 data sheet. If anyone is willing to look inside their unmodded device, the resistors in question are (looking top down, towards the rear of the unit) directly above pins 9 & 11 of the mc3361.

Thx
This seems to be too much of a coincidence that two radios show up with this 'modification'. It's as if it was a factory rework in late production. The modification is not detailed anywhere on the web as far as I know. So how do these unrelated radios show up with this undocumented mod?

Uniden did sell service manuals for the BC8500XLT a long time ago. One sold on Ebay a few months ago so they are around. Even having a service manual may not settle the issue with the modification if it was a late circuit change after the service manual was printed.
 

ratboy

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I wish I had known about this mod when I had my 9500XLT, the hiss it had annoyed the hell out of me. It had major intermod issues too, but the main problem it had was the hissing. It hissed even with the squelch closed. I solved the problem eventually by running an external speaker with an add on audio filter I found in a magazine. It had much cleaner audio and the filter worked great to get rid of the hissing
 

SCPD

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I wish I had known about this mod when I had my 9500XLT, the hiss it had annoyed the hell out of me. It had major intermod issues too, but the main problem it had was the hissing. It hissed even with the squelch closed. I solved the problem eventually by running an external speaker with an add on audio filter I found in a magazine. It had much cleaner audio and the filter worked great to get rid of the hissing
Yea intermod is an issue with the scanner sometimes. Using CTCSS seems to fix that
 
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