Uniden BC780XLT vs. Radio Shack Pro 2038 volume level

[tvengr]

I wonder how long NTE stuff will be around. NTE closed up shop earlier this year.
I have one NTE7000 left and one ST version of the TDA1905.

I don't know what we will do for parts if this trend continues.

 

[kruser]

Yep, its becoming harder and harder to source parts for older electronic devices.
Then you have eBay where you don't know if you can trust a listing for a semiconductor or not.
There's a lot of stories about counterfeit or mislabeled (purposely) chips and transistors coming out of China.

I recently repaired a 780XLT that had a bad audio amp. Caps were all good but I still replaced the same lytics in the audio stage that @pro92b's post mentioned.
The radio had been running non-stop for years but the only output was baseband audio all that time so who knows how long the actual audio chip has been dead.

 

[rhrailfan1972]

Caps changed,minus the 47mf,since did not have one handy. No change in volume. Only external speaker I had was the one that goes to my President CB base station. Long,rectangular speaker. Tried the plugging it in and unplugging it trick. Still no change. But I can say with the speaker plugged in it is loud! Maybe the speaker in the scanner is bad?

 

[pro92b]

No, I think the internal speaker is ok. For reasons only Uniden knows there is an unnecessary resistor in series with the internal speaker. The TDA1905 certainly doesn't need for it to be there and it reduces the audio volume greatly for no good reason. The attached diagram shows the remedy.

 

[rhrailfan1972]

I do think that did the trick. I can tell more about it once it is back in the car and I get trackside.

 

[Ubbe]

For reasons only Uniden knows there is an unnecessary resistor in series with the internal speaker.

It could be to protect the speaker from the power on and power off thump from the speaker amplifier. The speaker can probably not handle that thump spike. As you mention, it is not normal practice to have a 18 ohm resistor in series with a speaker that converts half the power into heat.

That TDA1905 could be too powerful for the internal speaker. My 780 are plenty loud at my desk at a 10 o'clock setting. If it would go twice as loud I would have difficulties to set the correct volume as it becomes much more sensitive. A modification could be to bridge that resistor, maybe using 10 ohm resistor, and lower the signal to the volume controller by inserting a 47k resistor in that wire that goes to the top of the pot. I've never used more than a 1/2 volume setting, but then I haven't used it mobile. If mobile I would use a 4 ohm external speaker that the TDA1905 can drive with 5W, very loud. Internal speakers are usually only 1W and 15 ohm.

/Ubbe

 

[pro92b]

I have never seen a 15 Ohm speaker in a Uniden base/mobile scanner. Not in Radio Shack either. They have all been 8 Ohms, rated between one and three watts. The TDA1905 is not too powerful for the internal speaker. With a 14 volt supply it has only a two watt output capability into 8 Ohms. Reproducing voice audio has low average power and there are many examples of the TDA1905 directly driving an 8 Ohm speaker rated at one or two watts. See for example the PRO-2004 and PRO-2006. Also see the Uniden-made examples in the pic below.



I have owned the PRO-2004, PRO-2006, BCT15X and others that directly drive the internal speaker from a TDA1905. There has never been an excessive thump on turn on or turn off. Nothing even close to causing speaker damage ever.

Aside from the BC780XLT I have not seen the addition of an 18 Ohm resistor in series with the internal speaker in a scanner. There is no need for it and it only reduces performance. Maybe the BC785D and BC796D have the same resistor since they are of similar vintage to the BC780XLT. I had a BC796D years ago but no longer own it.

 

[Ubbe]

It's a big thump in the 780 speaker at both power on and power off. My BCT15 has a tick-tick sound in its speaker so there's something else happening there, like a delayed function. But if I switch off and then directly on I get a big thump in the BCT15 speaker.

BCT15x came 10 years after BC780 and the other scanners 10 years before BC780. So they only added that 18 ohm resistor to the 780 scanner. It must have been some specific audio issue with the 780.

/Ubbe