RS 21-541 Amplified Speaker Modifications

[DaleHCook]

I recently purchased a Uniden BCD436HP and decided that I needed more volume from it in my Explorer. Now that I am (mostly) retired I have to make one scanner serve for portable, mobile, and home. I have two vintage Radio Shack 21-541 amplified speakers but discovered that they did not have sufficient volume when driven by the Uniden so I have modified them.

The reason why is that those speakers were designed for CBs and scanners that had 8 ohm speaker outputs. The BCD436HP jack is designed for modern headphones with impedances in the 32 ohm to 100 ohm range. I have modified the RS input to support the Uniden. In addition, those speakers had a VOX circuit - the amplifier IC was powered up only when there was input audio. I never cared for that VOX circuit and have disabled it as part of my mods. Note that this mod applies only to the original 21-541 - the later 21-541A may be different but I don't have one to make a comparison.

The current iteration of the RS site no longer seems to have support documents so I have posted the schematic to my site. All of the parts are marked on the PC board.

To raise the input impedance replace R3, a 100 ohm resistor, with a 1,000 ohm resistor. To disable the VOX remove C1 and Q5. Solder a jumper wire between the Q5 collector and emitter pads - the two pads closest to the edge of the board.

I am now running one speaker in the Explorer and the second in my living room, and I am pleased with the results of the modification.

 

[iMONITOR]

Good info, thanks for posting. Welcome to Radio Reference!

 

[DaleHCook]

As with many systems the Roanoke City/County VA P25 system that I listen to the most has very inconsistent levels - for example, Town of Vinton Dispatch has lower audio than Roanoke County Dispatch, even though both are part of the County section of the system and the same County radio techs are responsible for both. When I have a chance I will work up a simple compression circuit to add to the speaker circuit to keep audio at a more consistent level. I did that decades ago (when I was a CB/scanner tech) for some CBs and I still have an example somewhere in the shop if I can find it.

 

[Ubbe]

You could probably simplify the modification just by bridging the emitter-collector of Q5 and snip away R3. You don't need to remove other components or put a 1k resistor on the tranformers secondary.

/Ubbe

 

[DaleHCook]

You could probably simplify the modification just by bridging the emitter-collector of Q5 and snip away R3.

I would rather not have unused components connected to power. I have spent better than four decades minimizing possible electronic failures as part of my career.

 

[cmdrwill]

I would rather not have unused components connected to power. I have spent better than four decades minimizing possible electronic failures as part of my career.

Good point, Sir.

And the 100 ohm resistor, R3, on the secondary of the transformer DOES need to be at least 200 ohms to match a 8 ohm output of the scanner.

 

[Ubbe]

DaleHCook: I don't know what the difference could be bridging the emitter-collector with or without the transistor in place. Just leave it in place, it will not harm anything if you look at the schematics.

cmdrwill: The modification included adaption to low level earphone output, as opposite to the standard 8 ohm speaker it's original designed for. Using a 1k resistor or none will have little influence on the operation of the amplified speaker.

Why make things more complicated than neccesary. If you need to revert to the original design you unsolder the bridge and solder back the one leg of the resistor that you snipped.

/Ubbe

 

[DaleHCook]

And the 100 ohm resistor, R3, on the secondary of the transformer DOES need to be at least 200 ohms to match a 8 ohm output of the scanner.

A little experimentation showed that 1,000 ohms produced the most satisfactory volume level with the scanner volume at about 8-10.

 

[DaleHCook]

Using a 1k resistor or none will have little influence on the operation of the amplified speaker.

I didn't try it with an open R3 as excessively decreasing the load on the secondary of the input transformer will eventually cause degradation of the audio. That transformer has some wiggle room on the load impedance but I only wanted to push it far enough to provide satisfactory performance.

If you need to revert to the original design ...

That is of no consideration. Those amplifiers are about 40 years old and haven't been used for about 30 years. They are left over from the mobile repair shop that I owned 40 years ago when I was self-employed doing scanner, CB, communications receiver, and small two-way system service. They are going to get additional modification with an AGC circuit and will never be returned to stock.

 

[cmdrwill]

You are correct, the resistor R3 at 200 ohms was for a 8 ohm. Earphones are up in the 32 ohm range, so the 1000 ohm would close..

40 years old, but they work...

 

[DaleHCook]

They do indeed work. They served me well for many years until they were retired more than twenty years ago. I'm an electronic pack rat, so they got stored away against possible future need, which has now arisen. The one in the Explorer has enough volume available to be heard over the car stereo unless I'm listening to King Crimson (my car stereo volume goes to 11).

 

[Ubbe]

One interesting approach you could do with the original design, is to bridge emitter-collector of Q4 and put a double pole switching relay between Q5 collector and +12v.

Then you put the relay switches between car stereo and front speakers and bridge the connectors with 22 ohm resistors so that in normal relay position the resistors are bridged, giving full power to car speakers but when the relay engage there is 22 ohm in series with the speaker and mute to about 1/4 volume which makes it easier to monitor the scanner audio.

Some car stereos have a mute pin for phone use and most of them mutes the audio completly and are too annoying used with a scanner. But if one choose to use that pin you only connect a wire from Q5 collector to the mute pin in the car stereos connector instead of a relay solution.

/Ubbe