Baofeng UV-5R low TX modulation mods
Update: The “Vox Transistor” mod by Colin G4EML is now up on this site here:
This is a hardware modification for the Baofeng UV-5R. It deals with surface mount components of 0603 and 0402 size and you will need a very small soldering iron tip, preferably with temperature control, unless you’re a soldering jedi. If you don’t feel comfortable in disassembling and/or modifying your radio, then don’t do it. I can’t be held responsible for any damages.
Now, with that out of the way….
While I’m happy with my UV-5R, I think it could do with an outgoing audio boost. I have had good audio reports in QSO’s, but it is still relatively low compared to my Yaesu FT-7900R. Here I have a couple of suggestions to boost the audio.
In the good old days, you would just adjust a deviation control on the PCB. Greater deviation on the transmitter yields a higher amplitude output swing from the discriminator in the receiver, translating into louder received audio. Unfortunately, setting the FM deviation isn’t a possibility on this radio (with the exception of Narrow/Wide modes) since it is a register parameter in the RDA1846 IC programmed by firmware, so we have to find another way.
Essentially, we’ll be either bypassing frequency shaping components in the TX audio chain by using a large value capacitor or by just removing them.
EDIT 25th November 2012: Someone tried modification 1 below, but discovered that it can be done from the opposite side of the PCB – meaning that if this is true, the PCB does not have to be removed completely. Check the link here:
I didn’t do a disassembly video, so I’m pulling this from memory – please forgive me if I forget something!
- Remove the battery.
- Remove the belt clip, if installed.
- Pull off the volume knob.
- Remove the nut around the volume potentiometer.
- Remove the nut around the antenna connector.
- Remove the two screws on the back, near the battery release switch. Careful – there is a spring under it.
- Remove the two screws on the back, near the bottom of the radio.
- Pry up the bottom of the radio where you removed the screws in step 7, about 5mm or so.
- Pull the back section of the radio down and up to remove it from the front case, being careful of the speaker wires and button material.
- Desolder the speaker wires.
- Flip the board over and remove the screws attaching the PCB to the metal shield body. There are two screws under the display. See #14.
- Remove the two screws either side of the antenna connector body.
- Remove the screw down by the flashlight LED – this secures the battery connector.
- Remove the two screws holding the display onto the PCB and rotate it upwards, removing it from the PCB. Try to keep it in one piece.
- Remove the final two screws from the PCB (under the removed display).
- Locate the battery connector on the back side of the radio and pull it straight out. Don’t bend the pins.
- Lift the volume control side of the PCB off the shield first – this will allow the antenna connector to come out without needing to desolder it.
- Flip the PCB over and you’re ready to begin the modification(s).
Looking at the microphone area of the schematic:
The components are as follows, tracing back from the microphone itself:
MIC1 – Electret Microphone element.
C132 – RF Decoupling capacitor. Shunts any RF noise to ground. Not needed if there is a built-in capacitor in MIC1. Impacts audio level.
J1 – External microphone jack.
R71 – Power supply load/limit for external microphone.
C133 – Bypass for external microphone.
R69, R70 – Power supply load/limit for internal microphone. Impacts audio level.
C137 – Couples audio to VOX detection circuitry.
L44 – RF choke. Almost zero reactance at audio frequency. Doesn’t impact audio level.
C134 – DC block and frequency shaping (high pass). 100nF @ 1kHz = 1.6kΩ series. Impacts audio level.
C135 – Frequency Shaping. 47nF @ 1kHz = 3.3kΩ shunts higher frequencies to ground. Impacts audio level.
R72 – Limiting resistor. Impacts audio level.
C136 – Unnecessary capacitor (really?). Same function as C134 but no DC block needed. Impacts audio level.
C216 – Couples audio output/beep circuitry into TX audio feed.
Refer to the full schematic for more information on surrounding components, etc:
Baofeng UV-5R schematic diagram
Schematic Diagram for the Baofeng UV-5R. Accurate as far as I can tell, with some strange mistakes like using a BJT symbol instead of a MOSFET in the case of the final driver and output transistor, etc.
Here are a few modifications to try. These mods likely don’t “boost” overall audio level – they do change the tonality and frequency filtering characteristic. Anyway, the result is still more pleasing to the ear.
1. Add 10uF coupling capacitor
The red capacitor in the modification schematic is a 10uF tantalum capacitor. What this does is to bypass all of the frequency shaping and filtering components and add more coupling between the microphone output and the MIC1 pin on the RDA1846 IC. Since this is only adding one component, I consider this to be the easiest modification with the best results.
2. Remove components and bypass circuit with short
Remove C135 from the PCB. Bypass R72 and C136 with solder.
If you look at this image and compare it with the one further up the page (with the components present), you’ll see that I’ve removed C135, R72 and C136 (previous locations shown here in red) and I’ve effectively solder-bridged C134 output to the PCB track which leads to the RDA1846 MIC input pin. This gives a decent increase in TX audio. It didn’t seem to be quite as potent as the bypass capacitor modification, but this is the one I went with – mainly because I didn’t have a 10uF capacitor small enough to do the first mod.
3. Replace 0603 ferrite
Another one I considered (but didn’t try) was to complete modification #2 but also remove C134 and L44. Short the pads left by C134 and replace the L44 ferrite with a 10uF 0603 size ceramic capacitor, such as this one from digikey:
- or an 0603 Tantalum capacitor such as this one:
This would have the effect of removing any filtering which C134 provided, any attenuation that L44 provided, and maintain the DC block. I suspect that this modification might yield the greatest gain.
4. Adjusting values for R69 & R70
I haven’t tried playing with the resistors which provide power to the Electret Microphone element yet. It is feasible that adjusting the resistance values for these parts will have an effect on microphone output level.
I’m sure that playing around with the values of any of the components I marked above as “Impacts audio level” will have a change on TX audio, whether in tone or level, more or less. They key is to play around to see what you get. The RDA1846 datasheet doesn’t specify this, but I’d keep at least one capacitor between the microphone and the MIC pin on the RDA1846 in order to block the DC voltage which drives the microphone element from reaching the RDA1846. Just in case. If you blow the RDA1846, it’s game over until you can get another chip, or more realistically I think, a new radio.
I’ll be expanding this article in time with different mods.
Watch this video in full screen, HD if possible.
Good luck and 73