BV131 Modification
Here are some modifications to a BV131:
- An external PTT input to control the amp directly from the transmitter,
- An internal 12 V power supply to power a ventilation module* as well as a backlight,
- The backlighting circuit for the power meter.
*The ventilation module is not described here. It is a configurable trade model with a temperature probe, made to supply a PC fan for example.
The schematic diagram
The one on the left is the unchanged version containing the specifics of the amplifier I had in hand. There are other versions with different components values.
The one on the right is the version incorporating the modifications made. They are framed by dotted lines. I specify that the input attenuator as well as the resistance of 180 ohms (cathode) have been deleted knowing that the transmitter used has an output power not exceeding 5 watts.
List of components
One of the components is specific, the Stereo 3.5 mm jack socket (35RAPC4BH3) found under the FS352 reference at Gotronic and surely at other suppliers.
It will also need a small thermal dissipator for the regulator. The one I used was a bit long and I had to cut the plastic parts out of the top of the transformer a bit.
The LEDs used have been recovered from a Samsung screen backlight. These are TS731A, SPBWH1732S1B (3v 0.5W 7032 white). Dimensions: 6.35 x 3.4 mm for a thickness of 1.2 mm. With around 2.7 V per LED, their brightness is ideal and consumption
Printed circuits
The first contains the components necessary for 12V power supply and the jack of the external PTT input. The difficulty was to find a location in the amp and it is ultimately the space present between the transformer and the main board that was chosen. This provides two fixing points: the jack socket, this being fixed to the back of the case, and a spacer.
The 1500 µF capacitors will have to be short or will be placed along the printed circuit (this is ultimately what I did).
The second printed circuit receives the SMD LEDs.
The connections
The 10V7 and 12V outputs should only be used to supply elements that are not related to the general ground. If a fan control module is used, it is necessary that the supply is to be taken between +12 and - and especially not connect its ground to that of the amplifier.
It is first necessary to isolate the the emitter terminal of the 2N2222 and link it to the ST BY switch (which is therefore no longer connected to the anode of the detection input diode).
- The "MASSE" (ground) terminal goes to the ground of the amp.
- The "6V3AC" terminal is connected to the track leading the 6.3 V to the lamp filament.
- The " +7V" terminal is connected to the track bringing the +7V to the relay.
- The "base 2N2222" terminal is connected to the base of the relay switching transistor.
- The "Anode diode Vox" terminal is connected to the anode of the diode located at the input to the detection, that which was connected to the ST BY switch.
The PTT input
Originally, the switching was carried out via the detection and then the rectification of part of the RF signal present to amplifier input. A transistor is thus polarized and commands the relay. The standby function acts on signal rectification.
Adding a transistor controlled by an external PTT is simple but how to inhibit this action when the amplifier is in a stand-by position? Finally, the simplest method was to move the stand-by switch to the switching transistor emitter and act on the base of the latter via the external PTT.
When the jack plug of PTT cord is inserted, the detection is inhibited and the switching transistor is only controlled by the 2N2907.
When nothing is connected to the external jack PTT, detection works normally. In both cases, the stand-by command is functional.
12 V power supply
The solution adopted consists in designing a voltage doubler of Latour from the 6.3V available in order to reach the desired 12V. The tension of 10.7 V is suitable for LEDs used so that the brightness of the backlight is pleasant even with sifted ambient lighting. One of the two output diodes can be replaced by a resistor whose value will be defined according to the LEDs you choose.
The backlight
I found it practical to design a printed circuit which comes to connect via two pins with both tracks located at the back of the meter and which previously fed the filament lamp. The 6.3 V can be deleted. Above all, have in mind that the minus (-) output of the Latour's doubler should absolutely not be connected to the general ground. It is necessary to connect a double wire of the output + and - 10.7 V directly to the input + and - of the printed circuit of the backlight.
The deflector is designed at the start of a simple cut copper sheet and then folded and finally tinned on its internal side. Above the folding carried out. Fold according to the little dotted line fine so that the other faces are visible and then fold them down according to the big dotted lines. With the internal faces as well as the printed circuit itself tinned, this gives a fairly uniform light.
We could also increase the width of the strip so that the deflector completely closes the space with the meter. It would then be necessary to reduce its angle of inclination a little.
