Circuit Notebook 107 - Modifying the EMPREX Digital Media Box BMP-001

Introduction

The EMPREX BMP-001, shown in Fig 1., is a cracking little box. It will accept either an SD card or a memory stick containing your videos, pictures, test cards etc and output (through phono connectors) composite video, L&R sound. A VGA output is also provided.

The internal clock/calendar can be set, as can the video brightness, contrast, hue etc by using a remote control (supplied). NTSC or PAL operation can also be selected.

My interest was to use the Box as a source of test cards and captions independent of a PC and to power it from 12V. To test it out, I downloaded some test cards from Google, (enter 'test cards') and copied them to an SD card. I plugged this into the Emprex and connected its plug-top power supply.

Initial Testing

The video output was connected to a normal colour monitor. The parameters for the SD card were set up using the on-screen menu. The results were fine (1V p-p) providing the output was not terminated. But, when terminated in 75R it dropped to 0.6V p-p. This indicated a low output voltage and an output impedance of around 35R.

The BMP-001 requires a 5V supply of 500mA and this is normally provided by a switch-mode plug-top power supply. But, for portable operation from 12/13.8V a 5V regulator would be required.

Modifications

To overcome these two shortcomings, I decided to replace the video output stages with two video op-amps and to install the required voltage regulator. In fact, I fitted two regulators, a 5V one for the existing circuits and a 9V one to feed the proposed video amplifiers.

The 5V regulator would have to dissipate a lot of heat. 13.8V - 5V = 8.8V x 0.5A = 4.4Watts The Emprex box is plastic so a suitable aluminium heat sink would be needed. The one from my junk box was thermally larger than required but it fitted comfortably on top of the Emprex box. I fitted the two regulators on the underside of the heatsink, as shown in Fig 4. and wired them as shown in the circuit diagram, Fig 2..

I cut the live tag coming out of the power connector from the PC board, as shown in Fig 3. and wired this through D1 to the regulators. I took the 5V output from the regulator and connected it to the 5V input to the PC board. Using a variable power supply, I applied 12V to the input power connector and checked that the output voltages of +9V and +5 were correct. I find it prudent to use a variable power supply and monitor the 5V output whilst gradually increasing the input to 12V. If there is a problem then you can stop in time!

Dual video op-amp

I used two AD817AN video op-amps. I prefer EL2020 op-amps but these are no longer available. I laid out the circuit on Veroboard as shown in Fig 5. ('x' is a cut in the track, a 'blob' is a soldered joint). The components were loaded as shown in Fig 6.. I connected the Veroboard panel according to the circuit and fitted it inside the box, as shown in Fig 7., taking care not to foul any of the existing components.

I cut the link between the 'video' output socket and the PC board, also the 'Y' output socket from the PC board. I connected both inputs on the Veroboard to the video output on the PCB. This would feed video to both op-amps. I then connected the two video outputs from the Veroboard to the 'video' and 'Y' sockets respectively. This would provide a 'video' output from both the 'video' and the 'Y' sockets.

Final Testing

Using my 'test card' SD card, I powered up the box from 12V/13.8V. I connected a monitor, terminated in 75R, to each output in tern and adjusted the relevant RV1 or RV2 to provide a 1V p-p output. The rear of the completed Emprex box is shown in Fig 8..

Thanks

My thanks to David, GW8PBX for his helpful advice on the project.

The Emprex BMP-001 is available new from Amazon at £23.99 www.amazon.co.uk/emprex or 'refurbished' from the Emprex eBay shop www.ebay.co.uk/emprex at £12.99 inc. postage. My refurbished unit looks immaculate and works extremely well.

Note: These modifications will result any warranty becoming void.

Figures

Reference only: (copied from Emprex Leaflet) Model No. BMP-001 Support picture format JPEG, BMP, GIF Support video format Standard MPEG ½, Xvid Support audio format MP3, WMA Support memory cards SD, MMC, XD, MS, MS-Pro Remote control Remote control Interface USB2.0 interface Output format RCA, Y Cb/Pb Cr/Pr, VGA TV type NTSC/ PAL Power supply DC5V/1.5A Dimension 136.4 x 100.2 x 41.5mm