VHF/UHF SDR from a DVB-T dongle

Many people on Internet are talking and documenting "GNU radio" ability to create a SDR out of a USB DVB-T TV tuner. [I apologise for too many acronyms in one sentence.]
A 20US$ device (Ezcap EZTV668 in my case) is supposed to tune, display and decode transmissions between 65 and 1700 MHz. Sampling resolution is only 8-bit and there is no front-end filtering for this direct conversion receiver (can be added externally, of course), but this will let me have a peek at 23cm band.
Also it could be useful as a poor man's spectrum analyzer in my lab.

Who knows? Time - and experimentation - will tell (me).

Well, first it has to come from Far East.

Locating R1235 on FT-817

Two weeks ago I proposed a "play safe" procedure for testing the health FT-817's ACC port (Vcc line only). If R1235 is blown, what can be done? First let us find out where it is.

Remove all power sources to the FT-817, including the internal battery.

Unscrew the top cover (note, this procedure is the same of the "optional filter installation") and gently unplug the speaker cable: a shiny PCB full of SMD components is exposed. Well, that's not the correct side for R1235.

FT817ND without top cover.

Remove the five screws that hold the board (red circles in the picture above), the flat cable near the DATA connector (yellow rectangle) and the two coax connectors (yellow circles). Now the PCB can be lifted and rotated 180° above the front panel.

Check out the video to understand where you have to look for R1235 on the bottom side of the top board.

 

And here is an annotated picture of the area of interest:

Area of interest to locate R1235

It is tiny, isn't it?

A Vcc line shorted to ground though this 10 ohm resistor for a couple of seconds increased the resistance to about 7000 ohm. Voltage without load is the same of the supply, but as soon as some current is drawn the voltage reading drops.

Fix? Stay tuned.

Checking FT-817 ACC port health

Probably not many FT-817 owners know that the rear ACC port is not much tolerant to misuse. This is particularly the case of the +V line protected only with a tiny 10 ohm 1/8W 1/16W resistor (R1235): if this line is shorted to ground the resistor blows and goes open circuit (I've already heard of two blown resistors).

So, even testing the health of the ACC port +V line can be dangerous. But since that line is always connected to +V of the power supply or internal battery, you may use the following safe trick.

Power off the FT817. Unplug the internal battery. Remove any external power supply but leave the power cord connected to the radio. Get hold of a 1.5V cell and find out how to use it to power the FT817 (button cells can be held in place with a clothes peg).

The RTX will obviously not power up, but an accidental short circuit on the ACC line will not damage immediately the internal 10 ohm resistor (P = V^2/R = (1.5)^2/10 = 0,225W, about 1/4W instead of 14W at 12V!).

Extract +V and GND lines from the ACC socket with two insulated wires and measure voltage across them with a voltmeter. If you read your battery's voltage out of the ACC port, then everything should be fine.

You may also try to guess the internal resistor value, in case you have doubts it was damaged before. How? Connect a 10 ohm resistor across your loose wires and measure the voltage: it should be half of the battery reading. Or use Ohm's law to do the reverse engineering.


This procedure only tests the health of the +V ACC port line, not the whole ACC socket health.