Improving PlayTV reception Part 2

So it turns out that my PlayTV stuttering issues are probably not linked to signal strength, having verified the aerial (new), cabling (RG6) and signal strength reported by 2 other digital receivers as very high (one reports 100%). During tuning, PlayTV seems to briefly indicate 100% also. It's not the HDD (off during live TV) and I've tried 720p and 1080i output with no difference.

But no matter - I'll wait for poor weather to test if an amp makes the difference.

The following setup shows what I've tried with a 5V masthead amp and it doesn't seem to make things worse at least, with or without 5V injection from 2nd USB port.

So because I don't have a DTV spectrum analyser, I'm just going to assume the masthead amp is doing something here. Certainly I see the signal input on the PlayTV jump to +5V potential in the above scenario, but can't determine the nature of the AC (MHz). I'll have to wait for some more testing.

Onboard Power

This is the ultimate goal - powering the amp from PlayTV. On the PCB there seems to be 1.9V and 3.3V rails (see comments on previous post - thanks Rowan). Finally I found the voltage divider - an IC indicated in the image with pin configuration below.

5V   __.___.__  3.3V
       |   |
Gnd  __|   |
       |   |
1.9V __|___|__  1.2V

So basically I need to either tap the mini-USB jack directly or the pin above to power the aerial signal (see marked solder point).

Dissecting the PS3 PlayTV for Poor Reception

I recently purchased a PlayTV (see here also) and am fairly happy with it (many software improvements could be made - see later) but I'm having previously unencountered reception problems. It would seem I'm not alone and have even spoken to friends worse off than me where they have a working STB, but can not even use the purchased PlayTV due to weak signal.

In my case, whereas my old Sony DVR is able to receive all channels perfectly, the PlayTV breaks up and skips, mainly on ABC/ABC2. I'm no expert, but put this down to a cheap design without pre-amplification.

I had a thought; with USB connected to the unit (and therefore 5V +/- 0.25 and capability to deliver 500mA), why not devise a simple in line amplification solution drawing on no external power source? Sure I can buy a cots distribution amplifier, but yet another power pack when USB is available?

Time then to investigate the PlayTV PCB. Here's the front:

and here's the back, flipped for convenience:

I'm not an electronic engineer and know little more than basic electrical theory, but the first thing apparent to me is that the board uses contacts 2/3 only (USB data) and does not use the 5V bus power (contact 1).

That is, from the micro USB spec:
1 VBUS Red
2 D- White
3 D+ Green
4 ID <Ra_PLUG_ID
5 GND Black
Shell Shield Drain Wire

Not knowing anything about USB, I can only guess power for the USB and DVB circuitry is drawn directly from data signal itself (order of a few hundred millivots on/off depending) and assume likewise the LED is powered by some sort of rectification of this.

So far so good then - 5V seems available for use. After some further research, it seems that some STBs have the ability to inject the source coax with 5V DC to power an upstream masthead amplifier. Such an amplifier is shown here. I have ordered one and now just need to figure out how to solder onto that tiny tiny 5V rail. Any tips are welcome. Also, without a schematic, I'm not sure of the implications of placing an additional 5V DC onto the DTV signal. I presume however that with other types of masthead amplifiers being common fare, this is not doing anything unusual.