[mythtv-users] HDPVR intermittent failure

[Stephen Worthington]

On Sun, 10 May 2020 20:20:30 -0700, you wrote:

>On Sun, May 10, 2020, 7:58 PM Greg <gregl at nycap.rr.com> wrote:

>Ok thanks I will investigate that.  But it's been working fine with the
>current power source for like a year now, and others on this list use it
>too.

USB cables often fit badly into the sockets.  If so, the connection
generally gets worse over time as you get dirt or oxidisation on the
contacts.  So badly fitting cables will degrade with time.  The result
is that the voltage drop across the cable will increase markedly.  And
they can be too thin - the amount of copper in the wires is too little
and that causes high resistance and a big voltage drop across the
length of the cable.  Some (most?) USB cables are designed only for
data transmission, or to run very low power devices.  For a high power
device, you need a better (thicker) cable.  The high power devices do
a negotiation with device supplying the power and request high power
mode.  If the cable is not capable of high power, that negotiation is
not supposed to work and the device should either only work in low
power mode or it should turn itself off.  But USB cable makers often
make cables not capable of high power transmission that will allow the
high power mode negotiation to succeed.  So even though the device
supplying the power is sending high current, the voltage drop in the
cable means that at the other end, the voltage can be below the level
required for proper operation or to fully charge the device's battery.

I have had two notably bad experiences with USB cables.  One was a USB
DVB-T tuner, and it was very like your experience - it would go for a
number of days just fine, then suddenly stop.  If I unplugged it and
plugged it in again, it would usually work again.  When I finally
investigated properly, I found the cable was just a little loose in
the PC's socket.  I replaced the cable with one that fit more tightly
and the tuner was much more reliable.  It still occasionally caused
trouble, but only when I had bumped the cables (or in one case, after
we had a small earthquake).  So because of that and because I needed
more DVB-T tuners, I finally replaced all my DVB-T tuners with an 8
tuner PCIe card.

The second bad experience was my Samsung Galaxy Tab S2 tablet.  Its
USB charging cable was supplied with it by Samsung, so I assumed it
was a good one.  But right from the start, the tablet took a long time
to charge, and the time gradually got longer and longer and the
battery life on one charge was getting less and less.  And then I
started to have to jiggle the cable in the socket to get it to charge
at all.  I actually called the Samsung help line about this, and they
said it sounds like a bad cable.  So I bought a expensive (NZ$30)
Pudney & Lee charging cable, which was a fair bit longer than the old
Samsung cable, but fit very tightly at both ends and was significantly
thicker - it has more copper in the wires in the cable.  Then suddenly
the battery charging times were what was specified for the tablet,
rather than three times as long.  And over a number of charging
cycles, the battery life came back again.  So the original Samsung
supplied cable was clearly bad from the start - it is probably less
than the specification required to charge the tablet properly as it is
too thin and has too much voltage drop even when the plugs fit
properly.  So definitely NZ$30 well spent.  But I am surprised that a
reputable company like Samsung would supply a bad cable with an
expensive top-of-the-line product like my tablet.  But they did - so
now I always suspect any USB cable I get and keep an eye on how well
it is working.