I have a Humax Freesat twin tuner PVR. All of a sudden in the Diagnostics screen I'm seeing 100% signal strength and quality for tuner 1 and 0% strength and quality for tuner 2, on various channels including 101-108. My dish/LNB and cabling are DIY but the connections all seem OK as far as I can see. Is it likely that one side of the dual LNB has failed?

Cables may look OK, probably worth physically checking.

Have you tried swapping the cables over to see what the readings say?

Check the cabling as suggested above. If you find that LNB2 cable works OK on LNB1 input of the Humax and LNB1 cable does not work on LNB2 input it is possible the Humax has put itself into single LNB mode.

To correct this perform a Factory Reset on the Humax which should force it to detect both LNBs again. It won't wipe any existing recordings from the hard drive doing this but will clear any future recordings you may have scheduled plus any other custom settings you have made will be returned to their defaults. So you'll have to set all those up again.

Even if the Foxsat had somehow reverted to single cable mode, both tuners should still show a signal. The box will simply incorrectly assume that tuner 2 is fed from lnb1 out, rather than an independent feed.

Single cable mode still uses both tuners, only if the loop cable is omitted will tuner 2 shown no signal when in single cable mode.

I swapped the cables and it caused the symptoms to swap between tuners. Then my brain kicked in (it does that occasionally) and I realised I needed to swap cables at the LNB (which luckily is easily reached). That did not cause the symptoms to swap so by my logic the problem is in the cabling. Looks like I need to get under the floor. Thanks guys.

The problem is most likely a f connector issue. Are they tightly screwed onto the coax cable ? If you remove the connector from the faulty cable at the Foxsat end is the screen well away from the centre core.

You do not need to this !!!!!!!!!!

If you swap the cable at the back of the box FIRST then you are checking the Humax

If you swap the cables at the LNB second after checking the Humax, then you are checking the LNB

You need to tell us what Channels are causing the problem as H & V channels work differently ,

If the OP switched cables at the Humax and the problem changed tuners, then the cabling (or very unlikely the lnb output is faulty), the Humax is OK.

If the OP then switched the lnb outputs and the same tuner is still affected it has to be a cabling issue. Most likely the f connector at the Humax end.

The perils of screw on F connectors. Professionals use crimp or compression.

Guess what: it was the f connector at the Humax end. One of them had the cable almost falling out of it but it was the other that was causing the prob, though it seemed OK in every way. I took it apart, trimmed the cable slightly, put it back together and now everything is 100%. Thanks again guys.

And you see plenty of those faulty - crimp/compression are used for reasons of ease, NOT for better reliability - and of course you can't refit a crimp/compression one

There's also no choice on the thin twin commonly used by Sky.

As a 'professional' I use crimp ones if I'm doing a lot of connections, but just for an odd one or two screw-on ones are better. However, if you've got 50-100 to fit, crimp ones save a considerable amount of time - and pain!!! - fitting loads of screw-on ones hurts your fingers.

Nearly always the result of using a incorrect size connector for the cable. You would get the same issues if you used an incorrect size crimp/compression connector.

Obviously, but then a professional would not do that.

Crimp connectors generally have better matching and lower return loss than screw on and compression are better still.