Only 0.4V at LNB 1/2 Input Sockets on Back of Foxsat HDR. Is this a fault ?

Mechnical Al

New Member
I have a 12 year old Foxsat HDR on a Sky antenna installation which is approx 2 years older, which has worked perfectly, albeit only doing the basics; watching + recording + playing. It lost reception of all channels…. on Christmas day would you believe. An auto and manual retune found nothing, along with our postcode becoming invalid, with the message check antenna connections displayed. I checked at the rear of the HDR and at the wall plate and they seemed secure, but I de-powered and remade them after a wishful spray of Servisol, but the fault persisted. I didn’t get the ladder out to check at the antenna as it’s a long way up at the top of a gable end, but I took a zoomed in picture, for what it’s worth, and saw no obvious disconnections or birdstrike damage etc.

As a best guess attempt at fault diagnosis between a U/S box, or U/S LNB, I checked for the placarded 13/18v dc at the rear connections, but measured only 0.4v on both, ( with the antenna leads disconnected of course). This voltage stayed the same with standard and high power (for long cables) selected in the hidden menu, I did read para 8 of the what every HDR owner should know thread, and this doesn’t clarify the 13/18v, but describes a 5v antenna amplifier supply. I hoped to test for that but the menu options have closed down on me so much, that settings is no longer available.

I’ve inspected the PSU, hoping to find something simple like bulging caps, or a blown fuse, but sadly not, and measured the outputs at 12.4v & 6.04v, and stable when loaded by connection to the main board, when displaying both boot and menu. A little above the stencilled figures, but seems close enough.

There is what looks like a tiny relay on the LNB input section PCB, stencilled ‘KC170’ so I audibly checked it with a bit of plastic tube as a stethoscope, and it does positively click at power up, and power off. But the PCB tracks are way too tiny to go poking around blindly with multimeter probes.

I emailed Humax Tech Support, describing the no signal problem, and specifically asking about the 0-4v, and if & when should the 13/18vdc be present at the back of the HDR. All they told me was they no longer support this model, but said from what I told them, it seems the box is working normally, but to be sure, I should try my box on another antenna, and/or another box on my antenna.

Woah…I only have one of each, but even if I had access to others I’d be cautious about whether a fault on the LNB could damage another receiver, or a fault on my receiver damage a neighbour’s LNB.

So if Humax are implying that 0.4v is normal, I’m wondering if that means it’s somehow ‘in idle’ with no LNB connected, and perhaps it needs to see a load to make it ramp-up to operating voltage. There’s a 750mA limit on the placard so with my basic electrics knowledge, I’d expect that I should be able to connect a 12v 2w car instrument panel bulb, (or 2 in series, just in case there is 18v on there) and at least get light!

That didn’t work. Current flow at 0.4v into a single 286 pea bulb was only 5.3uA, on both connections.

I don’t have a signal strength meter to test the LNB, so to see how they’re powered I looked up circuit diagrams for simple DIY ones. Bah…..It appears they use the receiver as a power source ……so no point me buying or making one!

But I’m also wondering; If the LNB was at fault, it’s at least dual output so is it likely that both outputs would fail simultaneously ?

I hope this isn’t too much info, and I have searched for similar topics, as per forum newbie instructions, but even the closest, (by BJ_Racer Freesat Tune Fails), still has some antenna function, so I haven’t found any useful clues in that thread.

It’s probably unrelated, but this fault occurred along with remote control failure, ( another long story) discovered when I found the empty channel list was mysteriously set to 21 Jan 2021, which I thought was simply because programmes aren’t listed that far in advance. I thought I’d be the hero by simply selecting to current date…..but things went downhill, as detailed above!

Hopefully you folks will have seen this all before and recognise the wailings of a novice, and be kind enough to suggest what I should try next, thanks.
 
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Black Hole

May contain traces of nut
I did read para 8 of the what every HDR owner should know thread, and this doesn’t clarify the 13/18v, but describes a 5v antenna amplifier supply.
That is written specifically for HDR-FOX and HD-FOX (which are terrestrial Freeview receivers/PVRs) not FOXSAT-HDR (which is a satellite Freesat receiver/PVR). The clue is in the title of that thread "Things Every HD-FOX/HDR-FOX T2 Owner Should Know".
None of what follows is necessarily applicable to any other Humax device, including the Foxsat HD or Foxsat HDR, although HDR-1800T and HDR-2000T have much in common with the HDR-FOX and users may benefit from some of the information.)
5V antenna power (HD/HDR-FOX) is for running a mast-head UHF amplifier.

I emailed Humax Tech Support, describing the no signal problem, and specifically asking about the 0-4v, and if & when should the 13/18vdc be present at the back of the HDR. All they told me was they no longer support this model, but said from what I told them, it seems the box is working normally, but to be sure, I should try my box on another antenna, and/or another box on my antenna.
That's a much better response than I might have expected, but "seems the box is working normally" has no supporting evidence so far as I can see.

Update: this is a red herring, see post 10.

Woah…I only have one of each, but even if I had access to others I’d be cautious about whether a fault on the LNB could damage another receiver, or a fault on my receiver damage a neighbour’s LNB.
Catch 22 for DIYers.

So if Humax are implying that 0.4v is normal, I’m wondering if that means it’s somehow ‘in idle’ with no LNB connected, and perhaps it needs to see a load to make it ramp-up to operating voltage.
Personally I don't think that's very likely.

Update: this is a red herring, see post 10.

I hope this isn’t too much info
Never. Usually we get far too little info.

It’s probably unrelated
Why? With the Foxsat not receiving data, all bets are off.

Hopefully you folks will have seen this all before and recognise the wailings of a novice, and be kind enough to suggest what I should try next, thanks.
No, I don't think so. And neither are you a complete novice because you've been able to conduct some useful investigation. The next step is for some kind soul on here with a Foxsat (not me) and a multimeter to disconnect their LNB feeds and measure the voltage on the open ports.
 
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Trev

The Dumb One
If you are not getting the 13/18v dc at the rear connections with nothing connected, then it seems that the box is broken. Now whether this has been a random box failure (most likely imo), or a failure caused by a duff LNB who knows.
But what is for sure, is that 'it won't work' on a neighbours dish/LNB without the 13/18V dc at the F sockets on the back of the box.
 

Black Hole

May contain traces of nut
If you are not getting the 13/18v dc at the rear connections with nothing connected, then it seems that the box is broken.
Have you confirmed this?

It seems likely there would be some kind of over-current protection. Now, whether the protection is a current-limited supply or a fuse is a moot point, but if it were a fuse (or a fusible link of some kind) it should be possible to find and replace it. The 0.4V is (I guess) just a bleed-through from somewhere.
 

Trev

The Dumb One
I haven't confirmed anything:roflmao: but as you intimated, a recheck would be confirmatory thing for OP to do. I also suspect there is current limiting citcuitry within the box, that a whisker of outer braid across one of the F sockets would kick into action.
 

Black Hole

May contain traces of nut
I haven't confirmed anything
Please do the experiment and report back. Until somebody does, we do not have positive confirmation that there is indeed 13-18V present on the LNB inputs when disconnected.
 
OP
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Mechnical Al

New Member
Hi all, and thanks for your thoughts. Hands up…. I misinterpreted the models covered by the what every owner should know thread. It was very late at night, after several hours of internet research of a subject I’m new to. Looks like all that Circadian Rhythm stuff studied in Human Factors training is true.

And apologies for my slow response time. I’m not ignoring anyone, it’s just that my memory impairment leaves me with, amongst other problems, slow access to vocabulary so I end up using 10 words because I can’t think of the one suitable word I need, so the post gets very bulky. And i forget to include some items, thus a lot of time is spent on re-reads and edits in draft.

Anyway, having done further research, I found some ads for battery powered satellite strength meters on which the battery pack plugs into the downstream F connector in place of the receiver cable. On that basis, despite Humax’s suggestion that 0.4v seems normal, i’m increasingly suspicious that the absence of 13/18v at my receiver’s chassis connectors, leaves the LNB dead, thus unable to return any signal….i.e; what Trev said at post 3 (thanks Trev).

To further investigate the problem I tried swapping the cables over, wishfully hoping to make one good channel out of two (possibly differently faulted) bad ones, but still no fix.

On considering the whiskered short possibility discussed at posts 4 & 5 ; This installation hadn’t been disturbed for at least 6 years, and the wall plate to receiver cables both measured infinity ohms centres to sleeves on removal from the system when the fault presented itself. I’ve only been able to find one fuse (F1 on the PSU board mains in, and it’s soldered in) but it must be OK or the whole unit would be dead. There’s nothing obvious which looks like mechanically resettable CB’s either.

Next I remotely checked the LNB via the wall plate as follows; Resistance across screen to screen, up 23M of cable, grounded together at the LNB, and back down the other cable, measured 0.8 ohms, so seems OK. Centre to screen measured 4.1 Megohms on one channel and 4.25 on the other, so taken as being not enough difference to raise suspicion. Then I slaved bench PSU power onto each LNB cable in turn and got 134 mA @13v and 136mA @18v, same on both channels, give or take 0.2mA.

At this point I wondered how the LNB distributes power internally i.e. all outputs become active if either channel is powered, or if they’re two totally independent channels. I'm guessing they're independant, so as to preserve the 13v / 18v selection function on each, but I thought I'd try just in case. I connected one channel to the receiver and applied 13vdc up the other wall plate cable……hoping to get some tuning function, but still nothing.


My next plan is to try and ‘inject’ 13v /18v via a bias tee, (a design for which I subsequently discovered on a website I’m not allowed to quote on here until I’ve done 20 posts). The site is called SotaBeams, with a very concise topic called ‘Bias Tee for LNB’, and coincidentally gave typical LNB current consumptions as 50-200mA….which put mine mid range. So it’s looking like one-nil to the LNB at present.
It also mentioned there’s a 22kHz 0.5v peak to peak square wave applied along with the DC to instruct the LNB to select between hi & lo bands…..Hmm, I’m not going to be able to replicate, or measure / check for that, but for fault investigation purposes I’m hoping the 13/18v experiment will at least positively indicate ‘two-nil’.

My concern with a bias tee is the risk of damaging levels of induced voltage spikes from the inductor into the antenna system when switching bench power off. I could put a flywheel diode across the inductor, but that creates an escape path for the signal…unless the signal is so small that the forward bias voltage of the diode is enough to retain it in the centre conductor.

Then there’s the capacitor to prevent externally applied DC going back into the receiver. It’s likely there’s already an internal one in the signal flow path, so adding an external one would be in series, so would reduce total capacitance….but to what effect?

I’m rambling on, out of my depth here, and of course; the trouble with test equipment is…..you have to know it works correctly in order to rely on its indications.

Anyone got any tips please?
 
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Black Hole

May contain traces of nut
Anyone got any tips please?
Yes: please would @Trev (or anyone else with a Foxsat) conduct the test I asked for before: measure the voltage on the satellite cable inputs (centre to screen) when the cables are not connected.

If it is confirmed that there should be voltage even when the LNB is disconnected, that confirms your Foxsat is broken and you need scratch your head no longer. You only need to be diagnosing problems with LBN, cable, alignment etc once you have a known working receiver.
 
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sine24

Member
Yes: please would @Trev (or anyone else with a Foxsat) conduct the test I asked for before: measure the voltage on the satellite cable inputs (centre to screen) when the cables are not connected.

If it is confirmed that there should be voltage even when the LNB is disconnected, that confirms your Foxsat is broken and you need scratch your head no longer. You only need to be diagnosing problems with LBN, cable, alignment etc once you have a known working receiver.
I've tried it.

It shows 0 or close to 0v
 

Black Hole

May contain traces of nut
Interesting. I wonder how that works (rhet.).

So we can't use the 0.4V as an indication the Foxsat isn't working, and it confirms Humax's assertion that the unit "appears to be operating normally". Excellent (or not excellent, according to your point of view).
 

Black Hole

May contain traces of nut
@Mechnical Al: I think you are barking up the wrong tree. As you seem able to tee off the LNB feed, it seems to me the next step is to measure the voltage on the feed when it is connected between the Foxsat and the LNB. It doesn't matter that the Foxsat isn't receiving anything - what you are confirming is whether the Foxsat powers up the LNB.

After that, I think the conclusion is easy: if the Foxsat doesn't power up the line even with an LNB on the end, the problem is most likely the Foxsat. If it does power up the line, the problem is most likely the LNB (unless the wind has knocked it off line or something). Simples. I would have recommended this before had I realised you are tooled up to take that kind of reading.

There’s nothing obvious which looks like mechanically resettable CB’s either.
Fusible links look like ordinary resistors, self-resetting links look like thermistors (or, indeed are PTC thermistors). Transformers also (sometimes) have thermal cut-outs inserted in their coils, which may or may not be self-resetting.
 

Trev

The Dumb One
I no longer have a foxsat in use, and I can't be asked to cobble it together to try.
I have been in hospital for a few days and am a non combatant for a while. But haw the hell anyone expecta an LNB to work without the correct supplies (13.5/18Vdc and tones for band switching etc) just beats the hell out of me. No dc from box = -broken box/short cct cables = LNB won't work.
I am too tired to read the ramblings as well..
I rest my case on this one.
 

Black Hole

May contain traces of nut
But haw the hell anyone expecta an LNB to work without the correct supplies (13.5/18Vdc and tones for band switching etc) just beats the hell out of me.
It seems that the Foxsat doesn't send power unless it detects a load. That's what Humax support were hinting at, and what needed confirming by experiment.

I have been in hospital for a few days and am a non combatant for a while.
Sorry to hear that.
 
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Mechnical Al

New Member
@Trev, Thanks for your assistance so far, and I think your assessment is highly likely......same as; take the battery out of your car, and it's not going to start! So I'm currently trying to make a bias tee to provide alternative power. But you rest and get well soon, no need to worry about this trivia. Hopefully you'll be OK ASAP. Best wishes, Al.
 

Trev

The Dumb One
I no longer have a foxsat in use, and I can't be asked to cobble it together to try.
What i meant to say was "can't be assed ".
How ever, unless my foxsat is in the attic, ill cobble it to the mains and check the F type skts for voltage
 

Black Hole

May contain traces of nut
So I'm currently trying to make a bias tee to provide alternative power.
I don't think it's a good idea to try to inject power, all you need to do with this "bias tee" (whatever that might be) is measure the voltage when the Foxsat is connected to the LNB (IIRC you are satisfied the LNB isn't a dead short). That should confirm whether the Foxsat is dead.

Trying to power up the LNB separately, but with the Foxsat connected, isn't likely to achieve anything - you would have to make sure the power source didn't shunt away the signal.
 
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