Faulty Preamp PSU Caused Humax Reception Problems on Some Muxes

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Newcoppiceman

Active Member
Our repaired HDR-Fox T2 had been working fine for almost two years until a recording of Mike Read's Heritage Chart Show (Talking Pictures TV, Freeview 82) made at 04:00 on 9 Oct was unwatchable due to picture/sound breakup. At the time this was attributed to co-channel interference due to unusual weather conditions (a problem we've had just once in the previous 10+ years). We're in Woking and get our signal (normally rock-steady on all 7 muxes) from Crystal Palace via a 25dB loft amplifier. (In January I fitted a 700MHz LTE low-pass filter which is now bypassed in case this was a factor - it wasn't.) Usually the PVR works fine but the problem seems to be getting worse and this morning (3 Nov) I gave up trying to listen to Today on Radio 4 the breakup was so bad. Investigations will begin in earnest this weekend (fortunately I have two working spare PVRs); I've already discovered that affected recordings don't have the break-up "recorded-in" as results differ when replaying the same scene. On the one hand this is an unwelcome fault, on the other I'm quite excited as this is another chance to plunder the wonderful resource that is hummy.tv - and probably an opportunity to install the custom firmware. I guess this post ought to be moved to or duplicated in another thread (I won't add further to this one).
 
Yes, let's get this split off. I'm not sure what to suggest as a title yet (but it can be changed retrospectively).

To be clear: you're getting break-up on live and playback, but analysis of the recording is clean?
 
Black Hole said:
Yes, let's get this split off. I'm not sure what to suggest as a title yet (but it can be changed retrospectively).

To be clear: you're getting break-up on live and playback, but analysis of the recording is clean?
Click to expand...
Yes, break-up on live and playback - on live, only occurs only during hours of darkness and some muxes better than others. Thus far, analysis of recordings limited to observing same sequence several times and noting while break-up is present each time it doesn't occur at the same points. Lounge and bedroom TVs, fed from separate outputs of the preamp's PSU (also in the loft), are working ok. I'll start a new thread.
 
Summarising posts 254-246 on p13 of https://hummy.tv/forum/threads/start-up-fails-when-hdd-connected.10164/...

Our repaired (see above thread) HDR-Fox T2 had been working fine for almost two years until a recording of Mike Read's Heritage Chart Show (Talking Pictures TV, Freeview 82) made at 04:00 on 9 Oct was unwatchable due to picture/sound breakup. At the time this was attributed to co-channel interference due to unusual weather conditions (a problem we've had just once in the previous 10+ years). We're in Woking and get our signal (normally rock-steady on all 7 muxes) from Crystal Palace via a 25dB loft amplifier. (In January I fitted a 700MHz LTE low-pass filter which is now bypassed in case this was a factor - it wasn't.)

Usually the PVR works fine but the problem seems to be getting worse and this morning (3 Nov) I gave up trying to listen to Today on Radio 4 the breakup was so bad. Investigations will begin in earnest this weekend (fortunately I have two working spare PVRs); I've already discovered that affected recordings don't have the break-up "recorded-in" as results differ when replaying the same scene. On the one hand this is an unwelcome fault, on the other I'm quite excited as this is another chance to plunder the wonderful resource that is hummy.tv - and probably an opportunity to install the custom firmware.

So the break-up is on live and playback - on live, it only occurs during the hours of darkness and some muxes are better than others. Thus far, analysis of recordings has been limited to observing the same sequence several times and noting while break-up is present each time it doesn't occur at the same points. Our lounge and bedroom TVs, the former looped-through from the PVR, the latter fed from a separate output of the preamp's PSU (also in the loft), are both working ok.
 
I may have misunderstood, but if the recording is clean, does that not indicate there may be something wrong with the playback/monitoring device(s)? Eg failing component or interference? I'm not sure if you've tried to playback the recording in 2 or 3 locations - lounge, bedroom and maybe 1 other - the original location where you made the recording?
 
Newcoppiceman said:
I've already discovered that affected recordings don't have the break-up "recorded-in" as results differ when replaying the same scene.
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Differ yes, but clean? Export a recording and try playing it with VLC. The video is compressed using differences between frames, so it is conceivable an error in the data might manifest differently according to the exact start of playback.

Newcoppiceman said:
it only occurs during the hours of darkness
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How do you reconcile that with a hardware fault? Sounds like propagation to me, or perhaps local interference.

For information: my signal is marginal at the moment (I've not had time/inclination to find out why), and a broadcast which seems clean live can easily hiccup on playback or timeshift.
 
Newcoppiceman said:
We're in Woking and get our signal (normally rock-steady on all 7 muxes) from Crystal Palace via a 25dB loft amplifier.
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Most people use an aerial. Do we presume you mean a loft aerial (what group?) followed by a 25dB multi-output amplifier? What about the cabling downstream?
(In January I fitted a 700MHz LTE low-pass filter which is now bypassed in case this was a factor - it wasn't.)
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No, it won't be with everything off CP down at the bottom end.
it only occurs during the hours of darkness and some muxes are better than others.
Click to expand...
More weight that it's an RF effect of some sort.
 
Thanks to everyone for their comments. The easy questions first:
  • Outdoor aerial: Antiference Extragain XG14A, so Group A (see pic)
  • Loft preamp: Antiference UP25A, so again Group A (single output)
  • Loft PSU for preamp: Fringe P1285/2 (two outputs)
  • All coax: Bieffe C-O-10R (foil-and-braid type, foam dielectric)
  • No intermediate connectors between PSU and Humax
  • Above installation unchanged for 10+ years
I'll respond to the other points later.
 

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Black Hole said:
Differ yes, but clean? Export a recording and try playing it with VLC. The video is compressed using differences between frames, so it is conceivable an error in the data might manifest differently according to the exact start of playback.

How do you reconcile that with a hardware fault? Sounds like propagation to me, or perhaps local interference.
Click to expand...
They differ in that, eg, audio mutes might occur on different words in a sentence but the playback is never clean - picture and sound
always breaks-up. Scrub my reference to "recorded-in" - I was still thinking analogue technology; it's basically the raw transport stream that's written to disc, isn't it (not decoded video)?

Exporting a recording and playing with VLC is something I might try later after some more basic checks like trying the other two HDR-Fox T2s I have(!)

Open mind at the moment as to whether a hardware fault or something else. Early days.
 
Newcoppiceman said:
  • Outdoor aerial: Antiference Extragain XG14A, so Group A (see pic)
  • Loft preamp: Antiference UP25A, so again Group A (single output)
  • Loft PSU for preamp: Fringe P1285/2 (two outputs)
  • All coax: Bieffe C-O-10R (foil-and-braid type, foam dielectric)
  • No intermediate connectors between PSU and Humax
  • Above installation unchanged for 10+ years
Click to expand...
Your installation hasn't changed in the last 10 years but the signal strength will have increased over that period. Have you considered the possibility of too much amplification?
 
Having two tuners complicates matters - you never know which tuner is used for a recording or live viewing, do you? A duff first RF transistor in one of the tuners might explain why the PVR sometimes works ok; a combination of the tuner with poor RF gain and impaired propagation during hours of darkness might be causing the break-up. Twice I've had to replace the first RF transistor on Pure DRX-601E DAB (-only) tuners because they'd failed.
 
MartinLiddle said:
Your installation hasn't changed in the last 10 years but the signal strength will have increased over that period. Have you considered the possibility of too much amplification?
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Very much so - and I'm aware of the problems of intermod distortion, the need for derating, etc. I used a Maxpeak TAM DVB-T meter (which I no longer have) to measure levels at the input to the preamp and at the input to the Humax. Arguably the level is too high on some muxes, but as our reception has been essentially error-free for years I figured any intermod products were negligibly low.
 

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25dB is quite a lot. 10dB would be more appropriate given your distribution losses appear to be about 7dB (if I've read your diagram properly).
Or do you even need an amp. at all? Try a passive splitter instead if you need two outputs?

You can check the signal on the individual tuners using the hidden menu.
 
Newcoppiceman said:
it's basically the raw transport stream that's written to disc, isn't it
Click to expand...
Quite so.

Newcoppiceman said:
...I'm aware of the problems of intermod distortion... Arguably the level is too high on some muxes, but as our reception has been essentially error-free for years I figured any intermod products were negligibly low.
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Easy to check: the signal checkers on the Installation and (hidden) Service menus don't only report strength, they report quality too. The problem with the strength figure is it maxes out at 100%, so 99% is good but 100% could mean 150% for all you know, and we don't know where saturation is. The quality figure is a report of how much error correction is going on. Anything less than 100% is not good, and it will plummet when saturation occurs.
 
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prpr said:
25dB is quite a lot. 10dB would be more appropriate given your distribution losses appear to be about 7dB (if I've read your diagram properly).
Or do you even need an amp. at all? Try a passive splitter instead if you need two outputs?

You can check the signal on the individual tuners using the hidden menu.
Click to expand...
Trouble is, there's about 20dB difference in received levels between the weakest and strongest muxes, so I figured I'd risk overdriving the Humax rather than allow the weakest mux to drop below the "digital window" (45-70dBuV) which is what would happen with a 10dB preamp. Over the years that's proved to have been the right call, with rock-steady reception on all multiplexes (which is the object of the exercise, after all).

Yes 7dB distribution losses: eg PSB2: 58+25-76=7dB

Thanks for that about the hidden menu (I've previously used it to show Crids) - will check.
 
Black Hole said:
Easy to check: the signal checkers on the Installation and (hidden) Service menus don't only report strength, they report quality too. The problem with the strength figure is it maxes out at 100%, so 99% is good but 100% could mean 150% for all you know, and we don't know where saturation is. The quality figure is a report of how much error correction is going on. Anything less than 100% is not good, and it will plummet when saturation occurs.
Click to expand...
I note the strength/quality bars of each mux whenever I do a rescan; I manually rescan for UHF22/23/25/26/28/30/35. Typically the readings are 70% strength and 100% quality. That would seem to confirm I'm not overdriving the Humax and is consistent with our (usually) rock-steady reception of all muxes. If the strength display is linear in voltage, 70% is 3dB down on 100% (20 x log0.7). It may well not be as it may be derived from the AGC control voltage (at least that's how it tended to be done in analogue days).
 
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Black Hole said:
Worth checking again shirley, if you're getting problems now which you were not before!?
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Indeed. A few hours ago I checked both tuners using the hidden menu option. It's a pity there's no indication of the UHF channel each tuner is tuned to but I did a crash-record of BBC One HD (BBCB/PSB3) and then selected C4 SD (D3&4/PSBS2) to view to ensure they were tuned to different muxes. Both signal strengths were stable at around 70-80% whereas both quality indications were mostly 100% - but would often dip to around 80% (which doesn't normally happen).

I need to investigate how the aerial input is split to feed the two tuners; there will be a 3dB (inductive split) or 6dB (resistive split) loss to each tuner, so maybe there's active circuitry upstream of the tuners to make good this loss. My wife tried watching Home and Away but switched to the TV's own tuner due to the occasional break-ups (which the TV didn't suffer from). The TV aerial feed is looped-through from the PVR.

As my wife is up-to-date with her viewing, tomorrow I'll install one of the spare PVRs and we'll run with that for a week or two. I can then investigate the current PVR (try unplugging the HDD, install the custom firmware, etc and look into copying content to the spare PVR if this proves necessary).
 
Took this prior to replacing the Humax. Note the quality level (green bars) and signal strength (orange bars). Selecting the same service (Talking Pictures TV) on the TV (ie changing input from HDMI1 to Antenna) produces problem-free pictures and sound. TPTV is on the ARQIVAB/COM6 mux.
 

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