Faulty Preamp PSU Caused Humax Reception Problems on Some Muxes

Thanks to Rodders53 for his (presumably, but you never know) helpful contributions.

We're quite low down to the west of Woking at GU21 8US, close to the Basingstoke Canal. Our aerial is on the side of the house because when I moved here in 1995 the installer couldn't get a satisfactory signal from CP with the aerial on the chimney. (I didn't want to use the Guildford relay as this is less well supported maintenance-wise than CP.) Certainly since analogue switch-off reception issues at any time of the year due to the usual suspects like variable propagation, CCI and trees (there are some nearby) have been few-and-far-between; we rarely saw a glitch on any mux. So our post-9 Oct predicament has come as a real surprise.

Actually, throughout the mixed-mode years of the 2000s reception of both analogue and digital was also ok - the 25dB Group A Antiference loft preamp was installed in Mar 2003, replacing an 18dB Fringe loft preamp installed in 1998 with the XG14A. Pic shows measurements made with the new preamp - note 7dB difference in level between analogue BBC1 and BBC2. The distribution losses were less then as the preamp PSU had only one output, not two. (Everyone keeping up?)

Re the Yagi 18A, worth considering - but its provenance isn't clear. I think I'll stick with a UK manufacturer (with a reputation to protect and a full-spec data sheet).

Mike Read's Heritage Chart Show is transmitted overnight (time varies) Sunday/Monday on Talking Pictures TV, Freeview 82. I've been watching my recordings of this for about 18 months with ne'er a glitch - but the 9, 16, 23 and 30 Oct recordings were completely unwatchable due to break-up. I'm currently checking this Monday's recordings made on the replacement (spare) PVR and also on the original PVR.

Notwithstanding that our two TVs seem unaffected by our current Humax predicament, a new aerial (and possibly preamp) seems a wise investment. Incidentally we have always used our lounge TV as a monitor (no aerial feed) as we rely on the PVR as the source for live TV.

The new meter arrives today ...and I think I've found the fault with the garage light!
 

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Certainly I intend to be helpful. I was (well) paid to do the same work in the '90s.

Give Justin at ATV a call, I'm fairly sure he'll tell you who supplies him with the specific antennas?

Guildford is now effectively a main transmitter (one of the 81 original DTT sites) - dual line fed (a 'daughter site' of CP), duplicated transmitters etc.,.).
CP will of course be far more resilient as it also has Croydon as a backup in the event of a disaster. Not sure if CP is a staffed maintenance base, post the Arqiva takeover.

https://www.freeview.co.uk/corporate/platform-management/planned-engineering-works lists an awful lot of main transmitters with the same 'Pixelation or flickering on some or all channels' effect. That suggests to me that it's nothing major works-wise but something being upgraded/tested/checked out all round the UK. {e.g. fire safety stuff following the Bilsdale incident}.
It would be quite unusual for testing/switching to be done overnight, rather than during daytime (and disrupting reception for more than a few minutes at a time would be a complete no-no). As your night-time recordings have been unwatchable there must be some other explanation, with the planned works a coincidence.

In your shoes I'd be fitting F-plugs to make swapping/removing the amp easy - insertion losses are minimal with them - to permit the full range of tests and measurements to be made. Note the PSU-splitter you use has an insertion loss of -3.5 dB due to the split.

Wolfbane guesstimates 55dBuV/m at 7m agl from CP and that is diffracted off the ridge some half to 3/4 of a mile towards CP it seems. I used 7m as the antenna is on the side of the house rather than 10m for a rooftop antenna; 10 m agl gained a couple of extra dB.
 
Rodders53 said:
It would be quite unusual for testing/switching to be done overnight, rather than during daytime (and disrupting reception for more than a few minutes at a time would be a complete no-no). As your night-time recordings have been unwatchable there must be some other explanation, with the planned works a coincidence.
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Not with us. The smeared recordings were generally from normal evening viewing hours.
As I said before, I'd wait until the notifications have been gone for a couple of weeks before rejigging the receiving apparatus. I wasted a lot of time and effort for nothing gained, except a little knowledge. My system is now fine again and exactly the same as before I attacked it.
 
An update on this saga. The replacement PVR is just as badly affected as the original one, so this isn't a problem with the PVR. Some services have again suffered bad picture/sound break-up throughout the week, including on Tuesday evening (so my recording of Thunderbirds on Talking Pictures TV was trashed - first time this has happened).

With bad break-up again this morning I fired-up the Digiair Pro T2 (fed from the loop-through output of the in-service/spare) PVR to discover the following:

UHF 22, 23, 25, 26, 28 (these are all DVB-T, 64-QAM modulation): poor post-error correction BER
UHF 30 (DVB-T2, 256-QAM): post-error correction BER ok and solid at <1.0E-08
UHF 35 (DVB-T, QPSK - aka 4-QAM): post-error correction BER ok and solid at <1.0E-08

QPSK = Quadrature Phase Shift Keying; QAM = Quadrature Amplitude Modulation.

So it seems that our problem services (when the problem presents) are those carried on the 64-QAM channels (COM5, PSB1, COM4, PSB2 and COM6).

More on digital modulation, constellation diagrams, etc at https://en.wikipedia.org/wiki/Constellation_diagram
 
Newcoppiceman said:
...
UHF 22, 23, 25, 26, 28 (these are all DVB-T, 64-QAM modulation): poor post-error correction BER
UHF 30 (DVB-T2, 256-QAM): post-error correction BER ok and solid at <1.0E-08
UHF 35 (DVB-T, QPSK - aka 4-QAM): post-error correction BER ok and solid at <1.0E-08
..
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I don't understand a lot of the tech discussion of transmission/reception in this thread. But doesn't that summary suggest most channels should be affected? (UHF 30 has the few HD channels, and I think UHF 35 has the handful of local channels). It should affect your main SD channels in addition to TalkingPictures TV. Have you discounted recently introduced (electrical) devices that may emit rougue EMI?
 
Black Hole said:
Just a cautionary observation: correlation does not disprove coincidence. That has bitten me before.
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The puzzle now is why the lounge TV (yet to check fully the upstairs one) appears immune to the woeful/varying BER measured by the meter - it had no problem displaying Talking Pictures TV this morning at the same time as the PVR's pic was riddled with break-up (this was after I had removed the aerial feed from the meter and plugged it into the TV).
 
bottletop said:
doesn't that summary suggest most channels should be affected? Have you discounted recently introduced (electrical) devices that may emit rougue EMI?
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I think they are - when the problem is present; it often isn't - that's why, for example, my Saturday (15:00) and Tuesday (19:55) recordings of Thunderbirds throughout Oct were clean.

Will consider the rogue EMI possibility, but unlikely I think.

The various levels of QAM represent different trade-offs between the robustness of the transmission channel (lower QAM number, the more robust) and data throughput (higher QAM number, the more data).
 
Signal levels measured on all those frequencies? NB you have cascaded another amplifier (the -T2 RF pass-through) in the circuit.
No chance this is related to the transmit mode unless you have two PVRs with the same fault ;) - especially as other tuner-receivers are OK.

How are you feeding the TV at the same time as the -T2?
Although error correction soft/firmware may have improved significantly in the 10 years since the -T2 was designed?

Have you read all of the Rowridge / Aura issue that has been mentioned? https://hummy.tv/forum/threads/picture-break-up-on-aura.11024/ as there are similarities? I also provided some -T2 readings here off Sandy Heath with attenuators. {An aside, I had some signal issues recently that were cured by removing the splitter and using the passthrough to the TV... I think some dodgy barrel connectors were involved.}
 
Rodders53 said:
Signal levels measured on all those frequencies? NB you have cascaded another amplifier (the -T2 RF pass-through) in the circuit.
No chance this is related to the transmit mode unless you have two PVRs with the same fault ;) - especially as other tuner-receivers are OK.

How are you feeding the TV at the same time as the -T2?
Although error correction soft/firmware may have improved significantly in the 10 years since the -T2 was designed?

Have you read all of the Rowridge / Aura issue that has been mentioned? https://hummy.tv/forum/threads/picture-break-up-on-aura.11024/ as there are similarities? I also provided some -T2 readings here off Sandy Heath with attenuators. {An aside, I had some signal issues recently that were cured by removing the splitter and using the passthrough to the TV... I think some dodgy barrel connectors were involved.}
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There is a danger of T2 confusion here! (i) HDR-Fox T2; (ii) DVB-T2; (iii) Digiair pro T2 (the meter).

This morning was very much a familiarisation session with the new meter. I haven't (yet) quoted signal levels as they would have been, as you say, with the PVR's pass-through amplifier included. Oh for a circuit/block diagram of the PVR. Not sure if its aerial input is split two ways (tuners + pass-through) or three ways (tuner 1 + tuner 2 + pass-through). Lid off, the two metal cans in the PVR look like the two tuners - but, of course, one has an input and the other the pass-through output at rear. The pass-through must be active as there is the option to switch it off in standby (to save what must be a tiny amount of power).

I'm taking the meter as the reference here - this morning it said all five 64-QAM muxes were u/s with poor/varying post-error correction BERs; the other two muxes it said were ok. The lounge TV is old ~2010 vintage (LG 32LE5300).

We've always used the TV as a monitor and haven't given it an aerial feed - until now, while investigating this problem; its feed is the pass-through output of the in-service/replacement PVR. This feed is sometimes used for the original PVR (set-up side-by-side with the other one) or, now, the new meter.

I will be taking more measurements over the weekend, including meaningful level measurements (ie raw aerial/preamp input and PVR input).

I will check out that thread you mentioned - thanks.
 
Pic showing UHF 22, 23, 25, 26, 28 and 30 muxes. The 64-QAM muxes' post-error correction BERs have improved since this morning and are now (15:00) varying between E-03 and E-07; the other two remain at a solid <1.0E-08 so very definitely QEF (Quasi Error Free).
 

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Newcoppiceman said:
Not sure if its aerial input is split two ways (tuners + pass-through) or three ways (tuner 1 + tuner 2 + pass-through).
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The only sane design would be to amplify first (usually they seem to use just a bit more gain than necessary) and then split three ways.
Now you have a meter you can tell us how much there is between input and pass-through!
 
Black Hole said:
Are you able to calibrate the Humax signal strength indication, in terms of uV per percent?
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I have a couple of in-line attenuators (6dB and 12dB, together 18dB) but to do the job properly I'd need a calibrated switchable RF attenuator with 2 or 5dB steps (time to head to eBay). The Humax signal strength indication might well be highly non-linear whether it's showing voltage or dBs.
 
prpr said:
Now you have a meter you can tell us how much there is between input and pass-through!
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Using UHF 30, unplugging feed to in-service/replacement PVR and connecting to meter: 65.5dBuV; reconnecting to PVR and using short, good quality extender lead to connect PVR to meter: 68.0dBuV. So Humax gain is 2.5dB - about what you'd expect.
 
Newcoppiceman said:
I have a couple of in-line attenuators (6dB and 12dB, together 18dB) but to do the job properly I'd need a calibrated switchable RF attenuator with 2 or 5dB steps (time to head to eBay). The Humax signal strength indication might well be highly non-linear whether it's showing voltage or dBs.
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Just how many uV are going in and how many percent that registers will help. It will give us a clue anyway.
 
Newcoppiceman said:
I have a couple of in-line attenuators (6dB and 12dB, together 18dB) but to do the job properly I'd need a calibrated switchable RF attenuator with 2 or 5dB steps
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I just bought some F attenuators - 2, 3, 6, 10 and I already had a 20. It's good enough to interpolate anyway.
 
NB The HDR Fox-T2 signal meter has some strange anomalies in its display of signal level --- see 'tother thread, specifically post #19 {and #27 where I had 0dB, -6, -12 and -18dB on my Sandy Heath signals}.
 
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