Re: I think I need channel filters - ok fixed it

As part of a larger ongoing project that will see a fair bit of additional wiring in the house, I created a new entrance into one of the areas of roof that had been voided off (this also opens up masses of extra storage space).

This provided access all along one side of the roof and a chance to get close to much of the coax. All I needed was someone skinny enough to fit through the small triangular bit of roof space under each of the dormer windows. Fortunately today my mate Martin came to visit and was coerced into void exploration mode!

So a nice new CT100 co-ax feed from the aerial itself is now in place along with replacement runs to some of the less well performing sockets.

Andy Wade wrote:

John Rumm wrote:

Looking at the feed to the distribution amp (with the gain on the masthead set to minimum) we now get:

Channel Signal Mux
(dBuV)
54 48 D5
56 48 D2

51 74 BBC1

So an A-D level difference of 26 dB. The 'operating window' here is quite small - starting from those levels you'd need the overall downstream gain (D/A gain less cable losses etc.) to be between -3 and +6 dB to keep to recommended signal levels at the outlets.

Which are at least 20 dB stronger than the digital muxes. (if you ramp up the gain on the masthead I can push these to over 99 dBuV - which is where my meter gives up!)

The proMHD11V is OK up to over 100 dBuV o/p though, so turning the gain up and using a passive network may be quite attractive. Hang on though,

Yup, this is the way I have gone for the moment. I am driving four outlets via a passive splitter, and have the masthead amp on close to max gain. The digiboxes (Netgems) are reporting approx 40% signal strength (Toppy says 99% on most muxes), but channel quality is at least 85% or better. SNR is typically between 22 and 26 dB on all channels, with BER below the limit of the box to measure in most cases. (the occational burst up to 1 in 1x10^6 - but within the FECs capability to handle it). Video and audio seem "perfect" (well as good as you can get for over compressed MPEG2 stuff!)

That will probably do until such a time as I need to bring more of the sockets back into commission. At which point I was thinking something like the Proception AP01828 or AP01824 would probably do the job nicely.

Would the single AP01838 PSU cope with the amp ad the AP01824?

something doesn't quite add up: the gain range of the preamp is around 15 dB (7 min. to 22 max.) - so as you're getting 74 at min. I'd expect about 89 at max., not 99+.

That's 75 measured at the output of the aerial itself, not the masthead amp. (74 was what I was seeing at the end of the soggy co-ax from the aerial with the amp on minimum gain)

Note also that these measurements have been taken over a few days and we are nearly 30 miles from the Tx - so there is a bit of variation in them anyway. Finally note that beyond 90 dBuV is over spec for accurate measurement range on the meter.

I have come to the conclusion that the labgear is crap anyway.

What model and vintage of Labgear amp is this? The only significant

CM7296 - not sure of the age.

problem with the CM729x or MSA series 'white amps' was that they took to using stupidly cheap electrolytics for the PSU reservoir capacitors, with the inevitable result that the caps dry out and lead to a mains hum modulation problem, clearly visible as a hum bar on analogue. That's

The visible trait on analogue was a vertical bar roughly in the middle of the screen about 20% of the frame width, which showed more noise. There was also a horizontal portion about 10% height starting 4/6ths down the screen. This gave an inverted "T" effect.

I might have it apart in a bit, and investigate.

simple to repair of course (470 uF / 25 V, IIRC). If the gain is low and noise figure high it's usually because the input's been zapped by lightning, in which case throw it away (oops, sorry, I meant recycle it at your nearest WEEE DCF).

That would be the grey one one wheels in the garden then ;-)

and the house co-ax wiring leaves much to be desired.

Therein lies your problem, I agree.

Yes - it was quite noticeable how the original coax from masthead to splitter managed to lose somewhere between 5 - 7 dB across the board and gently roll off a bit more with increasing frequency. As soon as the new cable was in there I could not really detect any loss. (as you would expect for such a short length). I get the feeling there were some interesting notches in its response as well, since even when decoding all muxes, there were a small handful of channels that were suffering high error rates even when others on the same mux were apparently fine.

(looks like the aerial rigger was not the only one being lazy - the sky installer had found another bit of old co-ax to reuse as well - at least he joined the cable with a couple of belling lee's and a barrel connector, and tucked it under some lead flashing! Since the sky box seems happy enough with that, I decided to leave that for another day, and just took the precaution of wrapping the join in self amalgamating tape and tucking it back under its lead umbrella)


--
Cheers,

John.

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