Unusual problem with RF output from Sky Digiboxes

MiketheMechanic

I'm having an unusual problem with the RF output from my Sky Digiboxes. Maybe someone can suggest a fix?

Here's my current setup:

Leg 1: UHF antenna --> Triax masthead amp --> Triax power supply i/o --> Labgear mains-powered six-way distribution amp --> AERIEL IN on 4 Digiboxes in central upstairs location.
Leg 2: Sky Dish with quad LNB --> Sat IN on Digiboxes.
RF OUT2 on Digiboxes running to downstairs TVs equipped with magic eyes.

Problem: Terrestrial Channels on TV are all near-perfect.
However, the Sky Digibox output is rubbish when output on Channel 68.
Moving the RF output channels down through the range to 21 gives slight improvement, but still very poor (looks like very weak UHF reception).

Notes:
CT100 cable used throughout.
Cable runs from Digiboxes to TVs are not excessively long, all >10metres.
Bypassing the Labgear amp gives poorer Terrestrial channels, but near-perfect Sky Digibox output.
This happens on all 4 digiboxes and on all cable runs.

Before I buy a new amp I want to try to understand why this is happening. Any ideas?

Thanks in advance.

MtM

zilog_jones

I dunno much about the digiboxes, but it sounds like it doesn't need amplifying. Can you try connecting the box's RF output somewhere after the amp?

MiketheMechanic

Hi ziliog_jones.

The digibox IS after the amp.
The UHF is being split 4 ways in a marginal reception area - so it will need some help.

MtM

conax

It is possible that the Labgear amp is excessively noisy but I have generally found them to be OK. However if you have a masthead amplifier before the labgear dist amp you are probably overloading the dist amp with excessive noise.

If this is the case you can either get a better masthead amp and split the UHF signal to the 4 boxes with a passive splitter or place an variable attenuator prior to the labgear, note if the labgear dist amp is providing bias voltage to the masthead amp you may also require a masthead power supply.

If you don’t have a masthead then it may be in order to put a UHF/UHF group diplexer prior to the input of the dist amp. Using one leg only on the diplexer to match you current group of UHF channels and the leaving the other unused this will help reduce the noise in the group unused by the diplexer. Then place the output of the digi boxes to match the unused group from the diplexer.

Eg if the UHF channels are on group CD use the group CD pass on the diplexer prior to the dist amp and place the digi box outputs on channels within the AB group.

MiketheMechanic

Thanks conax.

yes I do indeed have a Triax masthead amp and power supply before the distribution amp. sorry I missed that in the description of my setup.

conax, would this not make the terrestrial channels noisy also? Why just the digibox output? The terrestrial are on Group C/D Ch. 53 to 63 I believe.

MtM

conax

the nosie generated is wide band across the UHF spectrum and not only contained where the UHF carriers are. The amplifier is generating its own noise + amplifing junk from all other receivable transmitters.

The fact that you have got a Masthead amp probably a TA25w or similar would lead me believe that you are also slightly overloading the dist amp .since your terrestrial pictures are OK I would say that you already have a good carrier to noise ratio on the terestrial channels otherwise they would be affected by the overloading of the dist amp,you are close to the threshold of the dist amplifier where cross modulation will take place to such a level as to interfere with the terrestrial channels.

A variable attenuator will solve your problem. but I would try a passive splitter first.

Tony

I agree sounds like an overload problem

MiketheMechanic

Thanks for your input guys (no pun intended!). I will try this.

Any recommendations for where I might find a 4 or 6 way passive splitter for UHF?

Also, the Triax power supply for the masthead amp has 2 outputs - (TV1 and TV2). Is this a built in splitter?

MtM

conax

yep it got a spllitter built into the amp, if you can drop another lead from the amp and use 2 X 2 way splitters a 6 way splitter may be too much for the amp to drive and group CD losses would be high(i estimate that the amp is throwing out around 75-80dB) if you need the 6 points put the attenuator before the dist amplifier.
Splitters can be got at most DIY stores or try satellite.ie

MiketheMechanic

Cheers conax. I will try it out with 2 "Y" splitters and see how the signal fares.

If its ok, I might try one 2-way and one 3-way for my 5 feeds.

Thanks again to everyone for the help.

Mike

Tony

i'd avoid using Y splitters unless you want big attenuation, they are rubbish and prone to interference. They may also give you too much or too little attenuation, I always use a variable attenuator in these situations to get your levels exactly right

MiketheMechanic

Thanks Tony.

I am going to try 2 different fixes in my setup to see which works best.

1. Get rid of Labgear dist amp altogether, and split each of the 2 outputs from the Triax masthead to give the requried number of feeds, at still hopefully acceptable quality levels) to Aeriel In on Digboxes. [I know that bypassing the Labgear amp straight into 1 Digibox already works well]

OR

2. Try using a variable attenuator between the Triax PSU and the input to the Labgear dist amp (since it looks like I may be overloading it) Hopefully the resulting signals from the Labgear amp and hence Digiboxes will be cleaner.

Is it the use of Y splitters on option 1 above that you think is a bad idea?

MtM

Tony

Yes Y splitters are poor quality. Can you tell me again in what order devices are connected in your set up?

MiketheMechanic wrote:

Thanks Tony.

I am going to try 2 different fixes in my setup to see which works best.

1. Get rid of Labgear dist amp altogether, and split each of the 2 outputs from the Triax masthead to give the requried number of feeds, at still hopefully acceptable quality levels) to Aeriel In on Digboxes. [I know that bypassing the Labgear amp straight into 1 Digibox already works well]

OR

2. Try using a variable attenuator between the Triax PSU and the input to the Labgear dist amp (since it looks like I may be overloading it) Hopefully the resulting signals from the Labgear amp and hence Digiboxes will be cleaner.

Is it the use of Y splitters on option 1 above that you think is a bad idea?

MtM

MiketheMechanic

Hi Tony,

I've edited my first post. This should accurately reflet my setup.
Not too sure which Triax masthead was installed, but conax is probably right in his guess of a TA25W.

MtM

Tony

MiketheMechanic wrote:

Hi Tony,

I've edited my first post. This should accurately reflet my setup.
Not too sure which Triax masthead was installed, but conax is probably right in his guess of a TA25W.

MtM

If you unplug the aerial feed into the digibox does the picture become clear, if so you need to attenuate the input into the digibox IE aerial in

MiketheMechanic

Hi Tony.
Yes. I tried that. The Sat RF out of the digibox on Ch. 68 (or any of the channels) is fine on the destination TV with no UHF plugged into digibox.
Downside is no UHF ;-)

Also, the combined solution of UHF and Sat RF output works fine on destination TV if one Digibox is fed directly from the Triax masthead PSU connector (bypassing the Labgear dist amp completely).

MtM

Tony

Then as I said dropping the level of the uhf should cure it, also try unplugging the mains supply for the masthead amp and see what happens.

MiketheMechanic wrote:

Hi Tony.
Yes. I tried that. The Sat RF out of the digibox on Ch. 68 (or any of the channels) is fine on the destination TV with no UHF plugged into digibox.
Downside is no UHF ;-)

Also, the combined solution of UHF and Sat RF output works fine on destination TV if one Digibox is fed directly from the Triax masthead PSU connector (bypassing the Labgear dist amp completely).

MtM

MiketheMechanic

Thanks Tony.

It would seem that the UHF levels coming out of the masthead amp are fine for the Digibox when connected directly, (this works ok) but may be too strong for the input to the labgear dist amp.

It looks like the conlusion we have arrived at is the combination of both amps is overloading the digibox with noise - seen on the Sat RF output.

I will try both fixes and see how I get on.

Thanks,

Mike

Tony

Pleasure, let us know how it works out.

MiketheMechanic

Will do, thanks.

MtM