You mentioned a red light. That's normally an indication that something is On, and sometimes also indicates an "ON and working correctly" status. It certainly is the way with the masthead power supplies I use; so, with that in mind, and the fact that you've got fewer channels not the red light is out, I would suggest that you may have introduced another wiring fault.
To troubleshoot this, do the following.
1) Disconnect the amp from the mains - Gives it opportunity to reset, and it will only be off for a minute or two - Unplug the connections out to the TVs/rooms
2) Reconnect the mains power - Do you have a red power light? - If the power light didn't come on, check the fuse, also try disconnecting and reconnecting the feed from the TV aerial
3) Reconnect one output to a TV/room at a time - Not the status of the red light at each stage - Stop when you get to the one that causes the light to go out - This is the plug or cable with a short - Fix it
It's easy to introduce a short when fitting a new aerial plug if just one of the strands from the braid shield touches the centre core. The filaments are thin and sometimes hard to see in less-than-ideal light.
Losing some channels but not all could indicate a few things. My first thoughts would be signal overload from some muxes (too much amplification - try an attenuator to knock the levels down), or poorly-shielded cable is acting as an aerial itself.
If I was on-site with my meter, I'd be checking the direct feed from the loft aerial, then plugging that in to the distribution amp with no outputs connected and checking each output in turn. If the dist' amp turns out to be faulty, then replace it. Once I know I've got good signal in the loft, then I'd check each aerial outlet in the various rooms. I'd finish off with the one that is the problem. Where the signal is good in other rooms but bad here, I'd bypass the aerial wall plate (in case its not shielded) and instead I'd put a plug end directly on the downlead and test from there. If that's still bad, then it's time to replace the downlead to that room.
Without a good meter - not one of the £10 ones, they're useless - and not even one of the better Fringe versions - then the most useful device you have is the Freeview tuner in a TV. Make sure it's on Digital, and then use the Manual Tuning option. Where you know which local transmitter your aerial is aligned to, then you can do directly to the frequency channel numbers for your transmissions. If not, then you can change up a channel number at a time.
Most TVs will give some kind of indication about the signal status. Generally it's signal Strength (not so important) and signal Quality (very important). Bear in mind that the TV will often overestimate on both, but it's still more useful than a £50 domestic signal meter.
A smaller TV taken in to the loft for testing purposes could tell you pretty quickly whether the troublesome room output works direct off the splitter or not; and if it does, and you have a Quality reading at 70-80%/Good then you'll know it's time to change the downlead.
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