Checking wiring - help please

[Ex-SolentBoy]

This is the new TV amp I am going to fit to the boat.

The left cable is from the aerial and the two sockets on the right are outputs to the TV sockets.

I wanted to make sure that the input from the aerial is working properly, as this is where I had the problem with the previous amp.
To do this I thought I would check the continuity.

Using the multimeter I first checked from the outer (shield?) on the input plug to the outer on both of the outputs. Almost no resistance at all which I think is what I want.
However, when I check the inner core of the input to the inner of the outputs there is no change on the meter. It stays at one, so I assume no continuity.

Is this correct?
Surely there should be continuity?

Please be careful with your answer as my physics is only "A" level and that was some time ago as you can imagine.

 

[PCUK]

"A" level Physics?! In this forum you're the Professor.
Presume you had the amp switched when testing?

 

[lw395]

It needs continuity at Radio (TV) frequencies. That can be achieved with a capacitive connection. It's quite normal for DC to be blocked.
Think of it as a high pass filter on the input.
It is actually helpful to block out low frequencies to stop them loading up the amplifier.
It also stops you damaging the amplifier by applying DC to the circuit!
Equally things like that can be a short circuit at DC but pass high frequencies.

 

[maby]

There's no reason to expect continuity from the input to the output - in fact that would be very surprising and an indication of a probable fault. There's no simple test you can do with a basic multimeter that will prove an amplifier like that - you would need far more complex lab equipment. Plug it up to a TV and antenna if you want to check it before fitting on the boat. Poking around with a multimeter on the inputs and outputs will prove nothing and could possibly do damage.

 

[VicS]

I do not think you can gain any useful info by testing these things with an Ohms meter, gadget switched on or not without reference to a circuit diagram.

I'd sort of expect the readings you are getting.

 

[Ex-SolentBoy]

Ok, thanks all.

I will just install it and see if I can watch TV.

I clearly have too much time on my hands at the moment.

 

[yachtorion]

Is it a digital meter? Some of them just seem to show "1" is the resistance is outside of the range.

 

[maby]

Is it a digital meter? Some of them just seem to show "1" is the resistance is outside of the range.

I'm sure that is the case. Don't try turning the range up - admittedly most modern digital meters only have low voltage batteries, but you don't want to be encouraging it to push the voltage up in an effort to get some current through. There will be sensitive electronic components in the path between the input and the output and there's no sense in trying to force a DC current through them.

 

[Dave_Seager]

Logically, if you did find zero resistance between the input and the output then any gain from the amplifier would be lost as the input and output would always be at the same potential.

As has been said, the only practical test you are likely to be able to conduct is to "suck it and see". Use for its intended purpose and see if it works.

 

[VicMallows]

If you are installing the amplifier near the chart table (which appears to be the case), I'm not even sure what you expect to achieve? It will amplify the (presumably low) TV signal along with the noise. I Imagine you will only have a shortish length of cable from amplifier to the TV.

An amplifier is usually installed as close as possible to the actual antenna so as to boost a week signal BEFORE it gets even weaker and noisier through a long coax feed. Or do you simply want to split a reasonable signal between 2 TVs?

 

[maby]

Logically, if you did find zero resistance between the input and the output then any gain from the amplifier would be lost as the input and output would always be at the same potential.

As has been said, the only practical test you are likely to be able to conduct is to "suck it and see". Use for its intended purpose and see if it works.

There are a few obscure microwave amplifier designs using tunnel diodes that would appear to have virtually zero resistance between input and output as far as a multimeter is concerned - but I think it is safe to say that a cheap UHF TV amplifier is not of that type.

 

[Ex-SolentBoy]

If you are installing the amplifier near the chart table (which appears to be the case), I'm not even sure what you expect to achieve? It will amplify the (presumably low) TV signal along with the noise. I Imagine you will only have a shortish length of cable from amplifier to the TV.

An amplifier is usually installed as close as possible to the actual antenna so as to boost a week signal BEFORE it gets even weaker and noisier through a long coax feed. Or do you simply want to split a reasonable signal between 2 TVs?

Thanks. The amplifier is installed at the base of the mast. It splits there and goes to two separate sockets in the boat.

It was working fine before, but the amplifier has failed. I have now bought a new one so hopefully all will soon be back to normal.