Need Desperate Help with Masthead Amp/Power Injector

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pm

78 posts

Master Geek

#22350 24-May-2008 22:05

A noise issue has just been developed on all my VHF/UHF channels. There seems to be a thick horizontal line of noise scrolling down my screen (more pronounced on the UHF channels). On some channels it tends to disappear after 6 or more hours, on others it stays...

This is the setup I've used for my house:
VHF/UHF aerial (with masthead amp) goes to power injector.
Output of injector and Satellite signal goes into Zinwell Freeview Decoder.
RF OUT of decoder goes to 2-way distribution amp.
1 Amplified output goes to the rest of the house and the other goes to my TV next to the decoder.

Simple and its worked for a long time...until now.

I went through each device to find the cause of the problem and narrowed it down the power injector for the Masthead Amp. When I turn off the power injector all the VHF channels become crystal clear (and no noise) but I get NO UHF channels at all.
Turning on the injector causes that line of noise on all channels immediately.

What do I need to replace? The injector or the masthead amp? Its a big job to get on the roof to rip off the injector so I'd really like some opinions.

Thanks.

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43 posts

Geek

#133084 25-May-2008 08:41

What do I need to replace? The injector or the masthead amp? Its a big job to get on the roof to rip off the injector so I'd really like some opinions.

I assume you mean the masthead amp requires getting up on the roof, not the injector.

My guess is that the power supply capacitor in the injector has failed, sending a 50Hz component on top of the DC up to the amp. If you could test the power up the coax to the amp with a AC Voltmeter or oscilloscope (better) you could be sure, otherwise it's just my guess.

pm

78 posts

Master Geek

#133088 25-May-2008 10:44

Yes sorry the amp not the injector is on the roof...
I have an AC Voltmeter and placed that on the output of the injector, is that essentially the same reading I would get on the roof amp (besides a bit of loss)?
The AC voltage on the injector output is: 18V
There was 0 VDC. I would've thought a DC voltage was being sent up...

andrewcnz

961 posts

Ultimate Geek

#133093 25-May-2008 11:30

What kind of power supply is it? Have a look at this site it has commonly used power supplys listed...
http://www.gme.net.au/matv/power_supplies.php

pm

78 posts

Master Geek

#133100 25-May-2008 11:59

andrewcnz:
From that site it is:

PSK08

240V AC to 17.5V AC, 100 mA

PAL

jdbob

43 posts

Geek

#133107 25-May-2008 13:00

I'm surprised they would feed AC up to the masthead amp, but apparently that's what they are doing. In that case you meter reading makes sense. The masthead amp must then convert it to DC, since you can't run an RF amplifier off of AC. Sounds like you are taking a trip to the roof :(

pm

78 posts

Master Geek

#133116 25-May-2008 14:09

I've taken a trip to the roof and thankfully I made it back...

I looked inside the amp and couldn't see any rectification circuit that would convert the AC to DC - All I could identify was a passive looking T network that diplexes the UHF and VHF signals together. I suppose the amplification circuit would be on the other end of the circuit board (against the housing). Anyway, this amp looks to be in perfect condition (almost brand new) ie. there is no rust, dirt or any blown component.
Its really hard taking a voltmeter up there since the aerial is on the tip of a steep triangular roof - it requires balls I don't have! :)

I'm not prepared to call an aerial technician because they would naturally overcharge...I guess there really is no choice but to harden up.

jdbob

43 posts

Geek

#133232 26-May-2008 08:07

Interesting. You should check the connectors on the coax. If there is a high resistance (and it wouldn't take much) then that would put 50hz on the output of the amp too. You could have someone with the meter measure resistance between the center conductor and shield with the coax disconnected from the amp (it should be infinate) and then with the center conductor and shield shorted at the amp end (it should only be a few ohms). It's pretty easy for connectors to get corroded.

Hatch

Trade NZ and US shares and funds with Hatch (affiliate link).

pm

78 posts

Master Geek

#133243 26-May-2008 09:10

Are you talking about the coaxial connectors on the amp? Because they aren't F or PAL connectors - The cable is screwed down, ie. the center pin is screwed to the board and the sheild is clamped to the board too. Those cables also look in good condition too, connectors on the rest of the cable are high quality gold connectors. Maybe when the weather clears up I will take those impedance readings on the amp end.

cyril7

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#133253 26-May-2008 09:46

The active circuitry is on the other side of the board, there is a large electroc cap on the board that you can see without removing PCB, that has possibly dried up. Many of these diplexer/amp combinations only have an amp in the UHF leg.

Cyril

pm

78 posts

Master Geek

#133255 26-May-2008 09:54

I could see a large polyester looking capacitor (yellow in colour) on the front... again that looks in good condition but perhaps it is dried up inside. Although if it were faulty then I would expect this noise issue to be permanent yet it is more intermittent. By the way cyril, whats an "electroc" cap? Is that an "electrolytic" cap?

cyril7

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#133256 26-May-2008 09:59

A polyester cap should go for years, I meant electrolytic, typing not a strong point of mine.

The GME mast head amps (as you sound like you have) send 14VAC up to the amp, you could try getting a 15-20VDC feed and sending that up, this would negate the rectifier in the amp assembly.

Cyril

pm

78 posts

Master Geek

#133258 26-May-2008 10:09

If I send a VDC signal up there, would there be a 1.4V rectifier diode drop (2x 0.7V)? The amp shouldn't care about +/- 1.4V right?

cyril7

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#133260 26-May-2008 10:30

I am pretty sure there is a 12V regulator on the amp or there abouts, GME actaully spec 17.5AC or 14VDC, I dont think they are particularly picky as long as its around there.

Cyril

pm

78 posts

Master Geek

#134613 31-May-2008 15:43

Ok I measured the voltage at the mast and it was 17.8VAC which is correct. So I guess we can safely assume the power injector is ok.

I redid all the strippings on the coax cables at the mast-amp and reconnecting them into their corresponding ports. The problem still exists. It hasn't fixed my noise problems.

I suppose the only thing left is to replace the mast amplifier right? I flipped the circuit board over where the electronics is and couldn't find any faulty components. I played around with the UHF gain but that doesn't do anything besides screw up my UHF reception. I also didn't realise that masthead amps don't amplify VHF they just diplex it with the amplified UHF signal. Is it because the VHF antenna's have a higher gain?

cyril7

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#134622 31-May-2008 16:27

There are only a couple of models that only amplify the UHF leg, you have one of them. Its more an issue of high propagation losses as you move up the bands. I would try without an amp, its only $20 or so for a passive diplexer, if you need an amp add it seperately. While you are at it replace the cable with gel filled RG6 and get a Fconnectorised diplexer rather than saddle and clamp.

Cyril

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