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Help locating noise source, horrible TV reception.

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Renesis

Sun Mar 07 2010, 10:46PM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

Hi.

My fave CRT TV's aerial input took a dive on me a few months ago. I came home from school one day and couldn't find any channels, only noise. I didn't mind, i had better things to do anyways. Well that was then.

So today i hooked up my old Super-Nintendo to the TV just to see if the problem could be with the cable provider. For those who doesent know, Super-Nintendos use the aerial input, and have a relatively powerful transmitter. Everything i could see was a grey Mario-like figure trapped in an intense blizzard, and some bleeps of sound now and then. Which should mean that my rf receiver is fine, i just have a noise problem.

But since the TV isn't hooked up to an aerial or cable, and i know the nintendo works brilliantly, then the noise source must be within the TV's electronics? Has anyone encountered anything like this before, or dare take a guess at what might be the cause of my little problem?

I should point out that with my Xbox using composite cables everything works beautifully, and that my roomate has no problems with the reception on his flatscreen just across the hallway.

Thanks a lot.

Proud Mary

Mon Mar 08 2010, 12:28AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

All sorts of things could cause the symptoms you describe - f. eks. a fault resulting in extremely poor SNR, rather than interference.

The first signal amplifier may have failed, or the co-axial socket connection to the board . A sufficiently strong signal (such as your Super Mario game) could leak past a failed first signal amplifier, or cracked board at these frequencies.

The AGC may have failed. The first LO may be doing its own thing.

There is no sure way to find out what is wrong without systematic signal tracing, from back to front, until you find the faulty stage.

radiotech

Mon Mar 08 2010, 01:36AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Assuming firstly that your TV is the old analog type, the kind that Nintindos worked on : What channel does the Nintendo work on? Are you connecting it with the cable switchbox that comes with the game, and is that box one that switched from game to aerial? Is that switch set correctly?

Coherent

Mon Mar 08 2010, 06:35AM

Registered Member #1886 Joined: Sun Dec 28 2008, 02:55AM
Location:
Posts: 73

If you're feeling adventurous and love DIY repairs then go for it. Just don't get your hopes up on finding an easy fix.

I'm not sure how common CRT tv's are in norway, but it would probably be a lot less hassle to just get another crt tv unless the one you have is particularly collectible or nice. Here in the us you can get nice big screen crt tv used for like $50. You can find them free even. Everyone's upgrading to flatscreens and they're all dying to get rid of their old crt tv's.

Renesis

Mon Mar 08 2010, 11:48AM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

Proud Mary:
Thank you for your reply. I have to admit i dont really know to much about the theory behind these receivers, so i'll have to do some googling on the subject. You're suggesting that my problem might not be too much noise, but too weak signal or amplification? That makes sense, since the nintendo is the only signal source strong enough to be detected.

radiotech:
I cant remember what channel it uses; im at school right now. But it has been used on that tellie before, and it is connected correctly with the original switchbox that came with the game. Wether or not the switch is set correctly, im quite confident it is, but i cant say for shure until i get home.

Coherent:
The most logical option would be to scrap that old thing and get a new flatscreen, mostly to free up space in my tiny bedroom, but there's just something about these CRT's. People have too much money now so nobody bothers trying to sell their old TV's, they just throw them away. And i cant go dumpsterdiving for another one of for spare parts, because thats not allowed anymore. Beause everything have to be recycled, no matter what.

GeordieBoy

Mon Mar 08 2010, 12:34PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Practically anything could be wrong in the RF strip of the TV. Without a good working knowledge of RF design, a circuit diagram and a way of injecting test signals you are not going to be able to fault find effectively.

Why not get a component or composite video cable for your Nintendo box if this type of connection works fine?

Also if you have an old-fashioned analogue VCR lying around then try connecting that to your TV via a SCART or composite video cable. Then you can use the analogue tuner in the VCR to pull in your favourite TV shows and RF from the Nintendo.

You are essentially using the tuner of the VCR to bypass the knackered one in the faulty TV, but it's a lot less hassle than actually opening the TV and replacing bits!

-Richie,

Proud Mary

Mon Mar 08 2010, 03:07PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

If you could examine only one thing, I would suggest you look at the connection of the co-axial RF input socket to the PCB. In consumer goods, co-axial sockets are
often directly soldered to a PCB, so that all the stress on the socket is transmitted into the soldered joints on the copper traces, which sometimes fail as a result.

Renesis

Mon Mar 08 2010, 03:39PM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

I dare assume the circuits of interest are found on the inside of this shiny metal case, which has 10 pins connecting it to the PCB. Is this a standardized package with a standard pinout, so that i can swap it if i happen to come across a scrapped TV with a similar unit?

GeordieBoy:
I could use a VCR, but for now that will be plan B. Simply because i dont have a VCR... The Nintendo is used only for testing, my main interest is viewing cable TV. Since i am paying for it i might as well use it.

Proud Mary:
I can neither see nor measure any problems with the solder joints on the main circuit board.

Proud Mary

Mon Mar 08 2010, 04:18PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Renesis wrote ...

Yes, the box contains the RF input tuned circuit (usually striplines tuned by varicap/varactor diodes) the first Local Oscillator and the Mixer, so what comes out of the box is at the First Intermediate Frequency. I'm sure you're right to think that the same RF module is probably to be found in products from other manufacturers.

Renesis

Mon Mar 08 2010, 04:33PM

Registered Member #2028 Joined: Mon Mar 16 2009, 08:13PM
Location: Norway
Posts: 319

Proud Mary wrote ...

Yes, the box contains the RF input tuned circuit (usually striplines tuned by varicap/varactor diodes) the first Local Oscillator and the Mixer, so what comes out of the box is at the First Intermediate Frequency. I'm sure you're right to think that the same RF module is probably to be found in products from other manufacturers.

I'll be looking for a replacement then. I can see one of the ten pins is labeled AGC, which you mentioned in your first post. What voltage level should i expect to see on that pin while the TV is running?

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