Satellite tuner repair experience

[Jane]

Does anybody here understand how   hardware of tuner works?
As far as I  understand tuner sends output voltage( 13V/18V) to LNB(installed  in
satellite dish) and from LNB , input voltage  comes to tuner chip then
to demodulator chip and to data bus and those data goes to CPU of the
satellite receiver.
I have one tuner module that has signal ( measured by
oscilloscope) on data bus but sattelite receiver shows no picture, so the
data on data bus is probably decoded in a wrong way.
So, can anyone suggest a procedure how to find out which part( electronic
chip) is faulty?
Is there any literature about tuners?
 Thanks for feedback

[EMUD]

Hi,
here a little about this things:

1. The lnb in the dish is a frequency converter. It converts the incoming frequency band (about 11Ghz - 12,5GHz) to to 500-2000MHz. There is an electronic switch inside to switch between the vertical and the horizontal position. This switch could be controlled in many ways:

- by voltage (13 bzw. 18V DC)
- by a frequency of 22KHz sent to the lnb
- by diseqc, the receiver sends digital data via the coax to the lnb

The dc on the coax is used to supply the lnb, too.

The receiver has an internal converter to convert the needed frequency band to the range of its computer.
The signal is decoded an leaves the receiver to be heared or displayed.

There is no sense to try reading the databus with a scope. You should check

- is there a on screen message (this could proof that the computer and power supply is ok).
- does the search for chanels start?
- is the 13/18V ok and switchable?

If you would like to have a closer look inside a trainig handbook for tv repair could be helpfull.
 

EMUD

[babysitter]

Better problem description needed!

What picture on which output, error messages, other diagnostic relevant data ? (screenshots welcome - usually you can get a screen where it tells you the transponder frequency, symbol rate, error rate and RSSI.)

Listening to the data bus makes no sense (in fact negative sense as you can screw it up) in this state.

To debug the receiver, completely erase the transponder list and do a full search or preset to your sattelites known channels, frequency and symbol rate wise.
First:
Does another receiver work at the given LNB? Does your suspect receiver work at another LNB ?

[Jane]

Thank you for the replies.
I can use a different tuner with the same mainboard of the satellite receiver  and it works. So, the problem is with tuner only.
There is no picture coming from LNB, regardless of transponder  frequency.
My idea was to check voltage testpoints( that is a path : input coaxial  connector,input  tuner chip , output  tuner chip, input demodulator chip, output demodulator chip... .
Does it make sense?

[bithead9]

Checking inputs and outputs of stages is a good place to start.  As well as voltages on those stages.  Do you have a schematic to help with that ?  Otherwise you will need to get the chip numbers and data sheets, working backwards using that information.  What is the mfg. and model of your receiver ?   

[sunnyhighway]

I can use a different tuner with the same mainboard of the satellite receiver  and it works. So, the problem is with tuner only.

I know of some sat-tuners which cannot support all type of plug-in tuner configurations in their slots. Did you check the faulty plug-in tuner in a slot that worked with the other plugin-tuner?

[Towger]

Check the power supply voltages are correct and clean. Bad caps are very common on these after a few years, and can manifest as no signal etc.

[SeanB]

A common failure of older digital tuners is the ADC chip dies, as it runs really hot, and often it is poorly heatsinked and the long term overheating kills it. Typically a small BGA or QFN chip with a heat slug under it soldered to the board. They typically run at a temperature where you cannot touch the chip without burning yourself, and often a modification of a fan and a small heatsink is added to the unit.