How to confirm a TV tuner is at fault?

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re Teac TV ctm687 This is further to my previous post 17/7/2005.
After performing many tests on the circuit I find the AGC voltage to be
higher (7.84) than it is required at I/C TDA8305A (3.9).
I am not sure what other things to try before actually desoldering the
tuner.
If I can obtain the schematic of the tuner which shows what voltages to
expect at it's outputs and these match the readings I measure on the
tuner whilst still connected to the circuit board, will this confirm
the tuner being faulty?
Any thoughts on this?
Thank you
Al


Re: How to confirm a TV tuner is at fault?



"alitonto"
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 **  Try pulling that AGC voltage down to 3.9 volts or so.

 Use a suitably chosen resistor to ground or maybe a 3.9 volt zener.

 See what happens.




........    Phil







Re: How to confirm a TV tuner is at fault?


Thanks Phil
Although your suggestion sounds interesting, are you suggesting that
tuners should have a voltage adjustements in place?
I am sorry but I am confused as to what is there to gain from trying
your suggestion.
I guess what I am trying to establish is if the higher voltage
appearing at the AGC pin could be caused by another circuit part or IC
etc.
Thanks.

Phil Allison wrote:
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Re: How to confirm a TV tuner is at fault?



"alitonto"

Please do not  TOP  POST !!!!!!!


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 **  Try it anyhow  -   the only way to learn.


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  **   If the pic comes up lovely -  what does that prove ?





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Re: How to confirm a TV tuner is at fault?


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It's a test. If you drag down that voltage to 3.9V and the tuner works, you
know it is not at fault.

Ken


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Re: How to confirm a TV tuner is at fault?


Try the antenna lead its not the TV, if you sit down with all the facts
you have posted it has nothing to do with the TV, stake my house on that


Re: How to confirm a TV tuner is at fault?



Isn't a typical AGC voltage about 8 volts?

Tom

PS - I'm NOT sorry for the top-post. If it fits the screen...

On Tue, 19 Jul 2005 00:31:25 +1000, "Phil Allison"

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Re: How to confirm a TV tuner is at fault?


Why do you think the AGC is the problem and not a symptom?
Do some research to find out what the function of the AGC voltage is
for and how it works?


Re: How to confirm a TV tuner is at fault?




snipped-for-privacy@locl.net wrote:
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Thank you all.
I would like to do this test but I need to check about correct
procedure: would it be proper to solder a 3.9 zener diode with the -
end to the tuner's AGC pin and the + end to the tuner's ground.
I understand the idea is to clamp the voltage to 3.9; is this right?
I will wait for confirmation.
Al.


Re: How to confirm a TV tuner is at fault?


Iv'e had this fault with a similar teac model.. replaced the electro's in
tuner and this fixed problem.  this involves removing tuner from set to do
this fix.
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Re: How to confirm a TV tuner is at fault?


finger to keyboard and composed:

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AGC = automatic gain control

If the signal from the tuner is low, then the AGC circuit attempts to
compensate by increasing the gain of the RF amp. Conversely, if the
signal is strong, then the AGC circuit reduces the gain. Under typical
(ie good) signal conditions the AGC circuit would be producing a
control voltage of 3.9V. The gain is automatically controlled via a
closed loop feedback system that monitors the signal level at the IF
stage.

If you suspect an AGC fault, then disconnect the AGC input and connect
the tuner's AGC pin to a potentiometer via a series resistor, as
follows:


  tuner AGC pin    o---- R ------|
                                 |
                                \|/
                                 |
            tuner ground o--/\/\/\/\/\--o tuner supply (+12V)
                           potentiometer


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

Re: How to confirm a TV tuner is at fault?



snipped-for-privacy@locl.net wrote:
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Hello dkuha
Your comments have aroused my curiosity,
I am beginning to suspect the I/C to be shorted;
Am I in the right direction?
Thaks
AL


Re: How to confirm a TV tuner is at fault?



"alitonto"

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  **  Just keep saying slowly:

"  The rain in Spain stays mainly in the plain  ...... "




............   Phil





Re: How to confirm a TV tuner is at fault?



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Well, fuck me! The looney thinks that he is Professor Higgins!!!!!!!!!!!!

Re: How to confirm a TV tuner is at fault?




Phil Allison wrote:
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How many times do I have to repeated? so far nothing has happened> just
kidding.  I have now realised-thanks to many replies- that my supply
voltage to the AGC is too high, so now I will investigate further in
the right direction and I will report back findings.
Thank you all
AL


Re: How to confirm a TV tuner is at fault?



alitonto wrote:
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Re: How to confirm a TV tuner is at fault?



alitonto wrote:
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 Have you ever thought that it was your antenna or antenna lead. It
plays VCR tapes OK, yet the channels through the VCR are snowy. Duh.
You got a faulty TV tuner and a faulty VCR tuner or a faulty antenna
lead. Guess what its gonna be.


Re: How to confirm a TV tuner is at fault?


Now that you are thinking about it.  The AGC voltage goes high to
increase the gain of the rf amplifiers inside the tuner to compensate
for a weak rf signal.  Most go as high as 9v DC.

If you unsolder the AGC pin off the tuner and feed a direct 4v DC to
it, you will probably find that the signal gets worse as the gain of
the rf amplifer is lower.

If the tuner is in fact tuning all channels correctly and it appears
they are locked on but just snowy.  Odds are very high that the problem
is in the rf stage of the tuner.

Do you have lightning where you live?
If that is a possibility, pull the tuner from the tv, carefully remove
the shields without prying on the internal parts of the tuner.
Visually inspect the front end (where the cable signal goes in).  Look
very carefully for any signs of burnt traces, arc marks, or bad solder
connections to the grounds.  Then do a good visual to the solder
connections to the pins of the tuner and the rest of the grounds.

Unless you have a tv tuner signal generator with if output, there is
little more you can do at your end beyond swapping out the tuner.

Instructor David


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