How to confirm a TV tuner is at fault?

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

Reply to
alitonto
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"alitonto"

** 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

Reply to
Phil Allison

Reply to
alitonto

"alitonto"

Please do not TOP POST !!!!!!!

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

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

Reply to
Phil Allison

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

Reply to
Ken Taylor

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?

Reply to
dkuhajda

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.

Reply to
alitonto

Reply to
Max Harding vk3jin

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

Reply to
alitonto

"alitonto"

** Just keep saying slowly:

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

............ Phil

Reply to
Phil Allison

Well, f*ck me! The looney thinks that he is Professor Higgins!!!!!!!!!!!!

Reply to
Phillip Allison

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

Reply to
dkuhajda

On 19 Jul 2005 08:01:53 -0700, "alitonto" put finger to keyboard and composed:

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

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

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

Reply to
alitonto

Reply to
rrobor

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.

Reply to
rrobor

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

Reply to
rrobor

Isn't a typical AGC voltage about 8 volts?

Tom

PS - I'm NOT sorry for the t>

Reply to
Tom MacIntyre

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