Tuning volts but no SNR? (4001)

lagerlout

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Hi guys trying to fix a 4001 stuck in tune, followed TBC's stuck in tune guide but it doesnt really cover the fault I have. I had low volts (5v) on point b of the tuners so replaced the affected tuner.
Now I have 36v on point a and 22 v on point b, but the netid is still **** the upstream power seems to be stuck on 60db. There is no snr at all and no tuner locks.
Checked all the power rails and they seem ok. Swapped the cim but made no difference.
Long shot but does anyone have any idea what could be wrong?
 
The surface mount caps? I didnt because after I changed the tuners the voltage seem to be ok
 
The surface mount caps? I didnt because after I changed the tuners the voltage seem to be ok
Change the caps mate, they are the cause of 99% of stuck in tune faults. Start with C323 and C325, try the box again in e.m. A quick check which often works is to go into the snr reading in the e.m and heat the 2 caps I mentioned and see if the snr starts to rise.

Chookey
P.S not the surface mount caps, the electrolytics mentioned in tbc's tut.
 
Ok will do that mate, cheers.

Only strange thing is though the snr doesnt say 0, it doesnt say anything at all!
 
Another check that should always be carried out with a stuck in tune 4001 is to measure the voltage across C323. Expect 3.3v but I have had 2 x 4001's where this reading was little over .5v. In both cases a smd capacitor was at fault and I have documented this in a previous post. It isn't in tbc's tut as it is a recently found cause of stuck in tune.

Chookey

Chookey
 
Replaced all the caps in the tut but still no good, seem to have a short on c402.
It was bulging soon as I tried it! removed the smd cap across it but its the still the same :(
 
Replaced all the caps in the tut but still no good, seem to have a short on c402.
It was bulging soon as I tried it! removed the smd cap across it but its the still the same :(
Here is a bit of the circuit in case you don't have one. I'm away to get a chinkie.

Chookey
 
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do you have a mute error card in or out?
 
No mute error no, just that there is no number where the snr should be.

Dont think I'm going to be able to find this short, I had a mute error box with a short on the 12v line months ago and I never did find it !
 
there are a number of smd caps under the broadcom chip that can cause this kind of problem. on some of the signal pages i seen some that just had numbers missing like freq ect. you will have to check all the caps under that chip. it sits near the card reader and is a bga type chip.


tbc
 
Ok I'll try that, cheers. Can I do that with them still on the board?
Could this also cause a short across c402?

Edit : just checked against another box and this also reads short.
Maybe its meant to be like that?

Checked all the smd caps under the broadcom chip (without removing them)and cant find anything obvious no shorts that I can see
 
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Ok I'll try that, cheers. Can I do that with them still on the board?
Could this also cause a short across c402?

Edit : just checked against another box and this also reads short.
Maybe its meant to be like that?

Checked all the smd caps under the broadcom chip (without removing them)and cant find anything obvious no shorts that I can see
Homerj, check D8 in the power supply for short circuit. Then check L30 for 5.7v, this feeds the 5vfe supply to C402.

Chookey
 
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Checked D8, thats ok. Cant check the voltage of l30 though I've removed all the caps I put in and also tun400. Will have to get it all back together and retest. Cheers for the help guys ha a few too many now to do any more tonight :)
 
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