Issue with GE "Portacolor III"
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This set's been a real thorn in my side, so I posted to the classifieds looking for a set to swap parts with, and it turns out there were actually a lot of sets that used the same chassis, only problem is they are apparently unobtanium. Anyways, my scope is saying I'm not getting any video out of the IF amp ic, (nte1545) but I've replaced it twice, and still no signal. It's got the classic "clean" raster (no snow) symptom of a bad detector, and when I inject composite video into the first video amp, there's no black and white video, just color. I suspect this is because composite is 1v peak to peak and the amp expects 2v. I've ordered another replacement IF amp chip, as the other two were old stock. I'm a little perplexed at this point, as I don't see how two IF amp chips were bad, although they were from the same seller.
Attached is a pic of this section of the schematic. The little numbers scribbled on the chip are voltage readings I took, and you probably can't read them as the file size was reduced, but the only thing of note was that pin 3 was supposed to be .7v, but is 4.5v, and I can't figure out why. |
Pin3 would vary with R150 setting AGC. Try cranking on R150.
But I'd re-solder the entire area first. |
Resolder all of the griplets first from the top of the PCB. Usually if
griplets are the problem you can rap the chassis and make it come & go. Beyond that measure voltages & waveforms again. Since you have color & sync you know there is composite inside the IC. 73 Zeno:smoke: LFOD ! |
I have actually run a wire through all of the 50 odd griplets that act as vias... I re-soldered every joint in the IF amp area, and there is still basically no signal coming out of pin 5, (if there is, than it's like .005v pp) If I bang and bop the set, or rap on the board, it seems pretty consistent in it's current state. When I turn the agc control (r150) pin 3 actually goes from 5.27v to 5.44v and seems very consistent, just 5 volts high...
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Do you know for sure that the tuner is working?
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Having thought about it, I took the output of the broken set's tuner (hooked up to dvd player) and jumpered it to the working set's tuner input, and the signal doesn't look half bad, a little lossy, but alligator clip leads across the bench don't help, so.... I think the tuner works.
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If it's easy to get to without help, I'll check it out and PM you in a few days. I'm glad I quit the repair business 30 or so years ago. I don't miss those headaches. |
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OK first give a detailed description of the symptoms, I think I am
missing something here. Do you have a color picture that goes away to blank white when the color control is turned all the way down ? AGC When there is no signal or very weak signal both the RF & IF are turned on to full gain. With strong signals gain gets turned down. SO AGC voltages can vary a lot. Most SS sets the AGC control will go from snow to normal to a jittery pix as you turn it. Set it to where the snow just goes away.. TUNER Most tuner fails are snowy pix or whited out & wormy. Crude test is turn the channel. You should see "noise" in the pix & crashing in the sound. If you do the problem is tuner related. If not its IF related. Keep in mind sometimes tuners get so dirty they stop working unless you violently shake the knob ! 73 Zeno:smoke: LFOD ! |
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It's a model # 10AB3046W in decent cosmetic shape. PM me with your phone number, as I'm not that fast on a keyboarder. Phoning is easier. Dave. :thmbsp: |
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check your delay line
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You may try lifting pin 12 & see if you get 1-2V P-P of video on the
pin. You could also try hooking in composite video at TP 6 from a DVD. Just be sure to use isolation ! I just dont see having a run of bad ICs but its not impossible. If it were an expensive power pack then I would suspect bootleg Chi-Comm crap. 73 Zeno:smoke: LFOD ! |
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