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I gave this little guy another shot Monday. It was a way to rationalize not mowing the lawn on my day off. I had two anomalous resistance readings according to the Sams resistance chart. Pins 1 and 2 of the 6AL5 audio ratio detector (V11) were supposed to be infinity Ohms to ground, but were at 280K instead. I traced it down to the fact that C37 (pictured), a .005 mFd cap was not present. The wiring appeared completely original where the cap should have been. I assume it was a production variation, but I put one in anyway to obtain the infinity-ohm reading at the two pins. The set's behavior did not change. All tube-pin resistance readings are now accurate.
I originally pulled each tube and tested it, if it did not test 100% I replaced it. Stupid me, I didn't check them against the schematic. Three of the tubes were replaced with incorrect replacements. The 6AC7 video amp tube had a 6AG7 installed, the 2nd sound IF amp 6au6 had a 6AG5 installed, and the 6AU6 AGC amp had a 6AG5 installed. I was pretty excited when I discovered this oversight, but installing the correct tubes made little difference. The brightness and contrast controls are more resposive with the correct video amp tube. The picture and sound quality might have come up just slightly. But the audio level is still 10% of what it should be.
If I pull the AGC tube while the set is running, the picture goes gray and the audio level comes up slightly (along with lots of buzz). I've also noticed that the audio varies slightly with the contrast control (more gray=less sound). But all of it is a few percent variance around the crappy 10% audio level.
I should also add that with no signal applied, the sound of "snow" is just as muffled -- at full volume it's down there at 10%. Is this normal? I would think the sound of snow with no signal should boom, and if so, then the AGC theory as the culprit would be questionable.
This is a 2-chassis set, the power supply and audio amp are separate. After the 6AL5 ratio detector and before the 6SJ7 audio amp there's a removeable RCA-jack & cable between the two chassis. I plugged the output from a transistor radio into that phono plug, and the sound booms.
I've double-checked every resistor in the sound-if strip. All are well within tolerance.
I recapped the Sencore PS148 Vectorscope but it has other problems. The dot only appears on the screen at max intensity, and the intensity control makes the dot wander off the top of the screen. Horizontal position control works, but vertical position control doesn't. Focus control does nothing. Inputs don't deflect anything. Not worth bothering, I'll look for a Heathkit. So I still can't do any alignment work, but I've always been able to "eyeball" an alignment pretty close. The alignment slugs on this TV seem to be doing what they're supposed to be doing. I get buzz when I should, picture tear or fuzz when I should, I'm doubting it's just an alignment issue.
Really annoying. I can't bring myself to abandon this one. Already have $150 in parts and days of labor and it's a favor for a friend (after bragging that I could fix it). So my reputation is on the line. Sure would hate to pull defeat out of the jaws of victory, when it's this close. But I'm getting pretty bummed.
Any other suggestions?
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