Emerson 612 Grid bias problem
 

Hello, I'm working on Emerson 612 TV and found that audio is weak and the image contrast is not that good, after troubleshooting I found that the grid bias in final stages of both audio and video is lower than what the Sam's shows, in spite of having the correct B- voltage. The attached images have the Sam's voltages in black and the measured ones in red.
is this the reason for my TV poor performance and how can I fix it. thank you

Have you replaced the tubes?

On your audio picture I don't see where on the Sams info you're getting the -8.4v you have designated in black? I think -8.4 is in error, and there's likely a problem with some component/components in the negative supply line. Also the focus control is part of that circuit, so I suspect that negative bias could change some when you alter your focus setting.

What kind of meter are you using?

Agree on possible error in the SAMS.

OK now I do see -8.4 listed on the voltage chart for pin 6(no internal connection) of the 6v6. So -8v seems within tolerance for that audio output tube. So if we assume -8.4 is the negative supply, then that should be divided by the in series 33 and 180 ohm resistors feeding the grid of the first video amp. So the measured -1.4v makes more sense to me than -.2v, unless Sams -.2v measurement was taken with a lot of grid leak action from a very strong signal, but I have my doubts about it changing that much when it has a fixed bias through a 470k at the grid.

I did swap the tubes but nothing has changed.
I measured -8.4 on pin 6 of the 6v6 tube which is not connected inside the tube but used as tie point and is driven from the B- via voltage divider and it is in the Sam's (attached). I agree Sam's it could be wrong and the -8.4 should be ~-3v

Actually I looked at the specs for a 6V6 in my RCA tube manual and it says -8.4v on the grid with 180v plate supply for class A. So I think Sams -2.4v is the error.

Interesting, I'll do further troubleshooting and see if I can find any other issues.

With NO signal you should get strong , clean snow & a loud "water falls" from the audio. If you dont look tward the tuner, IF, or AGC. Could be 2 problems but thats rare.

73 Zeno:smoke:
LFOD !

Apologies, just found another voltage chart in "Beitman Most Often Needed 1950 Television Information" and it is almost what I have, so I'm back to alignment which I really don't want to do ):

Have you looked at your AGC voltage on pin 2 of the video detector to see that it changes with signal strength?

Does the schematic indicated what the ohms/volt of the meter is? In the days before digital multimeters and even VTVMs, a meter was typically 20,000 ohms per volt. That may account for the less negative grid bias.

Low gain in the audio and video simultaneously could be on the IF or RF stages. First thing to do is to test the tubes.

"Back to" alignment?:eek: If so, that's the last thing in the world you should be doing. :nono:

Thanks to everyone, I'll sure follow all recommendations I get here and try all I can before attempting alignment...

Another thing sometimes overlooked, if your set has the Standard Coil type tuner, is to adjust the local oscillator slug on the channel you're using for strongest picture and sound. You access the local oscillator slug for each channel in the tuner through a hole in the front of the chassis. Be cautious not to screw these in too far or the slug will disappear behind the threads, then you end up taking the tuner apart to get it back in place. Sometimes the slugs have already been tweaked by a previous owner beyond the threaded portion and appear to be missing. Usually the local oscillator adjustments are shown in Sams as part of alignment, but there is no harm in tweaking the local oscillator for any individual channel without doing the full video and sound IF alignment. What this does is to best center your RF source to whatever the current IF alignment is.