Emerson 612 Grid bias problem

[soliman15]

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

[old_tv_nut]

Have you replaced the tubes?

[Kevin Kuehn]

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.

[old_tv_nut]

What kind of meter are you using?

[old_tv_nut]

Agree on possible error in the SAMS.

[Kevin Kuehn]

Originally Posted by old_tv_nut

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.

[soliman15]

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

[Kevin Kuehn]

Originally Posted by soliman15

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.

[soliman15]

Originally Posted by Kevin Kuehn

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.

[zeno]

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
LFOD !

[soliman15]

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 ):

[Kevin Kuehn]

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

[Penthode]

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.

[Penthode]

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.

[old_coot88]

Originally Posted by soliman15

I'm back to alignment which I really don't want to do ):

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