Tech-Master NIB TV Kit #5516

[old_coot88]

When you put that jumper from pin 1 (cathode) to pins 3 & 9, it shunts the contrast control and R109 completely out-of-circuit, giving the tube all the gain it's gonna deliver. The tube's at max gain and doesn't "see" the contrast control and R109 at all.

That may be all it's capable of with the low B+. What is the actual plate voltage BTW?

You might try raising the value of R87 a little, to allow the plate a little more 'swing'. For the heck of it, also try lowering the value a bit.

[old_coot88]

The set uses a very primitive, nonadjustable AGC. You could bump the IF gain up just a tad, by doing this:

Jump a 3.3 megohm resistor from B+ to the AGC bus (test pt. B). If no change is noted, work downward in value (like 2 meg, 1 meg, 470K etc until a gain increase is noted. (the 12BY7 grid also becomes a tad less negative, which will compensate for the contrast control's not going full on).

Too much gain will probably cause sync clipping. But there might be some usable gain-increase before clipping or sound buzz sets in.

[Crist Rigott]

I'll get to those suggestions later. I have some scanning to. Stay tuned.

[old_coot88]

Another suggestion if you happen to have a bias box (a low voltage variable DC supply): Put the positive lead to ground, the negative lead to: (schematic's too blurry to read) the Test Point on the AGC bus, below the 2nd IF tube. Vary the voltage to see if you can get optimal contrast with no clipping or overloading.

Some early low end sets had no AGC circuitry at all, and actually used this method in lieu of a contrast control, sometimes calling it the 'Picture' control.

Or you could rig a 'bias box' very easily with a 9V battery, a resistor and potentiometer. (Of course it will disable the set's regular AGC action.)

[Crist Rigott]

OK, here's what I did.

1) Jumper R109 so it is out of the circuit. I don't think it is needed because the contrast pot doesn't go all the way down to zero. It only goes to 93 ohms.
2) Placed a 56K resistor between pins 2 and 8 (B+) of V6 12BY7A.
3) Placed a 3.3K resistor across the 5K contrast pot to effectively make it a 2K pot per the schematic. This gives me a fair amount of rotation or range with the contrast knob.

These mods are fairly easy and give decent results. Would doing any of this cause harm to the TV?

[old_coot88]

Wow, I didn't know you'd have to go all the way down to 56K. That'll be making the 12BY7 grid too positive, causing it to conduct, which you don't want. So scuttle that idea.

Maybe try the bias box further down the AGC line, as suggested earlier. This doesn't involve the 12BY7.

What is the value of the resistor between Test Pt. B and the next test point (where the bias box would connect)?

[Crist Rigott]

Originally Posted by old_coot88

Wow, I didn't know you'd have to go all the way down to 56K. That'll be making the 12BY7 grid too positive, causing it to conduct, which you don't want. So scuttle that idea.

Maybe try the bias box further down the AGC line, as suggested earlier. This doesn't involve the 12BY7.

What is the value of the resistor between Test Pt. B and the next test point (where the bias box would connect)?

That would be R10 470K.

I removed the 56K from the 12BY7A tube.

I don't have a bias box.

[old_coot88]

Bad info. Disregard.

[Crist Rigott]

Originally Posted by old_coot88

There's something else you could try in lieu of a bias box.

From that Test Point junction with R10, run a 10K resistor to ground. Add another resistor from the Test point to B+, using clip leads. Start with 2.2 meg for this resistor, working downward in value until the IF gain starts coming up. Find a value that gives good contrast without overloading. The regular AGC action will be largely disabled however.

With a constant level signal source, AGC might be less important than with over-the-air reception.

I need a little explanation. In your last statement about a constant signal source, are your referring to my converter box? And the same thing as my RF converter from my DVD player?

Would doing this mod disable the contrast control?

How would I know when the IF starts to overload?

Sorry for all the questions but as you know I'm weak on the theory and how all the components work together.

When you say take the 10K resistor to ground, you mean my B- rail?

Thanks.

[old_coot88]

Sorry for the bad info. Senior moment Please disregard.

Try shorting the Test point/R10 junction directly to ground, and see if gain improves. If it overloads, use a resistor; start with a 1K, working upward in values like 10K, 47K etc until the overload condition diminishes.

[Crist Rigott]

Originally Posted by old_coot88

Sorry for the bad info. Senior moment Please disregard.

Try shorting the Test point/R10 junction directly to ground, and see if gain improves. If it overloads, use a resistor; start with a 1K, working upward in values like 10K, 47K etc until the overload condition diminishes.

I'll give that a try.

[Crist Rigott]

I have another thing that is a bit annoying. There is a very noticable "clicking" sound that varies with either the Vert. or Horiz. hold controls. It seems it is coming from the HV section, but tell for sure. I'm not sure it was there when I first got the TV going. It has been present for the last several weeks at least. Do I have something mis-adjusted? Any good way to find what the problem is or a way to eliminate it?

Thanks.

[Crist Rigott]

I think the sound is coming from the Vertical output transformer area.

[tom.j.fla]

Crist am sorry for replying to your request this way, couldn't get the system to post the reply, so here goes. 1) do not have a camera or smart phone 2)in my set the CRT is mounted to the cabinet so no need for chassis brackets. 3)chassis is mounted in vertical plane(knobs up/down).4) yoke back cover has means for centering and clamp. 5)Son wants the set so got to mod for 128vac, auto-transformer from a junk Pencrest color set, going to use the 98vac tap to get 113vdc for damper and output which gives 7/8 inch overscan,all the other circuits use the 130vdc supply. All the best,Tom.J

[Crist Rigott]

Thanks Tom.