Motorola Color Television

[Stlouisian]

I have had to replace R145 a 100K resistor that was split into two that went to the focus control. I used my high voltage probe and only got 4KV off the focus rectifer. Also I had to replace C2 a 2Mf 350 Volt Electrolytic that is in the vertical section. Now I am only getting the picture showed my prior posts briefly because the picture fades to nothing after it is on a few minutes. The only voltage reading that I did take in the high voltage section that seems to be very low is on Pin 5 of the 6BK4. It should read 360 Volts and I am only getting 230 Volts. Anyone have any ideas where I should try and why I am losing high voltage? Thanks for any help.

Ross

[Chad Hauris]

There is usually a 1000 ohm resistor in the cathode of the 6BK4, measure the voltage drop across the resistor. Voltage drop should be around .8-.9 volts corresponding to a current of 800-900 microamperes. If there is much more than this the 6BK4 is probably drawing too much current.

Too much 6BK4 current will drag the HV down. Have you traced the voltages back through the 6BK4 grid/cathode circuit to a place where they are normal? I have seen several cases where resistors changed value in this circuit and caused problems.

It sounds like a thermal problem since the HV goes down as it warms up...try spraying freeze spray on the resistors in the 6Bk4 circuit.

Also just to rule out tubes have you tried a new 6BK4 and HV rectifier?

And, keep in mind that problems in the horizontal output tube/damper circuit can cause low HV as well. Measure the screen and grid (not plate) voltages on the HO tube too.

[Stlouisian]

Tonight I measured the voltage on the 6BL8 Horizontal Oscillator/Horizontal AFC. Pin 1 should read .5V and it reads 2.5V and pin 9 should be -15V but I get -27.5V. The rest of the voltages appeared to be fine. Also I noticed that the 6GK6 audio output was not even light. After I resoldering some connections thinking it was a cold solder joint problem on audio output tube I plugged in the TV. I got a rush of static in the sound and then the TV started smoking and I had to unsolder a shield by the 6BL8 Sound Detector/Limiter to look for the part that caused the smoke. Visually so far I do not see what was smoking. Thanks for any help.

Ross

[Chad Hauris]

Ross, it sounds like there may not be enough drive signal from the horizontal oscillator...check the negative grid voltage on the horizontal OUTPUT tube grids...if it is low, then there isn't enough drive.

Also could you identify the tube elements such as grid, cathode, etc. that you test rather than just the pin numbers...it makes it easier to give suggestions because it is hard to remember what pin goes to what element.

When you got smoke it was probably a resistor that burned...maybe a shorted capacitor in the circuit that connects to the resistor.

[Stlouisian]

Here is a partial schematic of the TV. Specially the high voltage section of the TV. I found the resister that burnt and replaced it. Thanks for any help.

Ross

[Stlouisian]

I checked the high voltage tonight. After the voltage fluctuates between 2000-5000 volts a few times, it settles at 5000 volts. I am referring to the CRT voltage which should be be about 23,000 per the SAMs schematic. What might be causing such a voltage drop? I have subsituted all the tubes in the high voltage section. The picture I posted in earlier postings will only show for about 30 seconds. Thanks for any help.

Ross

Hello St. Louisian,
One thing you could try to eliminate the shunt regulator stage as a problem is lifting the cap off the top of the 6BK4. You will have to keep the plate cap away from anything as it will have HV on it. You could also connect your HV probe to the 6BK4 plate cap to lessen the possibility of arcing. If that doesn't make any difference try scoping the control grids of the HO tubes to make sure the sawtooth peak to peak voltage is what it should be. Let us know what happens.

Jeff (PUNK ROCK DUDE)

[andy]

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[Stlouisian]

How do I measured the P-P Pulse Voltage? Is this something I can measure with a VOM? The 660 Volt and 790 Volt Boost Voltage is low when I tested it. Thanks for help.

Ross

[Stlouisian]

Could C522 or C523 be shorted or leaking to cause the voltage to be low on pin 5 of the 6BK4?

Thanks,

Ross