Focus coil resistor

[timmy]

I have this ts907 Motorola and the focus coil has a 100k resistor that has burned up I’m not sure what may have caused this nor have I ever seen this any hints would be appreciated.

[Electronic M]

Originally Posted by timmy

I have this ts907 Motorola and the focus coil has a 100k resistor that has burned up I’m not sure what may have caused this nor have I ever seen this any hints would be appreciated.

Saw your message. Could you post the focus portion of the schematic? I Don't have access to my Sam's file cabinets currently (if I even have that Sam's), and haven't worked on that chassis to remember what circuit they used in those. If I can see what type of focus circuit and where the resistor is in it I'll have something to go on.

[timmy]

Hope this works. Actually I had the same resistor on my silvertone focus coil was burned the same but I think I changed the 130pf 6kv cap and that was it the focus coil was ok just the resistor I don't think I did anything else. I don't know if it matters if the slug is turned all the way in or out if that might burn the resistor.

[Electronic M]

I think I have heard that if the coil is at it's extremes it will burn that 100K resistor. I'd also check R142-144 if they are out of tolerance for that could upset things.

[timmy]

So I guess it’s safe to say maybe I can see the slug and position it in the middle and thanks for looking at that I’ll check those resistors.

[timmy]

Well those 3 22meg resistors are 26 and 30 meg so probably a good idea to change them.

[timmy]

Can this resistor be used in place of 3 22 meg 2 watt resistors. These 3 equal 66meg the pic shows what I have and the one I have is 66 meg.

[Electronic M]

Originally Posted by timmy

Can this resistor be used in place of 3 22 meg 2 watt resistors. These 3 equal 66meg the pic shows what I have and the one I have is 66 meg.

It will probably work. One thing that would be good to check is the wattage of the original VS the wattage of the replacement. The other sets that used it probably used it in the same focus circuit so it's probably the same wattage, but it never hurts to check.

[timmy]

The 3 22 meg resistors in there now are 2 watts each but the other resistor I want to put in I don’t know the wattage rating for that one is not marked so hopefully it will work. The 22 meg resistors are 10% each and the stick resistor is 20%

[timmy]

I have a hv issue 6bk4 cap off only 18 kv at anode so maybe oscillator problem or a bad flyback have sound but no picture just a white screen I don't know if they are both related. Changed the caps at the oscillator which the old ones still checked good. Any input would be a great help.I’m not sure if the horizontal hold adjustment is at one extreme or the other would affect the hv.

[Electronic M]

If there is an H frequency slug and it's misadjusted that can reduce HV. What I do to calibrate the H osc is center the user hold control, connect my oscilloscope to a composite video line and get 1.5-3 horizontal sync pulses on screen, count the divisions between them( do not touch the horizontal time base knobs of the scope from here onwards ), then hook the scope to one of the H osc test points and adjust the horizontal osc cycles to have the same distance between peaks of adjacent cycles as the video sync peaks did...I usually follow the Sam's H osc adjustment procedure while I'm in there so it's working extra good.

This procedure usually works well enough that horizontal will be dead on sync when signal is applied.

[timmy]

Well Tom oddly enough there is an HC but it’s turned all the way to one side of the pot aka bottomed out yet the screen is full that don’t sound right I think someone got in here turning things that didn’t know what they were doing. So I pulled the chassis again to replace other caps and upon putting it back I can turn that pot and see what happens since I don’t have a scope. The hc is I guess horiz centering. Actually there is a horiz frequency slug and that's used for the horiz hold so who knows if that has been turned to far. And yet another problem the horiz centering pot is supposed to be 15 ohms and its 75 ohms so that pot is bad what could damage that pot it's a 2 watt.

[timmy]

horizontal centering pot is bad it's a 15 ohm 2watt wire wound pot it measures almost 80 ohms could this bad pot cause low hv not able to adjust more then 20 kv from the hv pot brightness down brightness up is around 18 kv.

[Electronic M]

Yes it could cause HV issues. That resistor is effectively in series with 2 series windings on the flyback. If the pot end to end resistance increases substantially it will limit the energy through the flyback which could reduce HV....Also the yoke helps tune the resonance of the flyback. Disconnecting the yoke WILL reduce HV, so putting a high resistance in series with the yoke should reduce HV.

A quick way to check is take a fixed resistor the same value/wattage as the pot and temporarily replace the pot with it. If you connect the wiper wire to one end of the resistor it'll be the same as setting the centering to one extreme and the set will work fine that way (you will just be stuck with only 2 centering settings until you can get the right pot).

[timmy]

Ok thanks tom I'm waiting for that centering pot and hopefully put the hv issue to bed so 15 ohms to 80 ohms don't seem that bad as resistors go up in value in these sets but I guess when it comes to the flyback that becomes criticle. I do wonder what may have caused that pot to go up in value it is a wire wound so it's tough I hope the yoke is ok. The original is a 2 watt I can get a 5 watt .