Motorola ts907 low hv

[Yamamaya42]

Originally Posted by timmy

But can that single 66 meg be used in place of 3 22 meg resistors. Why Motorola used 3 resistors when they could have used one. I have a bunch of those resistors.

yes, the big 66 HV ones are much better in this case than small ones cause they were made just for such this task, and multi small ones have to be rated 2kv each or higher, its much easier to get a big 66 meg 6kv one, as they are all over the place.

[timmy]

Originally Posted by Yamamaya42

yes, the big 66 HV ones are much better in this case than small ones cause they were made just for such this task, and multi small ones have to be rated 2kv each or higher, its much easier to get a big 66 meg 6kv one, as they are all over the place.

When I changed those 3 22meg resistors I just got 22meg 2 watt resistors I didn’t specify 2kv rating does those pose a problem of any kind. ??

[Yamamaya42]

Originally Posted by timmy

When I changed those 3 22meg resistors I just got 22meg 2 watt resistors I didn’t specify 2kv rating does those pose a problem of any kind. ??

It stands to reason that they should be, you have 4-6kv on one side, ground on the other.

they may have used the 3 22k ones for Creepage and clearance for high voltage reasons.

but at any rate if they are not rated for 2kv, they may be a source of trouble.

22meg @ 2w 2kv seems to be unobtainium, so you are better of with a big 66m 6kv one.

[timmy]

I’m going to put the single 66meg resistor in but if the resistors I had put in are just ordinary 22meg 2 watt resistors then is it safe to say they were passing excessive voltage to ground. After all there is 4-6 kv on them.

[timmy]

I dont get why sams don’t list certain resistors like the 3 in the focus circuit.

[Electronic M]

Originally Posted by timmy

I’m going to put the single 66meg resistor in but if the resistors I had put in are just ordinary 22meg 2 watt resistors then is it safe to say they were passing excessive voltage to ground. After all there is 4-6 kv on them.

Reading that I don't think you understand how ohms law or resistor voltage ratings work. The same 6KV will be across those 3 resistors regardless of their voltage rating. Resistors don't pass voltage, they pass current and that too wouldn't necessarily change with voltage rating.

The voltage rating on the resistors is the maximum Voltage that you can put on the terminals and expect voltage to not cause arcing across the terminals....If it arcs it probably will do so noticably, and if it arcs it will pass current NOT voltage.

[dtvmcdonald]

Tom: I don't believe that arcing across terminals is what really determines
voltage ratings of at least carbon composition and possibly metal film resistors.

The reasson is that for a given voltage rating, terminal strips of the same outer material of the same spacing have higher voltage ratings. For a given size (spacing) epoxy transistors and diodes have enormously higher ratings.

I suspect that for carbon composition reststors the voltage rating has to do with possible internal damage due perhaps to tracking. In any case, experience says that the actual usable voltage for one and two watt carbon composition resistors of high resistance starting about 1947 or 48 is enormously higher than the ratings. The two watt ones easily take 2 kV.

I've used 2 watt 1 meg carbon resistors pulsed to 2.5 kV run continuously for months and on and off for 20 years with zero failures. Ditto for 1/2 watt resistors at 1 kV. One circuit generates 20 kV pulses with 800 pS rise times using 22 series connected ordinary transistors (yes, all one type and all only two near-sequential batches from one manufacturer) in avalanche mode with the bases driven by pulse transformers with resistor bias networks. The pulse rate was up to 10 kHz. I was stupefied when the idea worked at all, but after 20 years with zero failures in numerous systems .....

They ran continuously to keep the temperature constant as they were driving crystal polarization rotators that needed to keep the resonant frequency constant to better than 0.5 ppm (and rotators, while being quartz just like radio crystals, are an abysmally bad temperature quality cut).

[Electronic M]

I figured Timmy was talking about modern carbon film resistors which probably are less resistant to arcing than the old carbon comps. I doubt he would be as likely to look up the voltage rating on a period part as he would a modern replacement being considered.

It makes sense the old carbon comps would have better resilience against over voltage since there's a nice straight path between the terminals unlike some of the film resistors now that have a spiral which can potentially arc between adjacent tracks and or the terminals.

[Yamamaya42]

Either way, there has to be SOMETHING causing the peaking coil L42 to get hot, and I had serious doubts that it had anything to do with the sync / AGC area (C59,C60), him unhooking them confirmed this, so it has to be something with the current path in the flyback transformer pins 1 & 5, which also includes the resistor net to ground off C96, there is no way to know if the 2w ones he put in there are not up to the job, but there is no way to know if they are as well, but if he does have a 66m 6k resistor that's made for just this situation, best to try it and see if it helps.

[timmy]

Originally Posted by Electronic M

I figured Timmy was talking about modern carbon film resistors which probably are less resistant to arcing than the old carbon comps. I doubt he would be as likely to look up the voltage rating on a period part as he would a modern replacement being considered.

It makes sense the old carbon comps would have better resilience against over voltage since there's a nice straight path between the terminals unlike some of the film resistors now that have a spiral which can potentially arc between adjacent tracks and or the terminals.

Actually I was talking about the modern resistors not the old ones

[timmy]

Well the 66meg is in place and the L42 is still getting exceptionally hot all that’s left is maybe the focus coil or can the coil even cause this L42 to get hot I’m sure it gets warm under normal conditions but it’s way to hot.

[Yamamaya42]

does it still get hot with the 1V2 removed? :O

[timmy]

Originally Posted by Yamamaya42

does it still get hot with the 1V2 removed? :O

I was going to say if the 1v2 is shorted which I doubt or the 130pf is shorted which is new cause this.

[timmy]

1v2 out still gets hot but with it out the screen bearly lights

[Yamamaya42]

Originally Posted by timmy

1v2 out still gets hot but with it out the screen bearly lights

That is normal for no focus bias.

This leaves only 2 things, problem with the focus coil or L42 or something weird with the flyback transformer.

The path is from pins 1 -5, if the focus coil was somehow not resonating correctly, L42 would take the hit for it, or if somehow pins 5 &9 on the flyback were swapped, then focus coil/L42 would be overloaded.

Not really sure what it is here, this is a weird circuit.