Motorola ts907 low hv

[timmy]

While the other 2 couplets are measuring 115k don’t seem to low ? And that 4 volt source with no load should be higher then 4 volts.

[Yamamaya42]

Originally Posted by timmy

While the other 2 couplets are measuring 115k don’t seem to low ? And that 4 volt source with no load should be higher then 4 volts.

again, it's very unlikely that the couplets are a source of trouble, they would have to be well out of 30% before you should even think to start to replace them.

now, they seem to be well on the edge of OK, and would be more of a pain to replace at the moment.

also, which 4 volt source do you mean?

[timmy]

The tuner pin 3

[Yamamaya42]

That shows .4 on my scanned copy, and by nature, AGC is going to change based on given input demand signal, so (if working) don't expect to see what it says there on the schematic, the AGC voltage should change reactivity to the strength of the rf coming into the tuner, adjusting it almost scan line by scan line,

[Yamamaya42]

You will note that AGC keying has a link to horizontal pulses via C59, & link to video via r58, this is how it monitors video level and times ACG control for the tuner & IF gain, with no video, it has no control reference.

[Yamamaya42]

http://www.earlytelevision.org/Deksn...implified.html

way more info on AGC :p

[timmy]

C59 is 470pf I measured 410pf does ceramic caps go down in value? As for the vertical it does have 640v boost.

[Yamamaya42]

Originally Posted by timmy

C59 is 470pf I measured 410pf does ceramic caps go down in value? As for the vertical it does have 640v boost.

ceramic capacitors rarely ever go bad, and most in the set are 20% values unless otherwise listed, and it's very had to test in circuit unless you have a tester that was specifically made to test capacitors in circuit with leads that compensate, then a simple DVM will always give bad (deviated ) readings, especially with lower value capacitors because the length and position of the leads themselves used will skew the test results.

Notice the leads on this $1200 pro tester.
https://media.digikey.com/Photos/BK%...os/LCR-600.JPG

And yes, and mentioned before the vertical uses the 640v boost in 3 areas.

Vertical Height pot R17, Vertical linearity pot R18 and it also goes to C5 to R109 / C3.

Any problem here, and you would most likely lose sweep.

[timmy]

If c59 was bad that would upset the video and possibly sound path.

[Yamamaya42]

The chances of being bad are very small, the way to be sure would be to use an oscilloscope on pin 3 of V6 to check for the 450v signal.

again I have never had to replace a Disc cap unless they were physically damaged, IE broken / burned, they pretty much never wear out in these sets..

[timmy]

I don’t know how the hv oscillator has anything to do with IF and sound again hv comes up no sound no pic pull the plug then back on and again one hot gets red about had enough. I could have 2 defective caps 0068 and 0039 for the oscillator so if it don’t start then the tube can get red plate. But the IF is definitely another on going problem.

[Yamamaya42]

The problem is multifaceted, look at the block diagram page 5, AGC uses pulses from horizontal output to control tuner gain, if horizontal oscillation is not running, or not correct, or AGC itself is not working, then tuner gain will be wrong and you won't be able to tune anything.
Sometimes, when AGC is not working for whatever reason, an external bias can be used ( battery or the like ) to force the tuner into the right range of gain for test purposes.

You don't know at this point if the horizontal oscillator is stable at the right frequency, or the right amplitude, or the right waveshape, all of this matters.

[timmy]

True I don’t know but I do know that recently I tuned the horizontal coil and was able to get a perfect stable -45 from pin 5 of the hot. Now it changed and cannot get it back to -45 so something is not right and maybe it’s defective caps because I found nothing in the circuit whatsoever that was wrong different tubes both oscillator and hot and still so all I have left is the caps in the oscillator circuit. So if it’s not a defective cap what else could this be it’s not a resistor I checked them several times and the horizontal coil 85 ohms.

[Yamamaya42]

Not having it in front of me, I really don't know what else to add, other than if the horizontal circuit keeps being unstable, go full shotgun, EVERY resistor, replace with new carbon film 2% 1w, there is 1/2w in there now, but the new type 1w carbon film are about the same size, and much more stable, it does not matter if the old ones stlll test ok, new ones will last another 50+ years, the same goes for the higher power ones, sizes have come down a lot, so if it has a 2w, put in a 3w , replace a 3w with a 4w and so on.

Replace all caps with higher tolerance and voltage ratings, except for ceramic discs, UNLESS they show physical signs of distress, disc caps almost never fail, as mentioned before, other types do, film types are less likely but the do sometimes.

Like resistors, film cap technology has much improved since the set was made, caps can be gotten with much better tolerance and voltage ratings at the same values now.

this all of course is just a suggestion !

[timmy]

So the bottom line is if the horizontal oscillator is not right then the agc won’t be right and that can affect both sound and video even though in the beginning I did have sound but was alittle scratchy. All the film caps I put in are rated for 630 volts.