Motorola ts907 low hv

[Yamamaya42]

This should be done with the crt hooked up and a signal being fed in the tuner, adjusting the AGC bias to see if it makes any marked improvement in the picture on screen.

[timmy]

Originally Posted by Yamamaya42

This should be done with the crt hooked up and a signal being fed in the tuner, adjusting the AGC bias to see if it makes any marked improvement in the picture on screen.

Ok let’s say it makes a huge difference nice picture what would be the next step because I’m lost at this point I have checked all the resistors in and around the tubes associated with the agc so if this proves to give a great clear picture what’s next.

[Yamamaya42]

If it does turn out to be an AGC related issue, the whole AGC / NOISE GATE system will have to be taken into account, as Old TV Nut pointed out, this is a feedback system, involving not just the 6HS8 immediate area, the whole point of this is to find if there is or is not a problem in this circuit.

if there is not, then on to the IF.

[timmy]

Well unfortunately the bias test did nothing I even jumped the pot and gave it 9 v no change. This is beginning to really hurt. So I guess the agc is trying to fix or adjust the input.

[Yamamaya42]

Since you have given test point A a bias sweep of 0-9v with no results, and been over the resistors in the IF section more than once from what I can see, this basically only leaves one thing left, the IF is badly out of alignment, this is likely causing AGC to overcompensate, as there is no video for it to reference to, causing the abnormal voltages.

Who knows, the de-tuned state may also disrupt the normal voltages, be that as it may, color sets are quite hard to align even for the most experienced techs with the right equipment on hand.

It took me 3 tries to align my BW Arvin set (45mhz), and that was just with a frequency generator and a volt meter.
But for color, you need a sweep marker generator and a oscilloscope plus lots of know-how.

But the one thing that must never be done is, try to visually align a color set's IF, this will always end badly and make is worse than when sttarted.

[timmy]

While I can understand the IF but what happened here today I still think there is something shorted possibly to ground maybe a cap that I didn’t change yet there are caps still in there originals because on the bench the hv jumped instantly as if there was an arc and at the same time the voltages jumped up on the IF but it was instant a bad alignment would not do this hv jumped 4kv and the IF 15v so I’m going to hope to find this it could be anywhere and besides the picture has flickered in clear on it own several times already so I don’t think the IF is out. I have no idea how I’m going to find this because other then the agc pot not working probably because it’s maxed already trying to fix the signal all other controls work even though something could be leaking somewhere or to ground. I’m sure it wouldn’t take much to corrupt the video signal. So bottom line is voltage still dips diode gets hot so something is not right. I have 2 4 amp diodes I tried they only get warm so it’s not a diode. To bad this forum dont have audio easier to talk then type.

[Yamamaya42]

For that I still suggest a longevity test, remove V1,V2,V3, hook direct video to point B as before, heck, hook audio input to pin 3 of the still connected volume control plug connector to R1 if ya want, you should get some sound, run video for 30 min ( do NOT leave it unwatched ).
If OK, try another 30, try longer, and so on.

[timmy]

Originally Posted by Yamamaya42

For that I still suggest a longevity test, remove V1,V2,V3, hook direct video to point B as before, heck, hook audio input to pin 3 of the still connected volume control plug connector to R1 if ya want, you should get some sound, run video for 30 min ( do NOT leave it unwatched ).
If OK, try another 30, try longer, and so on.

Ok but what am I accomplishing I already have good video at test point B and my goal is to have it work with its own tuner I know you said I could take the other route with video inputs but I would rather find this headache and be done with it. After seeing the video flicker in and out clear there is another obvious problem something is more then likely leaking to ground I feel it don’t know why but I do. And an arc somewhere can only do what it did in an instant I don’t think it’s a tube. I had an rca don’t remember what chassis but I had a disc cap short dead to ground and took out the flyback and ho transistor so those caps do go bad. In this case I just don’t know.

[timmy]

What I did try was disconnected both diodes and ran the heaters and checked the positive side of the power supply caps and I got a very low voltage reading on the 250v 260uf string and also the 450v 80uf but all the other caps were zero so I pulled each tube one by one thinking heater cathode short but still had that small reading. These caps do go to ground but still why only these caps.

[Yamamaya42]

The point is to eliminate any possible problems from the video det forward first, then work back.
If you can determine that you can run the set for extended periods of time with A/V input without any strange things popping up, then you can turn attention on IF/ tuner.

But if things still act strange with A/V input when run for long periods, address that first.

[timmy]

Well the sound does fade on and off but that’s probably because of the lousy video

[Yamamaya42]

Originally Posted by timmy

What I did try was disconnected both diodes and ran the heaters and checked the positive side of the power supply caps and I got a very low voltage reading on the 250v 260uf string and also the 450v 80uf but all the other caps were zero so I pulled each tube one by one thinking heater cathode short but still had that small reading. These caps do go to ground but still why only these caps.

"disconnected both diodes "

but c151 was still there, right?
that gave some charge to C1, not much.

C1 & C2A = voltage doubler when added with the diodes, the other caps are filters.

[timmy]

C151and c152 are disc caps and both diodes completely disconnected there wouldn’t be anything flowing out of those caps from the heater string.

[Yamamaya42]

Originally Posted by timmy

Ok but what am I accomplishing I already have good video at test point B and my goal is to have it work with its own tuner I know you said I could take the other route with video inputs but I would rather find this headache and be done with it. After seeing the video flicker in and out clear there is another obvious problem something is more then likely leaking to ground I feel it don’t know why but I do. And an arc somewhere can only do what it did in an instant I don’t think it’s a tube. I had an rca don’t remember what chassis but I had a disc cap short dead to ground and took out the flyback and ho transistor so those caps do go bad. In this case I just don’t know.

and about that, things of this era just do not compare to the SS one, as everything went solid state, it also went cheap cheap CHEAP, the components were just not built to the same standards, not expected to last, all disposable, hell, only a fraction of the CRTs from SS TVs even still worked after 10 years, let alone 30+!

[timmy]

All I know is there is something that I would call an arc because of how the hv changed and IF voltages in an instant and if the chassis was in at that moment the video probably would have been clear for that moment.