Motorola ts907 low hv

[timmy]

V12 pin 1 is -8.5v and pin 9 is also -8.5v this is nuts and this looks like if I had video these voltages would straighten out.

[timmy]

Well bottom line here is I’m not convinced that there is actually a problem with the IF I believe there is stray voltage from somewhere and it’s corrupting the IF as well as overloading the focus circuit at the same time taxing the power supply. I don’t know where to look at this point. Diodes didn’t help anything. Like I said the video would flicker in and out and the hv would jump 4 kv and back down in an instant. Even though I got video at v2 it was still very distorted whatever this stray voltage is if it is maybe it’s affecting v1 and still carries on to v2 don’t know. When this set powers up there should be enough voltage to run everything even though the thermistor is warming up the voltage is just not there in the beginning and not always does the voltages come up to full after a few minutes it seems intermittent it’s odd never seen anything like this. I wish if there’s something shorting it would burn up that’s a way to find it.

[timmy]

I still had the issue with pin 8 v1 resistance check it should be 3.2k but it measured 6.3k this may be the key here since the set still don’t work I can’t say it’s a typo in sams every socket I checked were perfect except for the pin 8 v1. What is the likelihood of one of the old caps to ground that were not changed be shorted or should I say has this been seen before. The last time I checked v1 pin 8 it was 97v should be 100v v2 was 101v.

[Yamamaya42]

very likely an error, as V2 has an near identical bias on the pin and it's in the 6.3k range, but this has been pointed out before.

[timmy]

Originally Posted by Yamamaya42

very likely an error, as V2 has an near identical bias on the pin and it's in the 6.3k range, but this has been pointed out before.

Yes they are nearly identical but v1 is not v2 there has to be differences between the 2 or they wouldn’t have v123 it’s just odd get some video on v2 but v1 nothing and that’s the one with a discrepancy. The difference between v1 and v2 is that v1 8.2 resistor goes to ground the 8.2 don’t go to ground directly on v2.

[Yamamaya42]

The reason for the bad video is that there is something out of tune between the input from the tuner, to the grid of V2, AGC has nothing to process, and therefore has no reference to set the gain of the circuit , but it is not an AGC fault, as was shown by the override test, the heating of L42 and any HV related issues, if they exist, are a separate issue entirely.


"The difference between v1 and v2 is that v1 8.2 resistor goes to ground the 8.2 don’t go to ground directly on v2. "

since the measurement is not taken from ground reference, but from the main power point, this difference is infinitesimal, very high chance the SAMS is wrong and the 6k reading is correct for both.

[timmy]

Ok so let’s say it’s a sams mistake and leave it at that I already mentioned that the video would pop in clear and then gone so v1 there’s really nothing there except a few resistors and a few coils so I can’t believe that those coils were touched because the video at random would pop in and then be gone again. So what’s really left to check. Perhaps the .001 cap in L4 is bad the coil couldn’t conduct properly if that cap is bad.

[jr_tech]

Have you cleaned the tube pins? Is the socket ok?

jr

[timmy]

Originally Posted by jr_tech

Have you cleaned the tube pins? Is the socket ok?

jr

I checked the pins and tightened them a bit move the tube side to side no change

[Yamamaya42]

The IF is the most sensitive tuned area of the set, having RC constant circuits, that are going to react to voltage and temperature, and you have no clue as to what was done to it before you got it.

It's not unusual for something that is out of alignment to briefly pass a signal as it starts up while voltages and things are cold, but stop as it gets into normal ranges.
Thus alignment is done when the set is warm.

HELL, all my sets that are able to tune in stuff take 60-90 seconds before the can tune correctly, then they run OK for hours, what you see in the first few moments does not mean anything.

[timmy]

Originally Posted by Yamamaya42

The IF is the most sensitive tuned area of the set, having RC constant circuits, that are going to react to voltage and temperature, and you have no clue as to what was done to it before you got it.

It's not unusual for something that is out of alignment to briefly pass a signal as it starts up while voltages and things are cold, but stop as it gets into normal ranges.
Thus alignment is done when the set is warm.

HELL, all my sets that are able to tune in stuff take 60-90 seconds before the can tune correctly, then they run OK for hours, what you see in the first few moments does not mean anything.

Oh I understand what you’re saying but this is when it was already warmed not just turning on so I don’t know I have new .001 caps maybe I’ll try one in L4 and see what happens. If that don’t work then maybe I’ll disconnect L4 and check resistance and see what comes up.

[timmy]

Would an RF fixed 3600uh inductor the same is a peaking inductor 3600uh

[timmy]

I kind of overlooked at L4 that it has 2 .001 caps where L5 only has one the 2 in L4 one is red and the other is regular color ceramic disc the red one may be a mica won’t know til I get up close a personal with it. So now there’s 2 possible reasons for no video.

[Electronic M]

If it's intermittent it could also be one of the tuner inductors has a lead that either isn't soldered or corroded badly enough that it's effectively open and only gets continuity occasionally. Intermittent conditions often spawn in unusual places.

[timmy]

Since mica caps are used a lot in rf circuits it looks like it’s probably a .001 mica cap is on the primary of L4 makes sense to have a perfect signal to send down the IF line.