Motorola ts907 low hv

[Yamamaya42]

Originally Posted by timmy

An alignment issue can keep the first 2 tubes from conducting the proper way or can they conduct and show correct voltages when an alignment is needed.

It does make some sense that if the alignment was off, that the tube dynamics would be upset to some degree, but how much, who can say.

[timmy]

So I am going to pull the first tube and check the voltages on the second and if the voltages are good on the second then that narrows down to the first tube having a problem if not then first one back and I pull the second and check voltages again hopefully I can get it down to one tube I can’t believe that both tubes developed a problem at the same time.

[timmy]

First tube pulled 7- 252 8- 136 3-5 2-4
Second pulled with first put back in 7-242 8-130 3-7 2-15v

[Yamamaya42]

It's almost as if the alignment is WAY off, and AGC is trying to compensate for it by messing with the gain of stages 1-2.

the numbers gave today are relatively normal, compared to the ones yesterday, namely voltage on pin 8, (130ish ) where it was 60v ish last time you tested.

This is probably being caused by.
AGC changing gain of these tubes.
Changes in voltage on the 275v line.
Changes in value of the 3w/2w power resistors on pin 8 as they heat up.
This added with the fact that the alignment is most likely off to start with.

[timmy]

Don’t voltages normally go up when a tube is pulled? And I checked voltages also with no antenna and no change.

[Yamamaya42]

Originally Posted by timmy

Don’t voltages normally go up when a tube is pulled? And I checked voltages also with no antenna and no change.

the draw of an IF tube is so small, it won't matter that much, in or out.

[timmy]

Well I don’t understand how a 100v drops to 60v that’s not so small of a draw I would think

[Yamamaya42]

Due to the action of the AGC, the 1st and 2nd IF is in constant flux, using horizontal pulses and video as reference points, even with garbage video, AGC will try to gain on those tubes via cathode and grid 1, this of course will have an effect on the other tube elements when this takes place.

Where it's hard to be sure why that 100v on pin 8 dropped liked that, it's unlikely that it was because that 275v was low, so it's prob tube biasing and / or related to the age of the resistors in the area coupled with the fact that alignment may be off.

Alignment is all but impossible to correct given the current situation, but upgrading to better resistors is a pain, but rather cheap.

[timmy]

I can’t help but go back to having sound stronger on channel 4 then 3 but the one day I had the set on and the sound just faded into distortion and static but can still make out voices and that’s how it still is so if alignment was the cause sound would not have been good in the beginning so I still believe the loss of sound has got to be the reason for bad voltages and no video.

[timmy]

So if you think the agc is trying to clean up a bad signal then what if I pull the 6hs8 agc tube then see if voltages get better on the first and second IF tubes.

[timmy]

Well pulled the agc tube didn’t make a difference.

[Yamamaya42]

Read page 8 of the sams, where is says “alignment instructions” , the better choice is to do as it says, connect a variable voltage to point A, and adjust to what ever needed, 3v 4v 2v, this overrides the AGC control, for alignment, this can sometimes be done for testing.

[timmy]

I think I’m going to try and get the 12 caps across the IF strip and change them. there are some that are shiny almost to say they got hot or whatever the reason is they get the oil or wet look. Something took out the sound that I had. maybe I’ll get lucky and find one that is leaking or partially shorted. I checked voltages this time with no antenna and pin 8 on both tubes still low.

[Electronic M]

It's not uncommon if a set was in a living room close to a kitchen or if there is a coil or transformer containing wax close to a part for it to get an oily or waxy look. One check you could make if you don't have a leakage tester is to put a cap of over 10X the value in series with the low voltage end of the ceramic caps ( a .1uF will work for anything smaller than a .01uF) then measure the DC voltage of the point between the two caps. The voltage may initially float up but it should 0 out within around 10 seconds of connecting the DMM...If DC voltage doesn't 0 out within a reasonable amount of time you have leakage.

Putting a large test capacitance in series with a small test capacitance will yield an effective series capacitance close to that of the smaller capacitance, and the greater the difference between the two the closer the effective value comes to being unchanged from the existing small value.

AGC does need manual bias voltage injection if the automatic part is disabled....Heck if the automatic part is not disabled, but isn't working right using a manual bias supply to force it to the value it needs to be can be used as a troubleshooting procedure...The exact voltage it needs to work may be different than the one listed in the alignment procedure as the signal level may be different and the alignment procedure may desire adjusting amps at min or max AGC gain for linearity reasons.

[Electronic M]

I would strongly advise against changing capacitors unless you can prove failure. Changing caps WILL make alignment needed if they were not defective before replacement.
Changing resistors to 5% or better modern replacements typically fixes alignment issues if the original was bad, or barely changes anything if the original was good (provided lead dress is carefully kept the same).
I have on 2 CTC-2 and 2 CTC-4 sets fixed egregious alignment issues by replacing all resistors that were off tolerance or at the edge of tolerance with modern tighter tolerance parts....On the CTC-4s the sound, luminance and chroma were all so far apart from each other on the fine tuning that you could only get 1.5 out of 3 in good at once (you could get any 1 of the 3 in alone good at a different fine tuning spot) before resistor changes, and normal fine tuning and operation after resistor changes.