Motorola ts907 low hv

[timmy]

That’s another thing with the little video I have the contrast bearly makes a difference is this because of possible agc problems. And of course don’t have a 500k pot I think I have a 700 don’t know if this would work.

[Yamamaya42]

if its 700k then it should be OK,, anything above 100k should work, i picked 500k, cause they were most common.

700 OHMs, may load the battery down a bit however, not sure.

[timmy]

There’s not much to this agc the 50k pot and a 33k and 47k the pot checked good so this pot adjusts the bias on pin 6 of the 6hs8 correct?? I redone the k1 and it’s in this time I got a 680k rather then 680 ohm.

[Yamamaya42]

https://www.amazon.com/RadioShack-10...s%2C242&sr=8-6


It's audio taper, but should still work for the AGC bypass test.

[timmy]

Pics from today does this look like the agc maxed.

[Yamamaya42]

It's most likely 1 of 3 things.

A) the IF is so out of alignment that you cant tune in anything.

B) AGC is messed up and you can't tune in anything.

C) combo of both.

applying bias to the AGC test point will rule out one of these.

[timmy]

What if there is a higher voltage at the test point holding the tube at full agc then the test won’t work using a 9 volt battery how would this setup lower higher voltage if that works that way. Or does it maybe need a certain amount of voltage like the battery to turn it off per say. I do know that test point has 7.5volts there.

[Yamamaya42]

If you look at the “alignment instructions” in the sams, it says to connect a variable bias supply to point A, this will override any voltage present and allow you to set your own due the the high resistance of the the source.
Using a potentiometer and a 9v as described, should allow you to set the test point from 0 to 9v, which should be enough to see if AGC is the fault.

[timmy]

Ok well I’ll put that together and try and from what I have read agc is really not needed anymore since we are not using antenna anymore but rather stable inputs like cable.

[Yamamaya42]

This would be correct if the circuit were working as intended, but it may also so out of wack that it won't let you tune in anything.

The intent of doing this bias sweep on point A is to see if it has any effect at all on the screen, better / worse.
If there is a noted change, then it prob is AGC related, if not, then it's IF and alignment.

[timmy]

I’m going to go over a few resistors in the agc circuit then on to the 9v get up.

[timmy]

Just for the heck of it chassis back on back but upon plugging it in have a test lead on v1 with antenna and just as the hv came up it jumped 4kv and when it did the voltages on v1 and 2 neared 100 v then hv dropped again did this twice this just about confirms something is getting in or shorting intermittently somewhere. I never seen this since working on this chassis.

[timmy]

Well it’s official the 9v setup proves an agc problem voltage jumps up 11 volts on v1 and that’s a huge difference using the only pot which was 900k

[Yamamaya42]

Originally Posted by timmy

Just for the heck of it chassis back on back but upon plugging it in have a test lead on v1 with antenna and just as the hv came up it jumped 4kv and when it did the voltages on v1 and 2 neared 100 v then hv dropped again did this twice this just about confirms something is getting in or shorting intermittently somewhere. I never seen this since working on this chassis.

The voltages when it first comes on really don't mean that much, it's once it is all warmed up and stable that matters.

There is a reason for doing the AGC bias test, this goes back to 2016 when I was working on my CTC-16XL and it too had IF problems, and old_tv_nut said the following..
---
The AGC circuit is a feedback circuit. This means that the 9 volts at the plate does not necessarily mean anything is wrong with the AGC circuit itself. It may be getting an incorrect detected video signal at the grid due to some other reason (like a dead IF) and trying to correct it by changing the AGC voltage. You need to check the detected video for starters. If it's missing, check and/or replace the AGC tube. And, you may be right that some component is bad. Check all the voltages in this area. Check resistor values wherever voltages are way off. Further trouble shooting could involve breaking the feedback loop by connecting an adjustable bias to the AGC line, as you would for aligning the IF. If video is missing or wrong amplitude and you can't get it by manually setting AGC bias, there is something dead in the IF or tuner.
---
so please try that bias override, and see the results.

[timmy]

Originally Posted by Yamamaya42

The voltages when it first comes on really don't mean that much, it's once it is all warmed up and stable that matters.

There is a reason for doing the AGC bias test, this goes back to 2016 when I was working on my CTC-16XL and it too had IF problems, and old_tv_nut said the following..
---
The AGC circuit is a feedback circuit. This means that the 9 volts at the plate does not necessarily mean anything is wrong with the AGC circuit itself. It may be getting an incorrect detected video signal at the grid due to some other reason (like a dead IF) and trying to correct it by changing the AGC voltage. You need to check the detected video for starters. If it's missing, check and/or replace the AGC tube. And, you may be right that some component is bad. Check all the voltages in this area. Check resistor values wherever voltages are way off. Further trouble shooting could involve breaking the feedback loop by connecting an adjustable bias to the AGC line, as you would for aligning the IF. If video is missing or wrong amplitude and you can't get it by manually setting AGC bias, there is something dead in the IF or tuner.
---
so please try that bias override, and see the results.

So I assume the voltages at the agc tube 6sh8
3 7.95v 9 32.53 6 116 7 83. Doing the 9v setup then check voltages?