Okay, can't move forward until I clear this up

[TinCanAlley]

Which connections are my cathodes and G1s? I know which are the G2s (the higher voltages of 400 - 600V), but the ones I've been thinking are the G1s might not be. Take a look at the for the CRT schematic and the circuits feeding it. Are those in order of cathode, G1, G2 and focus? Do they represent the actual configuration inside the neck of the CRT? If so, then the G1s should be the 36.4V?

If that's correct, then how exactly does the cutoff work? Is the G1 supposed to be a fixed voltage? If not, what makes it change? If those are the G1s in my setup, then I see no way for the voltage to change. They get their voltage from a step down of the 240V Boost and in that circuit there's nothing but the 2404V boost, two resistors, a capacitor and a spark gap. From there it goes directly to the CRT socket.

And if the G1s are the 36.4V leads, that makes the video output transistors the feeds for the cathodes?

If color of wires helps, there are three white wires with stripes (a red, green and blue), three yellow wires with stripes (a red, green and blue) and a red wire, green wire and a blue wire. I know the whites/stripes are the G2s, browns are the heater leads and the black is the focus.

I guess I'm still a little too new to all this. I thought I had this correct, but if I don't, I've been doing things wrong and looking at the wrong things.

[jr_tech]

Yes, it appears that the G1s are tied to a fixed voltage and the cathodes are driven by the 3 color video signals. The only controls that you have for cutoff are the G2 pots. Did you ever test the crt? were the cutoff voltages ok?

jr

[TinCanAlley]

Originally Posted by jr_tech

Yes, it appears that the G1s are tied to a fixed voltage and the cathodes are driven by the 3 color video signals. The only controls that you have for cutoff are the G2 pots. Did you ever test the crt? were the cutoff voltages ok?

jr

Yes, I used a B&K 467 and the CRT tested good on all tests.

See my other post (the one with 5 pages). I found that removing the color taps from their connections allowed me to get the setup line. I don't want to adjust the G2s based on this as reconnecting the taps would just throw it off (probably worse than before).

[jr_tech]

Originally Posted by TinCanAlley

I found that removing the color taps from their connections allowed me to get the setup line. I don't want to adjust the G2s based on this as reconnecting the taps would just throw it off (probably worse than before).

When you pull the taps, the cathode may "float" to some unknown voltage ... adjusting the line for cutoff under these conditions is meaningless.

jr

[zeno]

The schematic shows the elements in order, also physical order.
Hi cathode V=low bright Low Cathode = brite pix.
This is why it gets bright when you pull the taps.
You are putting a high RELATIVE voltage on the G-1
& tons of electrons go towards it.

G1 is fixed, its negative compared to the cathodes.
Make it more neg & brightness goes down. Its a control
grid, make it too neg & you turn off the tube. Make it
to + things get too bright. You can actually melt
some power tubes with a + G-1 voltage.

G-2 is positive related to the K's & G-1 so it pulls the
electrons along, more + means more brightness.
When electrons get there they see the focus at 6KV &
rush along to that & then the 30KV HV.

73 Zeno

[TinCanAlley]

Originally Posted by zeno

The schematic shows the elements in order, also physical order.
Hi cathode V=low bright Low Cathode = brite pix.
This is why it gets bright when you pull the taps.
You are putting a high RELATIVE voltage on the G-1
& tons of electrons go towards it.

G1 is fixed, its negative compared to the cathodes.
Make it more neg & brightness goes down. Its a control
grid, make it too neg & you turn off the tube. Make it
to + things get too bright. You can actually melt
some power tubes with a + G-1 voltage.

G-2 is positive related to the K's & G-1 so it pulls the
electrons along, more + means more brightness.
When electrons get there they see the focus at 6KV &
rush along to that & then the 30KV HV.

73 Zeno

So if a higher cathode voltage makes it darker, then that's why the setup doesn't work. When in setup, the voltage to the cathodes goes up to 188 - 180V from around 143 - 149V. Now all I need to do is figure out why the voltage goes up that high when switching from normal to setup. Anything in the setup circuit stand out as a possible cause of increased voltage?

[DaveWM]

180 is correct for the setup cathode voltage. I show 40v should be at the G1 (12,3,7) per the zenith color service manual pretty close to sams.

You would def want to increase the cathode voltage to avoid burning the CRT when the vert collapes. I don't know why your setup does not produce the lines if you are getting those voltages (180 and 40) unless you just had the G2 turned down. Have you ever checked the HV? should be around 29-30kv

[Electronic M]

I could be wrong, but I think the monochrome signal modulates the g1 elements in this set. I have that chassis and a hybrid chassis from 71'. I don't have the schematic for that, but the one for my hybrid has monochrome video to the g1s and the color difference signals to the cathodes.

Most good setup switches dim the electron guns because if the service man forgets to turn the brightness down the service line will be too bright and burn a horizontal line into the phosphors. I'd try increasing the brightness until the other guns start to light.

[DaveWM]

not this one, the mixing is done in the video drive transistors, the emitters get the luma, the bases get the chroma, the collectors drive the CRT cathodes.

[TinCanAlley]

Originally Posted by DaveWM

180 is correct for the setup cathode voltage. I show 40v should be at the G1 (12,3,7) per the zenith color service manual pretty close to sams.

You would def want to increase the cathode voltage to avoid burning the CRT when the vert collapes. I don't know why your setup does not produce the lines if you are getting those voltages (180 and 40) unless you just had the G2 turned down. Have you ever checked the HV? should be around 29-30kv

Sorry, there was a typo. The voltages go up to between 188 and 190V. Is that still correct?

As for the G1s, they're roughly 33V in normal and 34V in setup. Sams has them at 36.4, so in either setting they're lower than specs.

[DaveWM]

per Zenith 40-180=-140 overall bias

Per yours 33-190=-157 overall bias

extra -17 v neg bias does not seem like it should be too much, but I don't now that for sure.

the 40 or 33v on G1 is a simple voltage divider of a 390k and 75k resistor connected to the 250v source. if the 390k has drifted up that would reduce the G1 bias.

the 190 is a result of the bias and the load on the output transistors. for that I would look at those resistors on the service setup and maybe the 18k collector load resistors on the output transistors (they go to the 250v source).

[TinCanAlley]

Originally Posted by DaveWM

per Zenith 40-180=-140 overall bias

Per yours 33-190=-157 overall bias

extra -17 v neg bias does not seem like it should be too much, but I don't now that for sure.

the 40 or 33v on G1 is a simple voltage divider of a 390k and 75k resistor connected to the 250v source. if the 390k has drifted up that would reduce the G1 bias.

the 190 is a result of the bias and the load on the output transistors. for that I would look at those resistors on the service setup and maybe the 18k collector load resistors on the output transistors (they go to the 250v source).

I have new 18K and 1.8K resistors for the setup. I got them in case I needed to change them. I tried to test them, but they both come up as 1.6K. I'm sure it's because they share a connection. I'll have to life them and test.

[DaveWM]

you should be able the check the 18k with the service switch in the normal position.

[TinCanAlley]

Originally Posted by DaveWM

you should be able the check the 18k with the service switch in the normal position.

Which 18Ks? There's one on the setup side one on each collector side of the video output. I haven't tried testing the collector side ones, the one I was referring to is the one on the setup line. It shares a connection with the 1.8K and won't give me a reading other than 1.6K no matter how I try. Those are the ones I was speaking of lifting.

So are you saying I should be able to test the 18Ks on the collectors without lifting a leg? If so, that would save some time.

[DaveWM]

opps nver mind, I was looking at the 18k on the service switch and yes you would need to lift the 1.8k ground 1st then with the ser swich in normal you should be able to read them both.

One thing to remember, IF you do not heat sink the lead AND if you use a lot of heat to unsolder them, THEN its possible that you may heat damage them causing a drift upward that was not there before you lifted them. So use a heat sink to be on the safe side.