1968 Zenith 20Y1C50 chassis - advice needed

[zeno]

Focus voltage is to low. IIRC should be abt 5KV.
Measure at the yellow wire. Should be abt 6 KV IIRC.
If not lift the yellow wire & see if it comes up. If not its the stick or the green
3W resistor.
2) lift the black wire to the CRT socket. At the focus control
center pin you should get 4-6 KV at the center pin. If so suspect the CRT
socket. Voltage will be dependant on the control setting.
Hope I got that straight, been a long day !
Just remember its a simple voltage divider....

73 Zeno
LFOD !

[nasadowsk]

Slightly off topic - I see the Horz output right next to the HV box, and the Damper in front of it. What's that power handling tube right next to the damper?

[zeno]

HV regulator. 6HS5 or 6HV5 IIRC. On some Zeniths you have to match it
with the right damper tube. On those it will be noted on the tube
chart or a big warning label on the HV box as to which pairs you can
use.

Zeno

Originally Posted by nasadowsk

Slightly off topic - I see the Horz output right next to the HV box, and the Damper in front of it. What's that power handling tube right next to the damper?

[old_coot88]

Even back in the day, that regulator tube frequently went bad, causing low HV and bad focus, even tho the tube tested 'Good' on a tester. The only fix was to replace it.
Just a small nit, but it's not a shunt regulator (like 6BK4 etc.) but a pulse regulator for the new-fangled reg system Zenith came up with.

[joe111671]

Thanks for the input so far everyone. The hissing in the HV cage is coming from the area around the rectifier tube. I'll ignore that for now. Maybe it's even normal. The HV is only 4KV on the yellow wire.

Here's my plan:

Since I have low focus voltage, and it doesn't drop as the set goes out of focus, I'll hold off on condemning the crt or socket. I'll lift the yellow wire and see if the voltage goes up to 6KV. If not, I'll change the stick and/or green resistor. If it does go up, I'll lift the black wire and see what I get at the center pin of the focus control. If the voltage is between 4-6KV depending on the position of the control, I'll pursue the socket or CRT. If it stays low, I'll change the 6HS5 regulator tube. I hope I got that right, and someone correct me if I'm missing something.

[nasadowsk]

Hissing isn't normal. Clean the HV box by the rect up.

[joe111671]

Focus voltage at the yellow wire was 4KV. I lifted it and the voltage went up to just under 5KV.

Hooked that back up then lifted the black wire, and the focus control center pin measured between just under 4KV to just above while adjusting the focus control.

HV at the 2nd anode is 26KV so that should rule out the 6HS5 tube.

So at this point I think I just need the stick.

Originally Posted by Electronic M

The old selenium focus rectifiers tend to fail by increasing their internal forward resistance.

I can get a NOS ECG118 for $5.50 shipped. Do these have a shelf life, or am I good with NOS? I'm hoping Chester's still has one so I can just drive over and grab one.

Originally Posted by nasadowsk

Hissing isn't normal. Clean the HV box by the rect up.

Will do.

[Electronic M]

Originally Posted by joe111671

Focus voltage at the yellow wire was 4KV. I lifted it and the voltage went up to just under 5KV.

Hooked that back up then lifted the black wire, and the focus control center pin measured between just under 4KV to just above while adjusting the focus control.

HV at the 2nd anode is 26KV so that should rule out the 6HS5 tube.

So at this point I think I just need the stick.



I can get a NOS ECG118 for $5.50 shipped. Do these have a shelf life, or am I good with NOS? I'm hoping Chester's still has one so I can just drive over and grab one.

NOS usually will work, but most vintage parts are selenium rectifiers so reliability isn't guaranteed.

I typically try to keep 5-20 of these modern parts in stock.
https://www.mouser.com/ProductDetail...NU2tEYAg%3D%3D
I usually put 2 in series in focus applications to ensure the PIV rating isn't exceeded.

[joe111671]

Originally Posted by Electronic M

NOS usually will work, but most vintage parts are selenium rectifiers so reliability isn't guaranteed.

Chester had an NTE118 in stock so I went over and got one for 3 bucks. You were right that it's selenium, but it worked. The ones you linked to are cheap, so when I need to place an order, I'll grab some just to have.

The focus voltage on the yellow wire went up to a hair under 6KV. That brought it into focus, but the control still needs to be all the way up, so I can't tell if it's as good as it's going to get since I can't dial it in. It still needs a lot of tweaking, and most of the controls need to be cleaned. I'll let it run and at least do a setup on it and see how it goes.

[joe111671]

I let it run for a little while. It doesn't look bad but not enough contrast. The brightness control worked normally, just acted like it needed to be cleaned. The contrast control had no effect, other than make a bit of "static". I cleaned them both. The brightness is smooth now, but the contrast does nothing. The schematic shows that it's a 500 ohm pot, and when I measure it, it goes smoothly from 2-480 ohms so it seems good. When the set is running, there's less than a volt across the terminals of the contrast pot. I guess the next thing is to check R85, L41 and change C3 (which I have).

[joe111671]

I set the gray scale and put a good source on it. It's working so much better than at first even with no contrast control. It's still laying on its side and the purity is out of whack with the degausser not working yet.

[Electronic M]

I'm a little late to the party on this observation but sometimes when focus voltage is good but proper focus is unobtainable and picture is bright (especially if previous owners have been monkeying with it) it can be because the CRT G2 AKA screens are set too high...I had that on a Setchel Carlson monitor and was chasing my tail till I turned G2 down...That brought the HV way up and I had to readjust the regulator.

[zeno]

That looks pretty damn strong. The green may be a little off though.
Contrast looks good, almost as if its at max. Keep in mind these sets
wont show the high contrast of newer sets & keep a good pix.
They are meant to be used in a dimly lit room & not driven to the max.
BTW we used ECG sticks. ECG was a good quality sub line almost all
the time.
BTW#2 Flipping a set on its side will throw off the purity, thats normal.

73 Zeno
LFOD !

[joe111671]

Originally Posted by Electronic M

I'm a little late to the party on this observation but sometimes when focus voltage is good but proper focus is unobtainable and picture is bright (especially if previous owners have been monkeying with it) it can be because the CRT G2 AKA screens are set too high...I had that on a Setchel Carlson monitor and was chasing my tail till I turned G2 down...That brought the HV way up and I had to readjust the regulator.

I set the G2s in service mode for a barely visible line, so I believe they're good. I did have slight blue retrace lines at first, so they may have been set too high. I don't think I touched the focus after that though so maybe it needs a touchup. The good news is that it didn't take much to get a line on all 3 guns, not even half, so the CRT looks to be happy! I should get the HV set when it's straightened out, that's a good reminder. It's supposed to be 25KV, and it was 26 before I even worked out the focus and adjusted the G2s.

Originally Posted by zeno

That looks pretty damn strong. The green may be a little off though.
Contrast looks good, almost as if its at max. Keep in mind these sets
wont show the high contrast of newer sets & keep a good pix.
They are meant to be used in a dimly lit room & not driven to the max.
BTW we used ECG sticks. ECG was a good quality sub line almost all
the time.
BTW#2 Flipping a set on its side will throw off the purity, thats normal.

73 Zeno
LFOD !

I think the picture exaggerates the contrast. I'm almost certain it's too low. In any case, the contrast control has no effect whatsoever, so something is wrong there. I took some voltage measurements around the video output tube, and the values on the schematic vs. the actual readings are below.
All readings vary w/ brightness control, and Contrast control has little to no effect on any reading:

Pin 1 (connected to contrast pot) shows 10.5V. It's 10.7-0.5

Pin 2 shows 5.5v. It's 5.8-(-19v)

Pin 7 shows 300v. It's 280-390

Pin 8 shows 130v. It's 187-273

There are 2 electrolytics in the vicinity that I didn't touch yet. Can they cause an unresponsive contrast pot?

[joe111671]

I decided to try and answer my own question on those electrolytics since I already have them, and the 50uF C3 was directly connected to the contrast pot. I chopped it out, tested and it was open, and changing it made the difference. The contrast works great now.

Now the vertical, which always had a little bit of jumpiness to it for the first half minute or so after power up, is now constantly jumping/changing height/linearity. All this stuff must work together. So now I have a vertical thing to work out. I'm tempted to just change all the electrolytics at this point, but spending time on issues makes it rewarding to find the exact culprit.

Not sure what I'll do yet, but I'm happy to be bringing this thing back to life.