Zenith Advanced System 3 picture tube testing

[Kevin Kuehn]

I was hoping to test the picture tube in my moms Zenith Model SS2721H. This set was manufactured in June of 1991. The CRT is listed as a GA68ACT00X. My Sencore CR70 setup book dated 1992 does not list this CRT? I was told 12 years ago that these had junk CRT's, but I never figured out what exactly was causing the original problem of green retrace lines in the picture. We suspected it was a heater to cathode short, but I never had a good way to test it back then. Now 12 years later the set won't even power on unless I unplug the PC board that plugs onto the CRT base. My mom is still attached to the cabinet of this set. Guess I could put a flat screen in there for her.

[zeno]

To test the jug use the universal adapter & the "common"
settings ( dont remember what they were). Green & retrace
could be H-GK short, tap neck wile running & see if it
flashes. Can also hang a meter on GK & pull socket while running.
Normal K voltage abt 100V - 130V, if low with socket on but normal
off its the tube.
Set can shut down with excessive beam current BUT it must warm
up 1st. Can be caused by:
Excessive G2 voltage
loss of 200 V supply for the R,G, & B transistors.
A dead shorted H-K may also.
Sometimes these sets would come in with the power supply out.
Fixed them, looked OK then a few weeks later back again.
The CRT's arced & blew the supply. Most I saw bad were from
low emission or drifting grey scale followed by shorts & arcs.
It also affected cheap models more so I would guess they put
a better grade in the high end sets ( they deny that).

73 Zeno

[Kevin Kuehn]

Originally Posted by zeno

To test the jug use the universal adapter & the "common"
settings ( dont remember what they were). Green & retrace
could be H-GK short, tap neck wile running & see if it
flashes. Can also hang a meter on GK & pull socket while running.
Normal K voltage abt 100V - 130V, if low with socket on but normal
off its the tube.

Thanks for the suggestions. My problem is I don't have any service info for this set, and I can't find specs and pinout for this CRT.

[zeno]

You can usually see the filament pins then trace out the others.
Just put the 2 F clips on first & be sure it lights before going on.

Here is the pinout for a mid 80's Zenith 27" jug.
Odds are its the same.
F pin 9
F 10
KR 8
KB 11
KG 6
G-1 5
G-2 7
Focus 1
After I wake up I will see if I got a manual

73 Zeno

[rcaman]

you can bet that crt is junk.

[Kevin Kuehn]

I went over to moms tonight with my CR 70 in hand. I think I figured out the CRT pinout, and it actually seemed like all three guns checked OK for HK shorts. Cutoff and emission seems OK too. Took a few pictures of the set.



What's the little slide switch on the CRT PC board for, possibly setup?




I finally got the set to come on with the CRT board plugged in. This is with no input signal. Normal background hiss with the volume up.


These next three pictures are with a DVD Player hooked to a A/V inputs. I can see a vague image moving around accompanied by a loud hissing-scratching- popping sound from the speakers, with the volume all the way down. It almost seems like the video is modulating the audio power supply. Anyone recognize these symptoms? This is not what I remember this set doing 12 years ago when I had it on. But my folks had two of these same sets(one upstairs, one down), so it's possible I was working with the other one at the time.






I'm about to drag this set up out of moms basement and back to my place. It's a real pain trouble shooting it over there away from my shop. If nothing else it needs to come out of there so I can dispose of it. But it sure would be nice to get to the bottom of this problem before sending if off to the recyclers.

[andy]

...

[zeno]

Originally Posted by andy

Not so fast. Zenith CRTs didn't go completely bad until 1993. The earlier 27" ones were hit or miss, but many were fine. Anything from 1993, or 1994 is definitely bad.

I second that. Thats a 2 board set & they were not near as bad.
It looks like the FBT is breaking down. When you get it home unhook
the SPKRS & use a paper towel tube to listen for arcing.

73 Zeno

[Kevin Kuehn]

Originally Posted by zeno

It looks like the FBT is breaking down. When you get it home unhook
the SPKRS & use a paper towel tube to listen for arcing.

73 Zeno

Would checking the HV verify if this is the case? I searched the Sams website but nothing comes up using the sets model number. Did Sams not provide service info for these era Zeniths? I'm more less curious because I doubt this thing is worth investing in the paperwork. My fear is that when I fix one problem, another issue will soon surface.

[zeno]

Originally Posted by Kevin Kuehn

Would checking the HV verify if this is the case? I searched the Sams website but nothing comes up using the sets model number. Did Sams not provide service info for these era Zeniths? I'm more less curious because I doubt this thing is worth investing in the paperwork. My fear is that when I fix one problem, another issue will soon surface.

This is an SS model. Most were sold in the big stores or were
drop ins. Sometimes we could get them. If not we didnt even
get the spec sheets. Same case with S, T, ST, V & SV models.
Get the 9-*** numbers off the 2 main boards & I can see what I
have on it. It will be near identical to a regular line set.
As far as reliability goes these were decent. Some FBT's went
but completely, no raster followed by shut-down & the HOT
overheats.

[andy]

...

[Kevin Kuehn]

Originally Posted by zeno

Get the 9-*** numbers off the 2 main boards & I can see what I
have on it. It will be near identical to a regular line set.

Finally have the set at my place. Offhand I don't see those 9-*** numbers on the two boards. Are they possibly on the bottom side, or hidden by the board mounting tray?

[Kevin Kuehn]

Well duh, the board numbers were on a paper label on the inside of the cabinet.

9-959 Main
9-1075 SMPS/Sweep

Since I've gotten this thing to my place it's back to not powering on. When I push the power switch the green light on the front comes on, but the power relay does not pull in. Pushing the switch a second time won't turn the light back off. So I suspect there's something flaky with the on-off circuitry.

[edit] Actually the green light that comes on is the stereo indicator. But it won't turn off when the power switch is pushed a second time.

[Kevin Kuehn]

Pulled the PC boards out and was poking around looking for suspect capacitors. Just for the heck of it I checked the big transistor mounted on the heatsink next to the flyback transformer. Out of circuit it measures 53 ohms in both directions across the base-emitter. That can't be right, can it?

[dieseljeep]

Originally Posted by Kevin Kuehn

Well duh, the board numbers were on a paper label on the inside of the cabinet.

9-959 Main
9-1075 SMPS/Sweep

Since I've gotten this thing to my place it's back to not powering on. When I push the power switch the green light on the front comes on, but the power relay does not pull in. Pushing the switch a second time won't turn the light back off. So I suspect there's something flaky with the on-off circuitry.

[edit] Actually the green light that comes on is the stereo indicator. But it won't turn off when the power switch is pushed a second time.

I looked through my junk stash and I have a chassis assembly, that came from a set very similar to the one you're working with. The main board is a 9-959, but the sweep is a 9-754-1.
The original problem was the set wouldn't always start the first time.
I pulled the boards and found a few solder connections that were loose.
I returned and it still wouldn't work and the owner said forget it.
If you think you could use them, P.M. me.