Zenith Advanced System 3 picture tube testing

[dieseljeep]

Originally Posted by 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?

Those things fool you because of the built damper diode. If it was truly shorted, it would read 0 ohms in both directions.

[Kevin Kuehn]

Originally Posted by dieseljeep

Those things fool you because of the built damper diode. If it was truly shorted, it would read 0 ohms in both directions.

Well I was wondering about that too, but this thing shows .034 drop in both directions when my Fluke 75 is in diode check, but it also shows 53 ohms resistance in both directions.

[Kevin Kuehn]

Originally Posted by dieseljeep

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.

Thanks for the offer, I'll think about that. At this point I'm not too sure I want to keep beating my head over this thing. Without a proper road map these solid state sets are just a major frustration to me. Life's too short and there's a lot of tube sets that are more worthy of my time.

[Zenith26kc20]

Been a while but if I remember right, that is a C2 chassis. Look around the power supply area (middle of the board on the right for weak or open capacitors. Also, look around the video jungle IC on the other board. The picture you have shown looks like a bad capacitor on the Jungle IC. It is not the large IC heading to the back. That would be the MTS IC. You virtually have to have a ESR type capacitor tester for this set. Look around the small (1 uf up to 4.7 uf) caps for liquid leaking out on the PC board. These sets were very good performers when working. The PIP modules almost always have a dozen or so dead capacitors in them. Also, look around the front of the power supply board for larger leaking capacitors. Look under the vertical IC for bad solder connections. Also, vertical fold over is going to be bad capacitors around the vertical IC. There were a lot of these sets here until Katrina got them

[Kevin Kuehn]

Originally Posted by Zenith26kc20

Been a while but if I remember right, that is a C2 chassis. Look around the power supply area (middle of the board on the right for weak or open capacitors. Also, look around the video jungle IC on the other board. The picture you have shown looks like a bad capacitor on the Jungle IC. It is not the large IC heading to the back. That would be the MTS IC. You virtually have to have a ESR type capacitor tester for this set. Look around the small (1 uf up to 4.7 uf) caps for liquid leaking out on the PC board. These sets were very good performers when working. The PIP modules almost always have a dozen or so dead capacitors in them. Also, look around the front of the power supply board for larger leaking capacitors. Look under the vertical IC for bad solder connections. Also, vertical fold over is going to be bad capacitors around the vertical IC. There were a lot of these sets here until Katrina got them

Thanks for all the suggestions. I'm afraid I'm not familiar with the term PIP modules?

[Zenith26kc20]

Picture in Picture. It was a large rectangular box on the signal (left viewed from the back). Not all sets had them. I don't remember what terminals were jumpered if the module failed (disabling the PIP). If you get it running I'll look it up in my notes. The broken up picture was almost always a small capacitor by the video jungle IC. Intermittant startup/no start was a cap around the heatsink in the middle of the power supply. I liked these chassis. They can produce a great picture.

[Kevin Kuehn]

Thanks for the explanation. Is there a way to safely power the two main boards on the bench without removal of the front switch panel PC board? Any voltage test points to verify correct power supply operation?

[Kevin Kuehn]

Well I had to take the two PC boards out of the plastic tray to get at the solder side. Boy that white goo holding the connecters was a treat to remove. I see at least a couple caps that have leaked electrolyte onto the video board.

[Kevin Kuehn]

These are the caps I've found so far on the main board that show physical signs of leakage. It looks like the Nichicon PC(M) series are the culprits, the darker blue and black ones seem OK. In several places the electrolyte has seeped through to the copper traces and made a corrosion mess. Lots of PC trace rework is in order. I'm going to have to place a cap order.

[zeno]

The transistor is either the power supply FET or the HOT.
The HOT on most mid 80's up sets have a damper diode
E-C & a resistor B-E. 99.9% dead short when they fail.
Power FET's can dead short D-S or open. The one
board Zeniths had 2 small 'lytics in the PS that caused
int dead or dead. The 2 boarders were big sets so almost
all were done in home at board level til near the end.
For the control board it helps to have one of those 3 foot
long xelite 1/4 drivers. I still got mine but you will have to
pull it out of my cold dead hands.
I will look later today for the manual.

73 Zeno

[zeno]

I dont have the Sams so I pulled a similar one for a J line set.
Its got some circles that may help also. Keep in mind this
is part of the same family & some things may NOT be the
same.

The transistor is the HOT not to worry.
Power supply caps are C3407, 3440, & 3408 start here.
Jungle IC, IC2301 caps in hoz C2726 also circled R2704 & R2752 & the IC
The only VCC I see for the jungle is 8 volts at pin 42.
The switch at CRT is a service switch.

The other problem is you have a green screen. This set has AKB
that automaticly adjusts grey scale. No adjustments. Odds are
the R & B are down so you may have to rejuvenate..........
So dont spend much on it yet.

73 Zeno

[Kevin Kuehn]

Thanks, zeno.
Yeah, I realize replacing all the bad caps could be in vein if the CRT has issues. Although I doubt I'll have over $10 invested in caps, plus a few hours of my time. I actually thought the picture had a decent gray scale last time it was on. My pictures made it look more green than in person. Oh, by the way - does that Sams give the pin-out for the CRT? I'd still like to verify I was checking it correctly.

[zeno]

Originally Posted by Kevin Kuehn

Thanks, zeno.
Yeah, I realize replacing all the bad caps could be in vein if the CRT has issues. Although I doubt I'll have over $10 invested in caps, plus a few hours of my time. I actually thought the picture had a decent gray scale last time it was on. My pictures made it look more green than in person. Oh, by the way - does that Sams give the pin-out for the CRT? I'd still like to verify I was checking it correctly.

Pin out same as I gave you. K voltages a little higher than norm,
130-140 V. Ive got some tricks if the grey scale is off, we shall see.
Try a life test on CRT also, if checker doesnt have it just
kill the filament & watch the meters. I think u have a CR70 , IIRC
it has it. Time for a beer

73 Zeno

[N2IXK]

Originally Posted by Kevin Kuehn

Well I was wondering about that too, but this thing shows .034 drop in both directions when my Fluke 75 is in diode check, but it also shows 53 ohms resistance in both directions.

Many of the HOTs that contained built-in damper diodes also had a 50-100 ohm internal resistor across the B-E junction. I'm sure this is what you are seeing here.

If it was not blowing fuses and generating HV, the HOT is likely just fine.

[Kevin Kuehn]

Finally today I got around to replacing 31 leaky capacitors, and it seems to have done wonders. Basically the picture looks very good to me, and no sign of those green retrace lines I recall seeing 12 years ago. The only very minor issue I see is some subtle shading down in the lower right corner. It's hardly noticeable unless there's a solid color on the screen, but then again sometimes it looks fine then too. I can see it best on the two b&w pictures below. I want to thank everyone for helping out with the suggestions on getting this set going.