1991 Zenith Console SJ2722lk5 totally dead
 

Hi all, i recently got a 1991 Zenith SJ2722lk5 console tv and upon bringing it home it’s dead, won’t turn on or anything. All it does is it sparks when i plug it in, but nothing happens when i push the power switch. All the fuses are good. I’m stumped.

I there is a capcitor in the switch mode power supply that goes bad and causes the tv not turne on when pushing the power button. The cap is 100 uf @ 25 and this video might help you fix your set https://youtu.be/OyGbEfa9CHI.

Thank you! So i replaced that capacitor and the set came to life with a horizontal line across the screen, so i shut it off. Now when i push the power button it turns on for a second then shuts off.

There is also another cap in that same circuit. I can't remember exactly but it's a lower value, like 10uf or 22uf. Change that as well. Failure to do so will kill the STR 53041 regulator.

On that model, the LA7838 vert IC is known to have bad solder. Add some liquid flux and resolder the pins of the IC.

Generally, that doesn't cause shutdown so there's a chance the IC shorted because of the bad solder. If so, you'll need a new IC and probably the 1 ohm resistor off pin 2 of the flyback.


John

Alright, so i replaced most of the capacitors on the board, now it turns on but there is just a horizontal line across the screen.

I am trying to find more information on the Zenith SJ2722lk5 TV, as I like all models of Zenith TV, radio, stereo, et al. (I am particularly interested in the details on this particular TV, as it is one of the last sets made by the original Zenith Radio Corporation before the company left Chicago for Korea.) However, every search I've done for this model returns an error. What are the letters following SJ2722? I tried "lk5" but that, like the other searches I've done for this chassis, returned an error, telling me to check my spelling (which I did, several times). The error shows up even with everything properly spelled. Are the letters following the set's model number "lk5", "lK5", "LK5"...? I also did a search for just plain SJ2722 (no letters following the final digit), with the same results.

BTW, I wouldn't operate the TV with just a horizontal line across the CRT, as this will burn the phosphors, as it will in older b&w sets. This line is caused if the service switch, on the rear apron of the chassis, is set to the "service" position; however, if the TV does not have such a switch, the problem is almost certainly caused by horizontal sweep failure. This can occur in solid state televisions (b&w or color) because the horizontal sweep in SS sets has nothing (or very little) to do with the HV plant.

Also, I was surprised to see the Zenith "System 3" badge on the front panel of this TV. Until now, I was not aware that Zenith was still using that particular model name as late as the early 1990s; I thought they had retired it by then, or at least by the end of the '80s.

Thanks much.

"almost certainly caused by horizontal sweep failure. This can occur in solid state televisions (b&w or color) because the horizontal sweep in SS sets has nothing (or very little) to do with the HV plant."

You mean caused by vertical sweep failure.

Also, the high voltage production is very much dependent on horizontal sweep working, even in SS sets.

The set comes up as Sams 3008
There used to also be a main PCB to Sams cross reference on the
internet. Dont know if its still there. It covered all the one & two
board sets up til the end. Just look up the module
( format is 9-#### ) and cross it to Sams.

73 Zeno:smoke:
LFOD !

Did you solder the LA7838? It's in the back right corner of the chassis looking at it from behind the TV (flyback in the near right corner).


These *all* developed ringlets around the pins.

Every

Single

One.

If you resolder the IC and it's still no joy, you need to replace the LA7838, the 100uf cap, the 220uf cap, and a good chance the low ohm (like 1 ohm) from the 24V source off pin 2 of the flyback that feeds the vertical output circuit. If the 1 ohm is burned, check the diode connected to it and make sure it's not shorted.

John

My info says it has a 9-1132-03 one board chassis.

John

Thanks much for the corrections. I did not realize solid-state TVs also depend on the horizontal sweep being in working order, as do tube-powered sets.

I am nothing short of amazed there are no safeguards against sweep failure in SS televisions; in fact, I strongly believe there should be some sort of protection scheme, even one as simple as a fuse, to shut down the entire sweep system (both sweeps) in the event of failure of either sweep circuit. There are (or were, before flat screens), after all, federally-required HV shutdown circuits which literally shut down the set's HV production in case anything went wrong with the HV regulator, or anything else associated with the CRT second-anode voltage. This was mandated by the X-ray scare at the time (1970s, IIRC), often caused by defects in the manufacture of the HV regulator tube (generally a 6BK4).

The invention of the flyback high voltage supply meant the abandonment of earlier more expensive separate high voltage supplies. (Plus, the line-derived ones were potential killers.)

The only color "TV" I can recall with a separate H sweep and high voltage supply was a studio monitor. This allowed both to be independently regulated/stabilized so there was no image size breathing with beam current at all.

Disclaimer: I was no longer involved with this quite some time before switch-mode power supplies were introduced in TVs. Maybe someone here knows of a later TV that didn't use the flyback for HV, but it seems doubtful to me.

Most if not all CRT TVs that were actually true HD used separate HV and horiz deflection transformers.

A good percentage of Japanese analogue projection TVs also used separate systems and all HD CRT projectors did to my knowledge.

John

Most SS TVs made from the late 90s on did shut down the TV with loss of either sweep. Once projection TVs came into popular production, many CRT TVs used the same jungle and uPc ICs as the PTVs did, so they shut down because the ICs were designed with PTVs in mind.

The Zenith chassis series after yours will shut down and require an AC unplug after a vertical IC failure.

Regarding shutdown circuits, there is no safety hazard for loss of sweep, which is why there was never any gov mandated protection. HV runaway is a different story, which is why all SS TVs from the mid 70s and on had a rudimentary protection scheme of some sort to mitigate runaway HV. A lot of TVs wouldn't shut down, but would swing the horiz osc off which would lower the HV. Later ones shut the chassis down completely.

Projection TVs will destroy all CRTs in a split second with loss of either sweep, so designing shutdown circuitry was critical for warranty considerations, not safety. RCA in fact sent a negative voltage to the CRT sockets which did nothing until there was a loss of sweep. In that event, the -200v was immediately applied slamming the tube off faster than the shutdown circuit would allow a normal voltage decay.

John

Thanks for the later info!

Did the separate HV supply run at H rate, or entirely independently?