Zenith ChromaColor II 19KC52 won't power up (long msg)

1)
Months ago when this set was still working, I noticed that when we powered it on with the ON button on the front panel (that is, not using the remote to power it up) that we had to try repeatedly to power it up. At tht time I thought the switch was wearing out.

I'd press the switch and the set would try to power up, I'd get an instant of sound and maybe a slight disturbance in the screen (say black changing to dark gray). If I pressed the switch again the set would power up.

I'm referring to the power switch behind the front cover, not the circuit breaker.

2)
I tested the primary rectifier diodes on the plug-in circuit board (think it's 9-122, the board with edge contacts that plug into a finger socket on the chassis) and the diodes measure like good diodes, not shorted.

3)
I removed the horizontal output transistor and measured all combinations of one terminal to another, and found no indication of a short. I can't say it's free of leakage, but my DVOM on diode and on resistance ranges shows no short. I did not yet try to power up with the HO transistor removed.

4)
The 24V regulator was not shorted to GND (as I recall I tested this, but I don't have my notes in front of me).

Thanks for your continued assistance. I'll repeat some of my measurements and try to power up without the HO transistor.

updates:
1)
Testing the large oval cap (connected to pwr xfmr). With the two wires still attached to the cap (white wire to one terminal, red wire to other terminal).

resistance, white wire at terminal to metal case: stabilizes at 10.4 ohms after charging a few seconds
resistance, red wire at terminal to metal case: 4 ohms (after a few seconds)
resistance, red terminal to white terminal: 14 ohms (after a few seconds)

These readings were with the rectifier board removed.

2)
Installed rectifier board. Removed horiz output transistor. Connected Space Command. Plugged in cord. Breaker popped immediately.

My earlier test a few weeks back didn't find a short in the horiz output transistor itself.

[mr_fixer]

have you checked the circuit breaker, it might be weak. i.e. popping at .5 amps instead of 5 amps or whatever it is rated. i've seen it before. Logan

In some of my past tests, I disconnected the Space Command module, as I figured this would eliminate a current path through the primary of the power transformer, and I could eliminate it as a possible source of the problem.

The Space Command has two connectors - one is a harness with a connector (seems to be the AC side), and the other is a circuit board edge connector. The edge connector is a 4-pin that goes to the front panel (It must go to the power switch). Both connectors are now connected.

The circuit breaker is now popping as soon as I plug in the line cord. When I first started looking at this problem the breaker would pop only when the power on button was pressed.

Now I disconnected the black wire going into the tripler. I positioned the wire in the air away from metal parts. When I plug in the set it immediately blows the breaker again.

Someone had already scraped away the black silicone material that was supposed to be around the black wire terminal scraped away.

When diagnosing the set earlier, I had the SpaceCommand module disconnected. I figure this was preventing any AC from flowing into the main transformer primary. The breaker does not pop when the SpaceCommand module is disconnected.

Maybe the SpaceCommand can fail causing a short across the AC line? But now it pops as soon as I plug it in.

[analog]

** WARNING** I am not all that knowledgeable on TV circuits so someone chime in if this is wrong.

Isn't the space command unit always powered if the set is plugged in? Could it be the power transformer in the space command unit has shorted??

Just thinking out loud...

[radiotvnut]

Yes, there is a standby transformer in the S/C power supply that is on all the time. The transformer could very well be shorted.

The AC side of the Space Command has a 3-wire connector:
BLK - AC from circuit breaker

BLU - neutral from AC line

GRA - I think this is switched by the Space Command to feed line voltage from the black wire to the power transformer when the remote (or the front panel POWER button) tries to turn the set on.

I can try to tie the black to the gray with some resistive shunt (60W light bulb) and see if it turns on.

[radiotvnut]

I pulled the Sams Photafact (1850-2) on your set. Disconnect the S/C power supply and jump the black and grey wires. If the set powers up; you know there is a problem with the S/C power supply module. If the breaker trips; it's something else in the set.

[radiotvnut]

Also, you may wish to check the resistance of the primary winding of the power transformer and the space command power supply transformer. The main transformer should be 3.97 ohms and the S/C transformer should be 164 ohms. You could also have a shorted degausing coil (not likely; but, worth checking). Just unplug it and see if the set powers up.

[rca2000]

I believe your problem is in the SC control unit.

These sets use a saturable power transformer, as was said. Now, it IS possible that it may have a shorted primary(I think I have seen about 3 of them that were, and a couple of others, tha tpeople THOUGHT was the fault of the tranny, put in a new one, and no change.

In this set--if the tranny shorts, it will NOT pop the breaker immediately(at least on the few I have seen that were bad for sure, that was the case. This is due to the design of the transformer, and it is such that a short in the primary or secondary will only allow about double the current to flow at maximum. when these short--the breaker pops in about 5-8 seconds, not INSTANTLY. I proved this in the cases I had--I disconnected EVERY wire from the tranny other than the tuning (oval) cap and the primary--and it still tripped in a few seconds. A New(or old, good) one did not do so. I then installed the tranny-and was good to go. however- You said yours trips the INSTANT power is applied. That tells me something else is going on..and I think I know what it is....

In the space command module, near the Power Triac(I THINK yours has one--the "TO220" thing), there may be a MOV (surge absorber) in the AC line. I had one many years ago popping the breaker instantly. It was condemmed for a bad transformer. I disconnected the transformer..SAME THING. The MOV (or cap) in the ac line of the SC module was shorted.

There is ANOTHER posibility...and I mention it because you saaid the "set had a hard time coming on". And tha tis that the optocoupler is GOING bad, and causing the triac to PARTIALLY fire when the set is turned on, instead of going full on, and this results in a CHOPPED waveform to the transformer(IOW, partially DC) and it does NOT like that at alll--and the breaker trips immediately. The best way to prove that is to bypass the triac altogether, The "MT" terminals need to be shorted together, and if the set then comes on and does not trip the breaker..that is probably the problem. I saw this ONE time on an RCA with a transformer (NOT a saturable one, IIRC) and on a microwave oven, it would run fine, BUT when you stopped the timer it would make a loud "bumping" noise from the HV transformer and blow the fuse!!---the Optocoupler was indeed bad!!

[work908]

does anyone have any schematics for the space command control chasis/reciever especailly for the space command 400 500 or 600 that they can scan in if so please email it to me at [email protected]

[kx250rider]

Originally Posted by work908

does anyone have any schematics for the space command control chasis/reciever especailly for the space command 400 500 or 600 that they can scan in if so please email it to me at [email protected]

There are dozens of versions of those; same name different animals, as they were made with tubes in the 50s, and I believe the Space Command 600 lasted all the way to 1980 when the System III came out. Please post the model of the TV, or the chassis number of the TV. The SC subchassis is an addendum in the back of the regular service manual, and Sams.

Charles