Hi,
I didn't find the answers I need in the archived posts. So here goes.

Zenith model SK1989W5
chassis 19KC52
Chroma Color II
Space Command
built October 1978

This was owned by my wife's uncle, given to us after getting married, around 12 years ago. We used it daily for about 12 years. Last week it would not power up. I don't have any schematics or service literature.

I found that the circuit breaker was tripped. Reset it, powered up, immediately popped again. Opened up the set, and followed the AC wires thru the circuit breaker to the power transformer, and what appears to be the Space Command power unit A-7550. After the power transformer, there is a rectifier board 9-122-04, followed by a can electrolytic cap and an oval AC cap (3.5uF 440V 60Hz).

1) Terminals of each capacitor don't seem to be shorted to GND
2) Power transformer input resistance is 3.8 ohms
3) Removed Space Command power module, apply power --> set does not power up, circuit breaker does not pop (I suppose it is because power does not get far with the module removed).
3) Installed Space Command, but kept disconnected from the Space Command the 4-wire card edge connector (wht/red, red, wht/vio, vio) that is wired to the the front panel Power switch. Apply power --> circuit breaker pops
4) Removed rectifier board 9-122-04 (figuring this would eliminate most/all DC supplies in the set). Apply power --> circuit breaker pops.

Any recommendations would be appreciated.

Is there a small capacitor on the AC line somewhere between the breaker and the transformer? check for shorts inside the remote control chassis

Otherwise the power transformer is likely your problem.

Francesco

First, disconnect the B+ line going to the space command module and see if it will fire up. If not, measure the resistance between the secondary transformer taps to ground. Only the black wire should have a low resistance IIRC. It's been awhile since I cracked open a CC2.

You might also check the horizontal output transistor for a short. Check between collector and emitter for leakage especially. A multimeter is not enough to reveal leakage but will show shorts with a diode test.

Test the power supply rectifiers and input filters as well. You can connect an ohm meter across a filter cap and see if it charges up (displayed as a continuing rise in resistance) if it stays at a lower resistance and does not go up, replace that filter cap. A bad filter cap can also destroy the main rectifiers.

Sometimes these problems will only be evident under load.

Is there a small capacitor on the AC line somewhere between the breaker and the transformer?

There is a metal can, thumb-sized, one end soldered to the inside rear panel of the set, the other end has a hole with 4 wires:
--black wire in from the line cord
--black wire out to circuit breaker
--white wire in from line cord
--white wire out to power transformer and to Space Command module

This looks like some kind of safety device like a capacitor but there is no marking or part number on it.

As best as I can tell this is the circuit:

I figure the Space Command module provides a current path, via the gray wire, for the power transformer. The Space Command must always be hot (black, blue) so that it can respond to the power on command from the remote. When it gets the command it provides a current path on the gray wire to feed the line AC to the xfmr. The white wire (common) to the pwr xfmr is always connected.

I didn't take apart the Space Command A-7550 module. I would need to unsolder a 3-terminal TO-220 device in the module housing to disassemble it. I will take it apart and trace the circuit, unless someone can provide a schematic?

Here is what the A-7550 Space Command module looks like:

---

First, disconnect the B+ line going to the space command module and see if it will fire up. .
There is a 4-pin card edge connector that connects to the space command module. red, white/red, violet, wht/violet. I disconnected this connector, and powered up, and the circuit breaker blows. I assume on of these wires is the B+.

If not, measure the resistance between the secondary transformer taps to ground. Only the black wire should have a low resistance IIRC.
I believe I did this when I had the rectifier board out, measuring each sec lead from the pwr trans to GND, and saw no shorts. I'll double-check it.

Test the power supply rectifiers and input filters as well. You can connect an ohm meter across a filter cap and see if it charges up (displayed as a continuing rise in resistance) if it stays at a lower resistance and does not go up, replace that filter cap. A bad filter cap can also destroy the main rectifiers.
The rectifier board has 3 full-wave rectifier circuits (2 diodes each) and one half-wave rect. All diodes checked OK using diode check on a meter.

The 2-terminal "AC cap" (oval cap) is not shorted section to section or section to case. I'll recheck the 4-section electrolytic can cap. It's a Nippon Chemi-Con, 300uF/150V, 300u/150, 1000u/85, 500u/35.

You might want to check the secondary resistances with the rectifier board in, it may help identify which circuit is shorted. You probably do not have any shorts in the transformer. Also I would still check the Horiz. output transistor for short. This will caurs the breaker to pop as soon as the set is powered on.

Bill R

You should also check for a shorted CRT. With the rectifier board out the only things still connected on the secondary side are the tuner (havn't heard of many tuner problems), and the CRT filiments. Look for Heater to Cathode shorts. You could disconnect the CRT and see if the set powers up. There is also a cap between the Horiz. output transistor and the flyback. Seems I recall they could be a problem. Was this the chassis that they all had to be replaced, maybe not, I really do not remember?

Bill R

Carl,

In all of your troubleshooting, have you pulled the Horizontal Output transistor? This was the first step we used to perform on the old Chromacolor (we called them "Dura-module") sets. B+ is tied via the flyback to the collector of the Horiz output transistor. If the circuit breaker doesn't pop, check the transistor, the safety/retrace cap, and a 5 ohm, 5 watt resistor that's in series with the B+ to the flyback.

BTW, pulling the 9-122 module will usually pop the breaker - the transformer supply is a saturable-core and uses the 3.5uF cap to regulate the secondary voltages. No load from the 9-122 and the secondary voltage soars/transformer saturates, and induced primary current pops the breaker.

I've got some Zenith service literature for similar sets - Zenith had a "Sight and Sound" training program with cassettes and manuals. I still have some of the cassettes and a few manuals too - GC chassis stuff, similar to yours, but much more common.

Cheers,

I was sidetracked for a few days - I will have an update tomorrow.

First thing you do here is check The horizontal output transistor for short before you do anything else. This was not a hard chassis to troubleshoot.

Definitely look at that oval, oil-filled cap, which may be bulging on the top where the 2 terms are. Measure the 24-volt regulator transistor (in a socket near the breaker) for short. Next measure from the case of the Horizontal Output #121-831 transistor on the big heat sink at the top Rt of the chassis as you look at it from behind) to ground, and be sure it isn't shorted. If so, remove it and see if the breaker still pops. If not, you probably just need to go get an NTE-165 and replace it. If the transistor is not shorted, and if the oil-filled cap is OK, then next try the HV tripler. All you have to do to check that is to remove the blob of silicone that covers the terminal on the tripler where the wire from the flyback goes. Unsolder the wire, and pull it at least an inch away from any metal. Try the set, and see if the breaker still pops. If not, you need a new tripler. Obviously, there are other things that cause the breaker to pop, but these are the most common, and I worked on plenty of those.

Charles

After being sidetracked for a few months...
--lost my job
--computer failed due to bad motherboard; parts obsolete, annoyingly costly to repair...

Now I'm back on track, and will resume troubleshooting it. I WILL have our beloved 19KC52 back up and running.

Forget the CRT theory. This was a common problem with these sets and normally when you have a tripping circuit breaker, the first thing to check for is a shorted Horiz. output transistor. Check it with a VOM on the RX1 scale. You can also have a shorted tripler which can also cascade to short the HO transistor if someone keeps resetting the breaker. You can check this simply by disconnecting its' input. (Black wire from the flyback). Shorted triplers were easily checked by disconnecting them and you should draw a good arc by touching an insulated nutdriver to the black wire out of the flyback which would tell you that all of your horiz sweep is OK but just a shorted tripler. You can do this after checking the HOT and it checks good. Secondly, check for a shorted diode in the bridge in the primary power supply. There can be other things as well but these are the two most common things that will trip a breaker. BTW, don't keep resetting the breaker over and over again. You can cause other problems by perhaps the power transformer burning up. Julian

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.

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.

** 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...

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.

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.

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.

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!!

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]

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