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

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.

[Kiwick]

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

[jpdylon]

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.

Originally Posted by Kiwick

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:

[andy]

---

Originally Posted by jpdylon

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+.

Originally Posted by jpdylon

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.

Originally Posted by jpdylon

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.

[Bill R]

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

[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

[Findm-Keepm]

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.

[julianburke]

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.

[kx250rider]

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.

[julianburke]

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