Sizzlin' Zenith SC 400 - Videokarma.org TV - Video

leadlike · #1 03-31-2009, 04:33 PM

Just started to put this Space Command 400 back in working order last week. I have replaced any worn or outright bad tubes and did some resistance checks and found nothing out of the ordinary. Most of the lytics have been replaced with new units under the chassis. Those caps, while newer, were probably put in about thirty years ago.

I did a variac slow start and after about fifteen minutes on twenty volts ac, I detected a 'warm' odor, about how the set should smell when it is running full blast. I powered down, felt around and found that the choke was pretty hot, about 110 degrees F. Closer inspection reveals the breaker was jumpered across. Clipping that out, it now goes into protection as it should. I have a full set of new electrolytic caps on their way now, the power diodes don't appear to be shorted. Is there anything else I can check on while I wait for my caps?

These color tvs are really different to me, as someone who has only worked on radios and B&W sets! I really hope to get this one going as local roundies seem to be scarce.

kx250rider · #2 04-01-2009, 12:57 PM

What chassis number is yours? If the filter choke is getting hot, and the breaker is tripping, I'd bet it's your filter caps, as long as the B+ diodes aren't shorted as you said. I'm puzzled by the breaker... Most early Zeniths I've worked on, have Belfuses. They made aftermarket breakers with Belfuse bases... Is that what yours is, or is it a factory breaker in the chassis? Also, you could check to see that the degaussing thermistor isn't shorted. Just unplug one side of the degaussing coil from the chassis.

Charles

leadlike · #3 04-02-2009, 01:56 AM

kx250rider, interesting about the breaker. Mine is chassis number 24mc32. I checked on the degaussing coils. Right away, the remains of a thermistor crumbled away into my hands. So that's a problem. On this set, one leg of the AC goes right into the degaussing coils. In this case, if I pull a lead as you suggested, it will be the same as the breaker tripping. I can jumper it, or would that cause an issue elsewhere in the set, now that the resistance of the coils, thermistor and voltage dependant resistor are now gone?

leadlike · #4 04-12-2009, 05:38 AM

Alrighty, I've been working away at this set and while it has not had power applied since my first posting, it is looking pretty good.

The deadly short wasn't obvious at first. The previous owner was a Zenith service tech, and he said he gave up on it after a power diode shorted. Well, after replacing caps and diodes, I still had a breaker fault. tracing through the circuit, I found on the B+ line a ceramic cap that had been twisted repeatedly, shorting out its terminals to ground. Hence the tripping breaker. The guy said his kids hated the tv, maybe one of them decided to kill it.

There were a lot of repairs. Mostly new electrolytics stacked on top of eachother. A pair of wirewound resistors in the Vertical section were dangling due to cold soldering welds. Since they weren't hooked up, I removed them and traced out the circuit, replacing a resistor and a cap and now it matches the schematic.

With this done, I have begun testing resistances at different pins and I have found some items that do not match the schematic:

First, the brightness control is coupled to the cathode follower, before the delay line. On my SAMS, it is attached to the Video output tube after the delay line. As a result, several components are not present in my set and my readings differ considerably here.

Another curious item I found is that the cathode of the 3rd video IF tube is supposed to be coupled to ground through a small resistor. Well, the resistor is there, but rather than being coupled to ground it is attached to a diode whose other terminal goes to ground. This diode is not in my schematic at all. What is its purpose?

Other than some small differences in some components, it seems good. I am mostly concerned with the above modifications, ie that they were part of earlier repairs, etc. If anyone has any ideas if I should be concerned about these differences, I'd be happy to hear them. Oh, and Happy Easter, everyone.

leadlike · #5 04-26-2009, 03:47 PM

Still working with this one. It SEEMS to fundamentally be working now...with the exception that I am only getting a B&W image. I'm gonna guess something is not passing through on the chroma circuit. Are there any common fail points for this part? I only tested the delay line a bit and that seemed to be working fine. But I'll let Kolchak tell the rest of the story:

Adjusting Hue and Color Level also change nothing, by the way. All other controls seem to be working okay.

Scott594 · #6 05-17-2009, 02:36 PM

I was a Zenith service tech back when these were still in service, and we often found bad coupling caps in the chroma circuit, hard to find without a scope though. 3.58 mhz crystals rarely, but do fail as well, so make sure the oscillator is on freq.
Looks like it has a convergence issue as well, if the magnets havn't been disturbed, check components common with the chroma/convergence/pincushion circuits.