Zenith C1415U (16C20) Restoration
 

So, I'm very happy to say, after years of owning this TV set and not really doing anything with it, I have finally re-capped it and got it working. So I'm starting this thread to try troubleshooting a few more issues with the set.

The first thing I'm wondering about is the video signal. I've spliced in a composite video signal directly to the video output tube. The TV actually displays a very clear picture from this, but I've noticed areas of sharp contrast seem to 'smear' to the right-hand side of the picture tube. Do you think it would fix the problem if I implemented a video input stage like the one mentioned here?

http://ankythera.com/Projects/video_stage.htm

The only thing I'm worried about is the video output tube looks like it's supposed to take a 1V peak-to-peak signal, and that stage is supposed to put out 3V, it looks like. I guess I could modify it to be unity gain, though I'd have to research the topology a bit more, first. What do you guys think? Or would it work fine like it is, if I just turned down the brightness?

I think the smear you are seeing is the reduction in higher frequency response due to the capacitance of the DVD/cable. The tube has a much higher input impedance, so this mismatch causes the issue.

The circuit using the 6BN4 is a buffer-isolator that overcomes this mismatch. I will try it myself. :thmbsp:

I am only aware of a circuit using an Op-ampAttachment 174850

Cool, I'm glad you think it would help. I'm actually wanting to try building the circuit with half of a 6BZ7, since I already have some of those, and it would prevent me from having to obtain a 6BN4 on ebay. Although I'd still have to find those odd resistor values for the 250V version (since my TV's B+ voltage is 250V, but the heaters are all 6.3V in parallel).

Here's a picture of the results so far. The camera blurred the motion on the screen quite a bit, I'm afraid, so you can't really tell anything about the picture quality from this.

EDIT: I mean, this is before adding the video input amplifier stage. I'm still waiting on the odd-value resistors from Digikey.

An interesting development: I replaced a 120uH inductor in the IF stage with a 100uH inductor available at Radio Shack. I now get a very decent picture via the tuner! I don't think it's perfectly aligned, though, as I still get a fair amount of 'fuzz' from a directly connected RF source.

And, unfortunately, the picture still smears in areas of sharp contrast, even over the tuner. There must be some other problem in the video circuit that's causing this, rather than a simple impedance mismatch.

More progress. I replaced the 'weak' 5U4GB with a 'good' 5U4GB, and the picture looks slightly better, now. I also plan to replace the main filter choke, as it's getting fairly hot as the set runs.

Just spent a while trying to get the picture adjusted. Vertical linearity is slightly off, as is horizontal linearity, but it isn't really noticeable on a regular picture, just with a test pattern. I think I need to make the main power supply filter choke a priority now, though, as just before I turned the set off tonight, I noticed a small amount of smoke drifting upwards from underneath the chassis at about where the choke is mounted. I'm thinking I'll replace it with one of these units from Edcor:

https://www.edcorusa.com/p/770/xc50-0_5h-250ma

It looks about right. The original choke is 0.4H at 250ma, and this one is 0.5H at 250ma. Unfortunately, the shipping from Edcor is about as much as the cost of the choke itself, so it'll be a little over a $30 purchase, total. Also, I'm not sure about the dimensions yet, but I think if I want a brand new filter choke, that's definitely my best option.

I'd check the current draw through your original choke first before purchasing a replacement. It is possible that somewhere in your set something on the B+ line, that I assume that choke is feeding, is drawing excessive current. If that is the case the new choke will likely smoke too. Also if you are able to reduce the load on the original choke it may cool down and stop smoking.

Just my 0.02$.

Good idea. Although actually, I need a new choke anyway. I neglected to mention that the original choke had fallen apart - I just happened to have a 'mystery choke' of about the same size that I decided to use to test with.

Alright, I just checked the current immediately following the choke with my multimeter. It's only drawing around 175ma, so the 250ma replacement should be enough. I don't know what the choke I have in there now is rated for, but it's either rated too low, or it is sufficiently deteriorated after years of non-use that it can't handle the 175ma.

Hearing that I say get a new one and pop it in! Your power supply will probably thank you.

Yup, that's my plan! I just took out the choke I've been using and measured it. The dimensions of the one Edcor sells are actually slightly smaller. I don't think I'll have a problem fitting it in, although if the tabs don't stick out far enough, I may have to figure out another way to secure the 'free' side of the choke.

Did a little more poking around today. Replaced a 47k resistor that had drifted very far out of tolerance with a 33k and a 15k in series (for some reason I'm out of 47k resistors at the moment). Also replaced the capacitor across the contrast control. It's supposed to be 50pf, but I had a 47pf capacitor available, so I tried that. I still get streaking in the picture unless the contrast is set at minimum, but with the replaced capacitor, minimum contrast appears to be slightly more watchable. Troubleshooting this thing is starting to get pretty fun, since I see slight improvements with each change I make. I don't plan to leave it on for more than a couple of minutes at a time until I can replace the B+ filter choke, though.

Unless you care about the old choke or mount the old choke's frame to chassis (opening the set up to the possibility of a B+ short that would kill the power rect or transformer) there is no reason to run the set for only 2 minutes at a time because the worst that could happen is you blowing the old choke open (effectively harmlessly removing B+ from the set) and having to wait for a replacement to run it.

Alright, today I wired up that impedance matching amp. It actually made my picture far worse! I think the original input impedance on my video output tube must be much closer to the current composite video standard than other TVs of the period. I'll just keep the composite video input wired up directly, and I'll do what I can to improve the picture quality by continuing to test for out-of-tolerance resistors as well as replacing any of the old ceramic and mica capacitors which I think may be problematic. Since the tuner input exhibits the same streaking on the picture (as I discovered recently) it makes sense that the streaking isn't caused by an impedance mismatch.

EDIT: Well, the video amp does help with one thing. It makes the brightness of the picture flicker a little bit less. Unfortunately, the TV doesn't cope well with the amplified noise in the video signal and keeps losing vertical sync. The streaking in the picture is also more pronounced with the amp in place. I kind of think the brightness flickering of the picture will be lessened when I get the proper filter choke installed in the TV, as well.