Zenith 2250R
 

On the brink of a hurricane, with too many other unfinished projects, I find myself starting yet another one. This time it's the 12" Zenith porthole I picked up on my trip to Alabama. This is actually something that I've been looking for for awhile. In my opinion, the smaller screen looks better than the later larger ones.
Attachment 197672

Attachment 197673

I've spent the past two days recapping and cleaning the set in preparation for a power up. The B++ is a smidge high, running in the 470-475 range instead of the 460. Other than that, the "low" voltage section is working fine. The HV is running about 2000V, I think that there may be a few out of tolerance resistors in the horizontal output section. (Maybe?) Also what's the horizontal drive adjustment used for? Adjusting the HV?

I've gone through the HV section and Horizontal output section and replaced any resistors that are remotely out of tolerance. The Horizontal output tube is only receiving 90 volts on pin 4 when the voltage chart calls for 180 volts. All of the low voltages from the power supply are spot on. Any thoughts?
Attachment 197671

None of your links or images work.

I re-uploaded them, not sure if it fixed it or not

I'm seeing them just fine on my Galaxy S9 with Chrome, but they won't open correctly on my Linux desktop using Firefox.

Either way, anyone have any ideas? I figure one of two things, Is it possible that the listed 15k resistor is too high? If I did my math correctly, to drop the voltage to 180 volts from 300v, you'd need a resistor of 8k.

Or the Horizontal Drive potentiometer is testing at around 34k ohm instead of 25k. In that case could it be substituted by removing the 4.7k ohm resistor that it is in series with?

Sounds to me like your screen(pin 4) may be drawing too much current which would cause the screen voltage to be low. Have you checked the cathode voltage(pin 8) against what Sams calls out? The horizontal drive will adjust the cathode current and generally you want to adjust for lowest current by inserting a ma meter in series with the cathode. A Sams folder should give some specific directions for setting the drive control. Also I'd try another damper if you have one. Specifically what schematic are you using?

I'm using Sams Set:114 Folder:13 Replacing the damper did improve the voltage slightly, to around 7.5kv. Doing so improved the Screen voltage to around 115v. The cathode voltage was sitting around 11v (Sams lists 10v) before and after replacing the damper.

So your HV increased from 2kv to 7.5kv? Thats quite an improvement in my book. ;) Did you see the Horizontal Sweep Circuit Adjustments on page 10 of your Sams? Also check R103 (470k under the HV rectifier socket) to make sure it hasn't drifted high, as that can really drop your high voltage if you're measuring at the CRT HV anode.

If you have them, try a few other 6BQ6 horizontal output tubes. Riders calls out 10kv for high voltage. 12v on horizontal output pin 8, 141v on pin 4, and 12v on pin 5. Nothing magical about those numbers, they're just ball park and vary from tube to tube.

Well, I've checked all of the voltages on both the horizontal output and the damper and everything matches up except the screen voltage on the horiz out. I tried a spare 6BQ6 that tested NOS and it didn't change. R103 on the anode lead tested about +8% but it's noted as a 20% resistor on the datasheet.

Riders also calls for 15k screen grid. I suppose you've already changed the HV rectifier and checked it's 1.2 ohm filament resistor?

Interesting that Sams calls for a 3.3 ohm HV rectifier filament resistor, but Riders shows 1.2 ohm. And I don't see any notes in the Sams parts list specific to model type variations.

*Edit* Oops, I do see a note in Sams that says some sets use a 1.2 ohm. The Riders schematic is specific to your model/chassis.

Yeah, changed the HV rect and checked the resistor, mine is the 3.3ohm variant

I'm not sure if 3.3 is correct for your set, it's not according to my Riders schematic that lists your specific model. But I also don't know if that alone could account for your 35% lack in high voltage, I kind of doubt it.

I guess it's time for someone smarter than I to step up and make a suggestion. :yes:

Someone just check this and my logic. Assuming that the screen on the 6BQ6 doesn't provide any voltage, the resistor (R15) faces the whole 340v of the B++. Given this, we can determine the "should be" resistance of R15 based off of the given voltage drop from the Sams voltage chart. If I did my math correctly, the resistance should be about 8K ohms. Why isn't it 8K ohms then?

I don't see how you can calculate a "should be" resistance value without knowing the nominal average screen grid current.
The average screen grid current can vary with the peak current and the length of time that it draws current, which I would think is a function of the drive control. So, I would look at the drive control value and also the adjustment procedure in SAMS. I would expect the DC screen voltage measurement varies with the drive adjustment (check if it's so?).

Well, I gave it a shot. I can't find the alignment describing how to adjust the horizontal drive in my Sams. Either way, I've messed with it and it moves the HV up some, but only to about 7.5kv. I noted earlier that the horizontal drive potentiometer tested a little high (34k vs 25k ohms) Not sure if that could be affecting anything.

If the H drive pot can go to zero, then the max value of 34k is not a big deal, as long as the 4700 ohm is the right value.

Yeah, the Pot can go to zero and I had replaced the 4700 ohm resistor as it had gone bad.

Just for the heck of it unplug the HV lead from the CRT and take a reading on the HV lead connector. Just to verify that the CRT isn't somehow loading down your HV.

Actually, I've been doing most of the tests without the CRT, I hooked it up last night just to verify that the voltage wasn't high enough. Still couldn't get anything. I've also ordered a few more 6BQ6s just to verify it isn't the tube, though both of the ones I have tested NOS without any shorts. Just curious, what could possibly be loading down the voltage on the screen of the HV output? and what is the plate voltage of the 6BQ6 supposed to be? (Listed as do not measure)

Leakage current to ground internally in the CRT or possibly something interfering with correct bias on the grids.

The plate of HO tube has high voltage spikes that can easily take a meter out. It's generally not recommended to try measuring there.

I meant using a HV meter to measure it. Either way I'm still not sure why I'm not getting the proper voltage on the screen of the Horizontal output, is there anything I can do to isolate the problem?

Measuring the DC at the HO plate, even with a high voltage probe, is useless.

If you have service info that tells what the retrace time should be, you can compare that to the actual pulse width, using a scope. Just bring the probe somewhat near the HO plate lead, but not close enough to touch or draw an arc, and view the capacitively coupled pulse. If the pulse width is notably different from nominal, this could affect the HV. This would lead to checking components around the flyback and yoke.

7.5KV should be enough to get some kind of light on a healthy 12" CRT...Try pulling the Vert output tube and see if you can get a horizontal line on screen. Is the ion trap of the CRT adjusted correctly, and do the electron gun bias voltages look reasonable?

One thing to be wary of is that if the horizontal oscillator is not running very close to the correct frequency the HV can become low (yet all the DC voltages on the output tube can be about right). If you haven't adjusted the horizontal osc with a scope and a frequency reference (the sync pulses on the composite video output of your signal source is a good reference), and you have never got video on the screen then the osc is likely to be off freq a ways.

That explains a few things, the horizontal oscillator was at 15.35khz instead of 15.75khz listed on the service manual. This might be a stupid question but what exactly gets adjusted? Do I adjust the master oscillator? Or is there an individual adjustment for the horizontal oscillator?

Sorry for all of the questions, still a newbie at this
~Zach

The only horizontal frequency adjustment you have on this set is the horizontal hold control. Can you see the horizontal frequency change when you move that control to its extremes? I agree with Electronic M that you should see a raster even with 7.5kv. I'd try rotating the Ion trap around the neck and also forward and back in a dimly lite room, in hope that a hint of light shows up. Then the idea is to get it to the position where the screen is at it's brightest. Once you see any light, turn down the brightness and keep trying to bring it back with movement of the trap. Since you've had the CRT out there's no telling how far it's been moved from it's former working location.

Alright, I guess it worked because I'm getting some sort of picture. It's not the greatest or clearest picture I've ever seen and actually, the voltage worked itself out. It's at around 9kv which is better than it was before. Here's a picture of it displaying the Indian head test pattern.
Attachment 197676
Attachment 197677
I'm still pretty new to the TV category, what else needs to be done to improve the image? I'll center it completely once I move it back into the cabinet, and looking back, I think the focus could be adjusted to be way better.

Good job! :thmbsp:

Now try running though this horizontal setup procedure from your Sams folder 114-13 page 10. Hopefully you can get the pattern a little less egg shaped. Just remember these things were never perfect even back in the day when folks used them for their daily drivers.
Besides focus adjusting your fine tuning control may help clear up your image some too.

https://c2.staticflickr.com/2/1881/4...9748b076_b.jpg

So, I was messing around with the TV and letting it run for a little bit. Lost the vertical hold from a few resistors soaring in value. Fixed that, but still can't get a crisp picture from this set. I can adjust the focus magnet to get a pretty good image, but it sorta stops at a point. Any ideas?

Also, anyone know where I can obtain test picture tube? Brushing up against the CRT isn't my definition of fun. I haven't seen any round versions on the Bay recently.

Do you have the brightness most of the way up? If so it could be that the CRT is near the end of it's useful life, which at that point the electron gun looses it's ability to focus well. It's also dependent on HV, so it could be that your HV is still on the low side. Try dimming the room lights and see if bringing the brightness down helps sharpen the picture any.

Another thought is if the ion trap is on the neck reversed end for end it could be limiting your overall brightness.

Considering the amount of ambient lighting in your shop I'd suggest your CRT is doing it's level best at producing a decent picture. These early tubes were never intended to be watched in a well lit room. The screens simply can't produce a high enough level of contrast with the brightness turned up high. Just remember that the CRT face can only go as dark as the ambient light reflecting off it's face. In other words the CRT can't make it self go any darker, it can only lighten up. That's ultimately what limits your contrast. Hope that makes sense.

http://www.videokarma.org/attachment...7&d=1536895143

One of the 8" rectangular test tubes with the same base (8AXP4 IIRC) will work on the early sets, but the screen will not be filled due to the difference in deflection angle.

When I do most of the adjustments, I keep the lights down, I’ve found that the fluorescent lights are way too bright, but that a light on in the back isn’t too bad. Brightness isn't bad, I have it up at about 3/4 brightness. Any more and the image washes out. The HV is still running lower than spec. (still 7.5kv) I replaced a bunch of resistors in the both the horizontal and vertical sections to fix a few issues. I’m gonna try injecting a composite signal tomorrow to see where that gets it. Hopefully it’ll make it easier to make adjustments that way. Also, is there an easy way to tell which way the ion trap goes? I don’t think the one on the unit is stock, and it doesn’t have an indicator.

Ok, great to know. Murphy’s law, there’s none listed right now. I’ll keep a look out

When you say adjust the focus magnet are you adjusting the focus magnet itself or the focus potentiometer?

Focus magnet, to my knowledge the H22 series only has mechanical focus.

Two things that may be worth doing towards the end of fixing the low HV: 1.) If you haven't done this yet and have several good 1B3s try swapping different ones in (tube testers while good at IDing dead HV rects are a poor indicator of how good they will do in a set). 2.) If a tube rect does not help try an SS HV rect...They made SS plug-in R-3A3s that will work in place of a 1B3, or if that is hard to get 5KV (or higher) PIV rated diodes are inexpensive online I'd grab 3-4 5KV diodes, string them in series and install them in place of the tube...Elimination of the HV rect tube filament load from the fly by switching to SS rect may bring the HV to spec.

And don't forget there was some discrepancy between Riders and SAM's as to what the HV rectifiers series filament dropping resistor was suppose to be in this set. An ohm or two can make a big difference when it comes to a low voltage heater.

Alright, so I substituted a brand new (5 in carton) 1X2B in for the HV rect. and I'm still having issues. The HV is still low, I'll probably try replacing it with the 1.2 ohm, though the one currently in the set is a 3.3 ohm. If that doesn't work, I'll try the solid state idea.

For a quick test you can tack a second 3.3 ohm in parallel. That way you'll save all the work of removing the original in the event that it makes no difference. However there's also the concern of keeping the solder joins round and smooth in that high voltage area. Sharp points are more prone to arcing.