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Old 09-11-2018, 11:54 PM
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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.
20180826_200539.jpg

20180825_230419.jpg

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?
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Last edited by Zsuttle; 09-12-2018 at 02:10 PM.
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Old 09-12-2018, 12:52 PM
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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?
20180912_133256.jpg
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Last edited by Zsuttle; 09-12-2018 at 02:07 PM.
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Old 09-12-2018, 01:17 PM
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None of your links or images work.
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What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4
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Old 09-12-2018, 02:11 PM
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I re-uploaded them, not sure if it fixed it or not
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Old 09-12-2018, 03:58 PM
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I'm seeing them just fine on my Galaxy S9 with Chrome, but they won't open correctly on my Linux desktop using Firefox.
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Old 09-12-2018, 05:56 PM
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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?
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Last edited by Zsuttle; 09-12-2018 at 06:05 PM.
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Old 09-12-2018, 06:48 PM
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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?

Last edited by Kevin Kuehn; 09-12-2018 at 06:57 PM.
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Old 09-12-2018, 07:30 PM
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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.
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Old 09-12-2018, 08:09 PM
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Quote:
Originally Posted by Zsuttle View Post
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.
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Old 09-12-2018, 08:21 PM
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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.

Last edited by Kevin Kuehn; 09-12-2018 at 08:38 PM.
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Old 09-12-2018, 08:56 PM
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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.
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Old 09-12-2018, 09:03 PM
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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?
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Old 09-12-2018, 09:09 PM
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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.

Last edited by Kevin Kuehn; 09-12-2018 at 09:13 PM.
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Old 09-12-2018, 09:16 PM
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Yeah, changed the HV rect and checked the resistor, mine is the 3.3ohm variant
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Old 09-12-2018, 09:33 PM
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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.
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