Zenith 2250R

[Zsuttle]

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.
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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?

[Zsuttle]

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?
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[Electronic M]

None of your links or images work.

[Zsuttle]

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

[benman94]

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

[Zsuttle]

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?

[Kevin Kuehn]

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?

[Zsuttle]

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.

[Kevin Kuehn]

Originally Posted by Zsuttle

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.

[old_tv_nut]

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.

[Zsuttle]

Originally Posted by old_tv_nut

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.

[Kevin Kuehn]

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.

[Zsuttle]

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)

[Kevin Kuehn]

Originally Posted by Zsuttle

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.

[old_tv_nut]

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.