Prototype set is here

[miniman82]

I was asked recently to provide shots of the proto set in it's cabinet to use as part of an article, so I thought I'd share the same photo here so you can all see what the set will look like once finished. This is the TC-168 cabinet with doors, in oak.

[mpatoray]

That is some great news Steve, passing video and getting sweep is an accomplishment, can't wait to see everything as it should be with this set displaying CPA on the proto tube!

Nick,

The set will look great in the cabinet!

[miniman82]

Mmmkay,


I finally had some time to fool around with the proto tube, powered by my CT-100 chassis. I mounted the stuff in the cabinet same as at the museum, then powered it up. Luckily, the tube rim doesn't get close enough to the chassis to arc over with the chassis in the cabinet.




I found that I have the same issue with not being able to achieve convergence, despite the HV pot mod being done. I confirmed that ultor potential is not dancing around, even with the convergence control maxed out. In this shot I haven't really done anything with the static magnets, just powered it up and played with the DC convergence control. Same as at the museum- you can get close, but not close enough. This is with the 'stock' 50 meg string resistors installed in the chassis, next I will give the 100 meg ones a try.

[old_tv_nut]

getting there!

[Sandy G]

Yep. The glorious "End" is in sight...

[Aussie Bloke]

Wow you're really making excellent progress with this TV, it is so cool to see colour on the prototype tube, fantastic!!! Keep up the excellent work!!!

[Bill Cahill]

I'm very impressed with the work. How far is the original chassis?
Though the colors were wrong, you have color. Isn't that the CT-100 chassis? I get mixed up which chassis you are using... Sorry, guys....
But, my hat is off to you experts. Great job....
Bill Cahill

[miniman82]

The 100 meg resistors only made the situation worse (as expected), there was even less convergence voltage available. Ohm's Law wins again. Then I tried a 25 meg resistor as the first one coming off the ultor going to the DC conv. pot, and bingo it popped right in! Unfortunately dropping so much voltage is causing it to heat up in a hurry, so voltage drops as time goes on. I've decided to order five 5 meg resistors at 2 watts a piece, which will give 10 watts total dissipation. This will spread the voltage out over more resistors (and thus more surface area), and hopefully cure the drifting issue. Once I am able to keep the convergence voltage stable, I will finally be able to post properly converged pictures of the proto tube. Here's where I'm at right now.



As you can see, blue is still slightly high and red is a little displaced as well. These are static convergence issues, which are easily cured by twiddling the magnets on the neck of the tube. Ignore the faint blue bar at the far right, I think the tuner is receiving interference from the stray static field escaping the CRT. I will need to construct a supplementary shield, to make sure all the ultor potential remains trapped. Heavy plastic should work. Yay progress!

[old_tv_nut]

Beautifuller and beautifuller!

[Eric H]

It looks great for a 60 year old Prototype tube that probably wasn't intended to last a year.
Strange to think that probably everyone who worked with it back then is most likely gone by now.

[ChrisW6ATV]

Nick, how about trying a 25 meg on both ends of the convergence pot followed by a 50 meg to ground? Mouser has 1-watt 25M HV-rated resistors, but none with higher power ratings. I used two of their 50M 5-watt ones in my CT-100 and they work well, even fitting the cup on the HV rectifier socket.

[Aussie Bloke]

Wow, you are really succeeding in bringing this tube back to it's former glory, keep up the great work, those colours are looking very good and very accurate!!!

[NewVista]

Amazing Proto metal/glass CRT holding orig vacuum!

[miniman82]

Originally Posted by ChrisW6ATV

Nick, how about trying a 25 meg on both ends of the convergence pot followed by a 50 meg to ground? Mouser has 1-watt 25M HV-rated resistors, but none with higher power ratings. I used two of their 50M 5-watt ones in my CT-100 and they work well, even fitting the cup on the HV rectifier socket.

I was going to try that, but Ohm's Law tells me it's a bad idea. Reducing the resistance on the low potential side does a couple unwanted things. #1 it drags the low potential side of the pot down, which is counter productive (I need higher voltage). #2 it increases the total current drain of the string, placing added stress on the first resistor causing it to heat up and drift more than it already is. With a 100 meg as the second one, the low potential side of the pot is pushed higher. Total current drain is reduced, taking stress away from the other ones in the string. Here's a comparison chart, with 25 meg for both and on the right the way things sit right now.




In practice there's less voltage available at either end of the convergence pot, because of the first resistor drifting. About 10kv and 8kv respectively, by the time I measure it. So as you can see, it's beneficial to dissipate the majority of the voltage after the convergence pot.

[ChrisW6ATV]

OK, I think I misunderstood the original configuration. I thought it was originally HV>50M>pot>50M>(followed by HV pot or ground depending on the mod you described elsewhere), as is my CT-100. I had intended to propose HV>25M>pot>25M>50M>(ground or HV pot, etc.). You are already at 125M total not counting the pot, so now I understand the problem is mainly the 25M resistor's dissipation, not the total string resistance.