Very interesting! The aspect of my 48-2500 that I find most disappointing is that the picture doesn't stay in focus between light and dark scenes. Mine is so bad that I don't really consider it tolerable. Interesting that Philco acknowledged that this was a problem.

When I received my set, the HV supply had been removed and replaced with a homebrew RF HV supply on a separate chassis. I never understood exactly why, and since the homebrew HV supply had developed its own problems, I removed it and rebuilt the original circuit using parts from another 48-2500 chassis with 1X2s. It works, but the focus problem is severe, and the cause appears to me to be due to sag in the HV with scene brightness (perhaps made worse by using a triode CRT) affecting the focus.

Perhaps the reason it had a homebrew HV supply was because a previous owner found HV sag to be the problem, and built a better HV supply.

Sometime in the near future I'll try the removal of the DC restoration tube. On the other hand, I'm not eager to live without DC restoration, so I'll be interested in whether there is some other fix. I've wondered if it would make sense to try to build a 6BK4 shunt regulator like that used in early color sets to solve the problem?

First, I'll wait to see if Eric's set shows less of this focus problem than I am seeing. There may be problems with mine beyond what these sets typically show.

It seems that it might be possible to build a dynamic focus circuit to vary the focus coil current with varying conditions of cathode current and HV. :scratch2:
Hide the ics and transistors under the chassis somewhere...nobody needs to know!
jr

I would think it is not the focus coil current that needs to be controlled, rather the HV needs to be regulated. As the As these supplied to be rather anemic, I would think a shunt regulator would not function all that well. But it would be interesting to try it . Or, as the Philco HV supply is independent of the horizontal sweep, it should be possible to build a feedback design which compares the HV to a reference voltage and used the difference to drive an amplifier to modulate the screen grid voltage on the horizontal output drive to the flyback, or modulate the B+ voltage to the flyback. Either approach would be interesting to try.

How bout just sticking a 20Kv Neon light transformer inside the cabinet? :D

I haven't run my 2500 in a couple of years, but I don't remember having any big problems with the focus blooming. Now the RCA sets are a different matter, none of my RCA sets will hold focus with scene brightness changes. RCA "solved" that problem in the '49 sets by giving you a remote control brightness and contrast control so you could somewhat control the focus issue from your easy chair.


Chuck

I agree the origin of the problem is in the HV sag, not the focus coil. However, the HV is derived from the horizontal sweep flyback transformer, so I don't think we can apply the fix you've suggested. Or did I misunderstand your idea?

Interesting to hear that the RCA projection sets are worse. I'm surprised they marketed the sets with these design weaknesses. It can't be that hard to design around the problem.

:lmao:

Indeed, HV sag would be the most likely cause of the defocus of a bright picture on these sets, and should be addressed first. However, triode gun CRTs have a bad reputation for worse spot size growth with increasing beam current than tetrode designs.

Just saying that if that is the case, a little "dynamic focus" could improve the beam performance.

Of course, the focus coil would be difficult/impossible to drive at video frequencies, but it should be possible to provide focus correction fast enough to improve average focus.

jr

Don't know if this is the correct tube, but I came accross it whilst surfing Egay this morning: http://cgi.ebay.com/NOS-RCA-5TP4-Ult...27510791741787

Not affililated.

The Philco uses a 4" type TP-400 CRT. Philco was the only manufacturer to use this CRT. The 5TP4 is used in the RCA and many other projection sets.

The problem with these sets is most definitely high voltage sag with increasing beam current...

The sag is the result of the voltage drop across the 1X2s...

I had one of these sets once, and after experiencing the blooming issue to the point of considering the set unwatchable, I removed the 1X2s and their associated filament windings and installed a solid state high voltage tripler (extracted from an old color television chassis) in their place.

The results were stunning.

High voltage regulation was vastly improved, and the focus issue was almost completely gone (only with the brightness cranked all the way up.)

ETA: I also installed a retrace supression circuit in that set so I could crank up the brightness a bit without having retrace lines. It worked flawlessly. BTW, don't remove the DC restorer. It will just make the set more unwatchable.

If you can stand to do this to your set, this is the cure.

TV Engineer,

Any tips on where to get a solid state voltage tripler? Do a lot of modern CRT sets have them, or is it something a bit obscure to find?

Tom

The most frequent high voltage problem comes from bad "Presistors" in the multiplier circuit. They are special high voltage 2 meg resistors. There should be plenty around. I have some if all else fails.

Well I've replaced all the caps in the set except four small paper ones.

One of the paper caps I have left to do is a .005@1000 volts which I don't have in my stock, it's on the audio tube so it won't matter much as far as trying it out.

I was able to restuff all the electrolytics in the cans except for two that I had to mount underneath.

I couldn't find the box with the tubes for the set when I was leaving for work this morning, if I can find them tonight I'll probably give it a test run tomorrow and see what happens, I'm going to use the worst looking of the three CRT's in case something goes kerblooie.

Almost any American solid state color set (and a few hybrids) from the early 70s til the early 80s have them. Zenith, Magnavox, Sylvania, RCA, Admiral... they all used them at some point. You can also order one new with the ECG/NTE523 part number. They're about $25. Hook the output of your flyback to the in, ground the ground pin, and connect the output to your CRT. Ignore any remaining pins. That is all there is to it.

Somebody here mentioned the 2 Meg resistors in the original tripler circuit being bad...

They did fail, but if yours were bad, the regulation would be even worse, and you might not have any high voltage at all. I experienced this with my set too. I replaced them, but the HV regulation still sucked, which was when I came up with the solid state tripler idea.

Let us know how it works for you if you try it.