Philco 50-T1403 with focus problem

[dreyfoos]

Gentlemen, I’ve met my match. I’m at a dead end. I need to use one of my lifelines.

On the bench: one Philco 50-T1403 (Code 125). This 12” beauty was won on eBay a couple of years ago and has just now found its way to my bench. It first arrived as a UPS total whack job. The top of the arch top cabinet was crunched in and the back cover forced down to guillotine the neck of the previously working CRT. That provoked me to swear a bit. But time passes and I’ve done the initial gluing and clamping to begin restoring the cabinet. The power switch and interlock socket obviously needed replacement and were done. With a 5AXP4 test tube in place and all the lytics and Black Beauties replaced with shiny new, I brought it up slowly. (BTW – there were a number of minor variations in this chassis and that kept me on my toes to trace every component and make sure that my schematics (both Sams and Rider) matched this chassis. At full voltage, a badly out of sync raster and weak sound greeted me. But I’ve seen much worse. So I dug in and replaced a bad mica to fix the sync. I touched up the sound to get decent but not booming sound. At some point I lost HV and traced it to a broken wire off the fly. A delicate soldering job fixed that. Ta da! - a very nice 5” picture. I was ready to try a big tube. I had a high mileage RCA 12LP4 – but it came on bright enough. Just one problem. Bad focus. Of course, the design of the 5AXP4 gave me perfect self-focus when it was in place.

So, I thought, what could be easier than fixing a focus problem? Weeks later, I’m still at it.

Let me backtrack a bit. When the set was listed on eBay, there were photos and one of them showed a sharply focused but badly out of sync raster on this set. The previous owner had done some cap replacement and I double-checked his work. By the way I’ve had other business dealings with him and have no reason to believe that the focus was anything but perfect before he turned it over to UPS.

This model uses a yoke and focus magnet assembly that is bolted together – so the separation is preset with the whole thing snuggling up to the bell of the CRT. Because of that, I don’t believe it’s a matter of fore and aft positioning or of having anything installed backwards. All voltages appear ballpark. Where 330VDC is called for I get 340-350 depending on the position of the focus control. For the 325, I get 320-340. For the 310, I get 320-335. For the 250 which supplies the tuner, I get 280-290. For the -4, I get -3.3 to -3.5. I have checked the focus control and the focus coil and they have correct resistance. I have replaced the resistor shunting the coil (R127 on Sams) because it was a little too high (and I had a new one handy).

Just to be sure about the CRT, I replaced it with a N.O.S. 12KP4, but still had the same bad focus – on a blank raster as well. Turning the focus control doesn’t change the focus although there is a slight size change which I attribute to its slight affect on the B+ voltage.

When I put a meter in one of the leads to the focus magnet it reads 52-165 ma. – depending on the focus control setting.

This Philco also uses a permanent magnet that surrounds the electromagnet. As I understand, that is supposed to do the heavy lifting and the electromagnet does the final adjustment. Could it be that the permanent magnet (non-adjustable) is tired – even though it didn’t appear to be tired before the UPS ordeal? How would one gauge the gauss of the permanent magnet? I have no number for what gauss strength it is supposed to be. If it were found to be weak, then how would one give it a boost – since I imagine a replacement would be hard to find? So that’s it. I’ve skimmed the comments on focus in every one of my many repair books from the era and other than mentioning that high heat or a hard knock could affect the PM, nothing else is mentioned. Could this be an aging problem that wasn’t anticipated those many years ago? But how would that fit with the apparently sudden loss of focus? Anybody got a clue?

Roger

[Steve K]

Roger:

Did you try my suggestion that I posted on the Antique Radio Forum? You never know, it might fix your problem.

Steve

[dreyfoos]

Steve, my Philco does not have a DC restorer tube – at least not labeled as such. I did substitute for the 6V6 video output and the 12AU7 video det. – agc rect. – video amp with no change in the focus situation. As you said, worth a try. I’m wondering if your Stromberg Carlson really had the same problem? That is, did it have sharp lines on the blank raster or was the focus bad only with a picture? My focus is totally pooped out all the time and that leads me to question the P.M. encasing the focus coil – simply because I don’t know how to test it and I have no readily available substitute.

Roger

[Steve K]

Roger:

My Stromberg had very poor focus without a signal. I could not see the scanning lines. The focus control changed the image but never brought it into sharp focus. I finally turned my attention to the video output and found the bad DC restorer.

Steve

[dreyfoos]

Steve, I forgot to remind myself and all that I get an excellent picture on the 5AXP4 test tube. Wouldn't that take the video section off the table?

Roger

[Steve K]

Roger:

Yes, you are probably correct. Darn, it would have been an easy fix otherwise! Perhaps your focus coil has some shorted windings? It could still measure correct for resistance.

Steve

[bgadow]

I have a 12" RCA (2T51?) with this problem. It just uses a permanent magnet for focus. First I found that the travel of the magnet is very limited-after a lube job and with some help I can get it to go through the full travel. From looking at other sets this looks like a common problem. But, even after getting that figured out the focus still won't improve. I tried swapping magnet assemblies from another set (I do have some spares) but still no luck; just holding a strong magnet up to the neck didn't seem to change the focus either. I had guessed, in my case, that I had either B+ problems or a defect in the crt. I am going to follow this thread close. I will have to look for a possible DC restoration issue, I guess; without reading the ARF thread, wouuld it effect this set which only uses the perm. magnet?

Strange that the test tube would work. It seems like that would eliminate everything but the crt but it doesn't...

[dreyfoos]

Hello Bryan,

You mentioned the test tube situation. In my case, I used a 5AXP4 with an electrically identical yoke from a similar Philco set (but not a similar magnet). In that lash-up I also used a resistor in the connecting harness to sub for the focus coil which I didn't need since the 5AXP4 is self-focusing. Of course, a gassy CRT can also cause bad focus. My understanding was (before Steve's comment above) that if you can get sharp scanning lines on a blank raster then the appearance of bad focus eminates from a video problem. Now, perhaps a video problem does have to be considered. Somehow, I still think that my probem has to be in the magnet area. It would be nice if we could make a definitive measurment but I don't have a magnetometer in my toolbox and I dont have a factory fresh strength number anyway.

I may yet try to rig up the permanent magnet or even the electromagnet from the other set. I'd like to avoid all that if possible as the circuitry is a bit different.

Next step might be to sub a third 12"er just to make sure in that area.

Roger

[Don Lindsly]

Have you measured the high voltage?
Have you reversed the focus coil leads?

Don

[bgadow]

I have heard that magnets can lose their strength when dropped quickly. But even as hard as this set was treated in shipping it seems unlikely it would be enough to kill the magnet.

I couldn't get a good scanline on mine no matter what. Complicating the issue is that my set is designed for an electromagnetic speaker, which is missing, so I have experimented with various combinations of power resistors or other speakers to compensate. I have had the B+ too high and too low and the picture never focused. I need to find an original speaker or a proper choke, I suppose. A back-burner project.

[dreyfoos]

That's the ticket, Don. I owe you. I reversed the focus coil leads. Huge difference – went from 0 to maybe 90% perfect.

However, the best focus then appeared to be just out of range of the focus pot at maximum CW rotation (with blank raster or video) So then, I switched from the 12KP4 (which I bought as N.O.S. from Surplus Sales of Nebraska some years ago) to the (old RCA) 10BP4 and the focus went up to 99% - but still at CW max rotation - perfection just out of reach? Any thoughts? I had previously replaced the Rider C65 (Sams R127) 100 ohm, 1W focus coil shunt. The focus pot checked fine at 500 ohms.

BTW, the HV is 8900 VDC. Koinkidentally, I had been ruminating about those focus coil leads. IIRC, I had some reason to re-solder them a year or two ago. I had red to red and green to brown. That green to brown was a clue. Now it seems very logical that there is a polarity dimension that I had not accounted for.

Roger

[dreyfoos]

Bryan, I found a thread in the AK archives that might give you some ideas. About half way down this thread you will find a discussion of permanent magnets that perhaps weren’t so permanent and needed to be “goosed.”

http://www.audiokarma.org/forums/arc...p/t-37424.html

Roger

[Don Lindsly]

The high voltage looks fine.
Does the focus coil now need more or less current for a perfect picture?

[dreyfoos]

Don, more CW rotation gives more current - higher current yields best focus.

Roger

[bgadow]

Roger, thanks for looking that up. I vaguely remembered that thread but had never gone searching for it. I've printed it and will stick it with set.