did you test the CRT? this is very odd, perhaps a weld opened up. did the CRT take a whack at some point during the recap?

One of the pins on the CRT (sometimes called grid 2) should have several hundred volts on it. Is that voltage present? I hate to say that I think that the CRT went bad, but it is beginning to look like it. I have been assuming that the CRT is a new enough design to not use an ion trap. If it does use an ion trap, then that could be out of adjustment.

I inspected the CRT, but not really can test it for mechanical or vacuum problems, I treated it like a raw egg when I cleaned it.


As stated several times all DC voltages are ok, including grid 2 (on pins 2 and 6 which are internally connected), which is at 400V. How can I see if that apparently after market CRT has an ion trap? As also stated several times the set had worked before recapping, and I didn't do anything to the CRT itself.

Tomorrow I will swap the whole chassis against one of the two working ones. Thanks so much for all your help.

I thought that you mentioned the G2 voltage, but did not find that mention in a quick (too quick) scan of the posts. An ion trap is a small magnet attached to a band (looks vaguely like a hose clamp) slid onto the neck of the CRT between the yoke and the CRT socket. If there is such a thing on the neck of the CRT, rotate it and slide it up and down until you get light on the screen. If you don't see anything like that, then it must be that your CRT does not use one.

Sorry to be asking questions that have already been answered. With the voltages that you measured, the set just about has to work.

the test I was think was just a reg emissons test with a CRT tester. If a weld came undone it would measure ZERO emmissons.

As far as swappig around chassis, I would make sure all the voltages you read are the same on the pins between the two chassis 1st. I seem to recall some odd pinouts used for G1 and G2. Of course if they are the same I don't think anything will be gained by the swap.

the CRT tester really should be the next thing. Since it did work before and you can get the correct voltages per the sams, confirm CRT tester pinout to the sams pin out. If it test with ANY emissons then there will be nothing to be gained by swapping CRTs.

the recaping process was done with out removing any wires from the wirewraps or anywhere else right?

Hi chris.
i agree with DaveM-a lot easier to compare readings from 2 sets versus chassis swap.
perhaps i should get my crt tester to you-i will check to make sure it can test your tube.
have a good one,
RonL

This may be a dumb question, but I am not familar with this model. The question is driven by the picture posted showing the neck of the CRT. I don't think I saw an ion trap and was wondering if this model is supposed to have one? If it is supposed to and it is not there you can end up with a whole lot of nothing on the screen. You mentioned carefully cleaning the CRT and was wonding if it might have been removed at the time.

Curtis Eickerman

Since I have two Predictas without ion trap magnets, I had assumed that Predictas don't normally use them. But I just looked at the Sams for this set and it lists a "beam alignment magnet" in the parts list, with the note, "Used on some picture tubes."

So, apparently it's not inconceivable that a Predicta would have an ion trap magnet. Which doesn't tell us whether this particular Predicta needs one.

Phil Nelson

I think it is a function of the specific model number of the CRT. If you have that, someone here might know if it needs one or not.

Curtis Eickerman

Yes, you should be able to look up a CRT and tell whether it takes an ion trap magnet. Here's one source with a lot of tube data sheets:

http://tubedata.tigahost.com/tubedata/

Phil Nelson

I did a little more checking and it appears that the BP4 tubes required ion traps. The FP4 tubes are interchangeable and did not require ion traps because of the aluminized screen and also provided a brighter picture.

Curtis Eickerman

http://tubedata.tubes.se/sheets/049/1/17DRP4.pdf

No ion trap, given the short neck you prob could not get one on there.

be careful when checking the tube, make sure the filament voltage is correct. You should see what the filament voltage its getting is, if it was rebuilt it prob has a 6.3v heater, vs the orig 2.68v heater. If that is the case then there may have been a wiring change at the time of the replacement CRT to hook the heats to the 6.3v winding and not the 2.68v tap.

If this is a series filament set then any difference in filament voltage will take care of itself. I think that I recall reading that by eye the filament looked to be lit correctly.

A CRT tester would be a good next step. Those generally have some ability to repair a bad CRT.

I today connected one of the working chassis (10L43) to that CRT which was on the 10L41 chassis. Same result - lighting filament, 14.5kV at anode, sound but no picture whatsoever. I had checked before that all wires are connected the same way, that the heater in both sets is on 2.65V and pin 7 is the cathode. Sam's photofacts for this model (set 466, Folder 1) in fact shows a picture tube 17DRP4 with 8 pins, like the one not working, and like the one shown in the reference mentioned above (http://tubedata.tubes.se/sheets/049/1/17DRP4.pdf). The 7-pin 17DRP4's that are on the two working sets (chassis 10L43) look authentic too.
So, my conclusion - the CRT died from unknown reasons on my work bench. Of course an autopsy would be very educational, to say the least. I dismantled the CRT and took a few pictures (below). As to some questions:

Yes


I repeat, the set was working before recapping, the picture tube has a 2.68V heater.


This is a Predicta Princess 3410 with a mains transformer and parallel heaters.
If there are no new ideas after studying the pictures below, I would very much appreciate RonL's offer to use his CRT tester. I can bring the tube to town.

It looks like there is no ion trap and no place for one. The next step is the CRT tester. If a internal weld popped open perhaps the tester can re-weld it.

And looking up the 17DRP4 in the RCA tube manual, it says that no ion trap is used. I should have done that earlier. Trying too hard to find a cause other than a bad CRT for the problem.

I think it's been staring you in the face all along, there's a pin missing on the CRT.

Genrally these button based tubes have all the pins even if they aren't all connected to something, yours seems to be missing one.

Not unheard of for a pin to pull out of the tube base.

Does the schematic show anything connecting to that pin?

Edit, maybe scratch that, looked up a 17DRP4 and it shows pin 5 as blank.

Next question, it's a rebuilt tube, are you sure it's a 2.7 volt heater and not 6.3?
That would make quite a difference.

May be the thread gets too long, and we start repeating all the good suggestions.
It is a 2.7V heater, and the CRT was working in this exact combination with the same chassis, before I recapped the latter. It seems the CRT died from some unknown reason and in an unknown manner, since also a second chassis, that has been tested working, doesn't produce a picture. I think we agree that I need a CRT tester.
Thanks for your help.