How about some advice from some of you with exerience with 7JP4's?<P>We already discussed the problem with one of the tubes that I have: an apparent ion burn on the phosphor. I picked up some more of the old sets, at least partly with an eye toward getting some good 7JP4 spares. The first tube I tried in place of the tube with the ion burn, has a smaller raster and a bad keystone. Is this another lost cause??? Or is this tube just putting a heaver load on a marginal sweep circuit? Or does the tube need to be burnt in?? Or something else??? 20 years ago when I used to collect these sets, I never had a picture tube problem. Was I just lucky?? Well, I still have 4 more 7JP4 tubes that I haven't tried yet. What are the odds that I will get at least two good working sets (and maybe a spare 7JP4)? The heaters all have continuity at least. <P>------------------<BR>

Well, I learned more about the problem. The 7JP4 "rattles". Upon inspection, I discovered that the support for the vertical deflection plates is busted. Upon reflection, I think there is an important lesson in this. The set that this tube came from, was shipped to me without significant padding in the box. Undoubtedly the box suffered a jolt that was sufficient to break the internal structure of the 7JP4. The tube came out of a Motorola VT-71, and I would like to also make sure that others realize that the support for the CRT in these sets can be very poor after the picture tube mask disintegrates, as it always does. In the past I have begged people to take special precautions with these sets to protect the CRT when shipping. I'm thinking now that the big difference in my luck with these sets now as opposed to 20 years ago is the fact that the sets were shipped to me rather than being acquired locally. The internet has made it easy to find the sets, but the shipping process can be very dangerous.<P>------------------<BR>

ok, I've tested all 6 of my 7JP4's, and found only two with serious damage. I did find the bases loose on all 6 of the tubes. I've seen some discussion before about repairing loose tube bases, and the majority opinion was to use "super glue". I have a couple of reservations about that. First, I have vague memory of heat causing super glue bond failures. I may be wrong about that, but I do know that the manufacturer does not recommend use on glass due to "early bond failures". I have always used epoxy, but cleaning the excess is always a pain, and often unsuccessful. I think that epoxy may be the best solution in this case, since some of the sets support the tube by the base.<P>I'm getting concerned about the lack of availability of these tubes. In the past Hawkeye has told me they would rebuild them for their standard price of $250. That is more than a whole restored set will bring these days. I do plan to ask them what, if anything, they can do with my tube with the burnt phosphor, and the tube with the broken vertical deflection plate support. <P>------------------<BR>

There are still lots of 7JP4s out there - at most swap meets I see 7 inch Motorolas and Admirals at reasonable cost. I have bought several just for the CRTs.<P>------------------<BR>

Ken<P> I use epoxy to glue bases. Run a bead between the glass and base. Then wipe it away before the epoxy dries. Enough will have flowed between the glass and base that it will never come lose again. <P> If there are any signs of epoxy on the glass after it has cured wipe it away with acetone.. <P>------------------<BR>Norm

Thanks for the advice on the epoxy. I'm probably using too much in the hope that some is getting down into the base.<P>Years ago I used to see a lot of the 7" sets around and they were a dime a dozen. Anymore, they seem to be few and far between. When I get one, I'm reluctant to trash it for parts (see my other post today).<P>------------------<BR>

Here's an update on what Hawkeye can, and can't, do. They will rebuild tubes that are in good shape except for the heater. They won't recoat the phosphor or repair a broken support.<P>------------------<BR>

Video Display (800-241-5005) will rephosphor tubes, but I don't know if they will do the 7JP4. Ask for Chris.<P>------------------<BR>

I wonder if Hawyeye or Video display can rebuild 3KP4's? Has anyone ever had this done? I once called Video Display and was told that I had to go through a distributer in CA. I'd love to have mine rebuild if anyone out there can do it.<BR>Mark<P>------------------<BR>

Hi Ken,<BR>I definitly agree with using epoxy.<BR>When I am machining metal, a trick I<BR>use is to super glue two pieces together if they need to be machined as one unit. When finished, I heat them up a little with a torch and they easily seperate. The epoxy is just a little harder to work with but should be superior.<P>Scott<BR><P>------------------<BR>

Well, Video Display is out of that business now, and Chris no longer works there. Hawkeye is just about the only place left now. They will rebuild 7JP4's for their standard $250 price. That involves putting in a new heater. They will not repair the structure or rephosphor the tube. You can still find junk sets (with good 7JP4's) for less than $100 so it's hard to justify paying $250. On the other hand, if you are like me, you may have a hard time parting out a set that could possibly be restored. In that case, you're stuck. Maybe you could work out something with Harry Poster. I think he pulls out chassis and retrofits the cabinets as prop sets.<P>------------------<BR>

Since we are on the subject of 7JP4's who can tell me just how bright a strong crt should be in operation? Specifically,is a good strong 7JP4 bright enough to view in a normally lit room? I have restored a bunch of these sets, mostly VT71's and most recently a sentinel 400tv and I have never been happy with the performance . The only way to really watch them is in a dark or dimly lit room. Am I expecting too much from nothing much more than a scope tube?<BR>-Mark<P>------------------<BR><p>[This message has been edited by Mark Lucicosky (edited 12-28-2000).]

I own a Motorola VT-71 with a 7JP4, and it is viewable on a normal lite room (light bulbs) but if sunlight is comming through window, it will wash out.<BR>Though nowhere near as bright as todays sets, I find it quite viewable.<BR>===========================================<BR>Man, if someone had the equipment and knowledge on how to rebuild all these B&W CRT's, like the 3KP4's and the 7JP4's,<BR>they would have the ideal money maker, just think, take all those scope tubes, re-phospher them to a P4 from a P1, replace the guns, etc.<BR>From what I see here on the forum and other places, I am surprised no one has started a rebuilding company yet, money and knowledge, neither of which I have........hee hee.<BR>=========================================<BR>Bill<P>------------------<BR>

I've no experience with the early electrostatic deflection TV tubes you're tslking about, TV started in 1956 in Australia and "modern" 17" & 21" rectangular tubes were then standard.<BR>However I doubt if those old tubes would be anywhere near as bright. Although they only had a small screen area the EHT was only about 6 kV. Also I believe they didn't have an aluminium backing inside the phosphor. This is a thin (a few atoms thick) aluminium coating vacuum deposited on the back of the phosphor that both protected the phosphor from ion burn and reflected the light forward that would normally shine back inside the tube and brighten the image. The electrons could pass through the thin aluminium.<BR>I think they also had clear faceplates. The darkest part of the image is the brightness of the tube face in room light. It's only the light from the image that gives it contrast, you can't make the screen darker. Modern tubes have the face of the tube (or protective screen) tinted so only about 75% of the light passes through it and the phosphor is also a darker shade. The room light has to pass through the tint twice, in, reflected and out. This greatly enhances the contrast when room light is shining on the tube. You could try holding a piece of slightly dark filter over the front of the screen and see what difference it makes. The light will of course be reduced but the contrast should improve.<BR> Don Black.<P>------------------<BR>

I also have a VT-71 and it is quite viewable as well. I ahve a brightness meter and I will post some measurements over the weekend as to how many foot candles one can do. Even though it is quite viewable I experience front end overload on occasion as the VHF signal strength here in Salt LAke City is pretty high. This mainly happens on old sets with out AGC.<BR> The electrostatic CRT's do not have aluminized screens and this cuts brightness down a bunch. One thing however is that I really prefer CRT's that do not have alumnized screens. The image always seems sharper, at least it is on some of my old sets that use 10BP4's. Perhaps they attain a smaller spot size. The other thing old tubes lack is the anti glare faceplate. Old tubes simply used what ever glass the bottle was being made from for the viewing screen. Some of my sets have swirels in the glass face form manufacture. <BR> I have never gotten a great picture from anything with a 3KP4 or a 3KP1 substituted in place. On a couple of Pilots I modified the brightness circuit a bit to push the tube harder for more brightness. This made a big difference but it is definately not good for the CRT. I usually only run these sets on occasion to show them to people that have never seen one.<BR>Mark<P>------------------<BR>

On my prewar sets with electrostatic tubes (5AP4 and 5BP4), I have made a modification to increase the brightness. These sets have a HV transformer and rectifier to provide about 4 KV for the CRT. I removed the ground connections from the transformer and filter capacitors and connected them to the B+ line. This increases the voltage to the CRT by about 400 volts. I haven't tried this on a set with a 7JP4, but it might work.<P>------------------<BR>