how can I tell if my 7jp4 is dead? - Videokarma.org TV - Video

jpdylon · #1 11-01-2005, 12:10 AM

I've got a 7jp4 that appears to have fading vital signs. I get a dark gray raster with retrace lines. My screen voltages and HV is ok. Is this just lack of video input, broken cathode, grid, or something else related to the tv itself?

Right now the TV only produces a raster to my knoledge. I've got a bud with a good 7jp4 that he's willing to use to test my chassis, but I just want to get an idea of the life expectancy of this crt.

Any thoughts?

***Eric H*** · #2 11-01-2005, 10:39 AM

Hi Jordan,
about the only way to test a 7JP4 is to plug it into a good working chassis.
A raster won't tell you much with this tube, dark or bright.

jpdylon · #3 11-01-2005, 10:45 AM

Thanks for the advice Eric. I'll have to get the chassis working properly again.

Adam · #4 11-09-2005, 12:48 AM

This was the other black and white set I got this weekend, after I saw it in the tv swap board on Sunday, I just had to drive 300 miles round trip to get a tv this old. This is the earliest tv I have ever worked on, the first one I have ever seen with electrostatic deflection, and without a flyback transformer (I am assuming no flyback, because I didn't see a hv rectifier tube) I will know more when I get my schematic this weekend (all my sams are far away in michigan).
The set just gives a white screen full of retrace lines, and a slight buzz on the speaker. I have run other sets with the tuner disconnected, and this set looks exactly like that, they would display retrace lines with the tuner disconnected, but with the tuner connected, no retrace lines. It is acting as if there is no signal at all, either audio or video. But I know the tuner is not simply disconnected, as adjusting the fine tuning will change the pitch of the buzz. Assuming this set was made back when there were seperate IF sections for picture and sound (again I don't have the schematic yet), and as I have neither, this would place my difficulties back in the tuner. But as it receives absloutely no channels at all, I wouldn't think the problem to be internal to the tuner. More like no power to the tuner, or no power heating the RF tube? Any thoughts?
Also, at some time this set was converted to have the tube heaters in parallel rather than series, using seperate transformers, and removing the ballast tube. It was done rather cleanly, and I don't plan to change it back, it actually seems like a good idea to me if you plan to use the set alot. But I wonder if this isn't the source of the current problem, like I said before, something like the RF tube not heating up.
As I am not that familiar with these really early sets, I am not going to take it apart until I get the schematic, and I will probably fix up that RCA remote set I was talking about elsewhere first, as I have already started fixing it, before I start work on this one. I will write more as I go.

***Eric H*** · #5 11-09-2005, 01:14 AM

Hi Adam,

The VT-71 doesn't have a Flyback but it does have a high voltage transformer and a 1B3 rectifier (under the metal box)

I don't know which chassis you have but I have a basic schematic for the TS4J chassis posted here:

http://www.vintagetvsets.com/schem/ts4j1.jpg

http://www.vintagetvsets.com/schem/ts4j2.jpg

Eric

jpdylon · #6 11-09-2005, 01:19 AM

That TV he is talking about was one of my parts sets. I'm glad it went to a good home. I didn't have the time or space to get it working right.

It does have the TS-4j in it

Best of luck to you Adam

Adam · #7 11-09-2005, 01:51 AM

Thanks for the schematic. It did seem odd to me that you could get by without a hv rectifier. But it wasn't shown on the tube diagram glued inside the cabinet, neither was that 25L6 for that matter. And there were no visible holes to vent heat in that metal box, and as I had never seen a 7" electrostatic set before I was willing to believe about anything. It is the Motorola TS-4J, 1948 is stamped on the back, but sams gives a date of 1949 for the TS-4J? What's going on with the only 8 positions on the tuner, are some of the channels doubled up?, I can't read the numbers on the cabinet anymore.
I see the audio section actually branches off after the video amp tube, so the trouble could be in either the RF, or IF sections. It still seems to me like tubes not working, once I had a set where the only thing broken was that all 3 IF tubes were bad, a much newer set, but it displayed exactly the same problems.

jpdylon · #8 11-09-2005, 02:01 AM

It is only an 8 position tuner. All the way counter-clockwise is ch 2-3, then 4, 5, 6, 7, 8-9, 10-11, 12-13.

Back in the day the tech who installed your TV adjusted the tuner selectors to receive your desired station. Thats what all the pots are above the tuner. The later models had a full 13 channel tuner.

Adam · #9 11-13-2005, 06:08 PM

I've taken a better look at this tv. The modified power supply basically consists of adding an isolation transformer, 2 new silicon diodes, a 100 ohm choke coil, and two filament transformers, 6 volts and 24 volts (no 12 volt!). But the tubes are not connected in parallel to them, mostly they are still connected in their original series circuits, with the transformers connecting to the series circuits in some way I can't really make sense of.
When I went to measure the filament voltages I didn't measure them according to the voltage readings given on the sams, measuring from one side or the other to ground, as this depended on all the tubes being in series, in a sense measuring how much voltage was left over at any point in the circuit. As all the tubes were no longer in series, I didn't think this measurement would tell me anything, so I put my volt meter across the heater of each tube and measured AC volts, assuming that if the correct amount of voltage was getting to the tube, however it was doing it, a 6v tube should read 6v, a 12v tube should read 12v, ...
The 25L6 HV osc. does get the 24 volts, and somehow the 12SN7's in the sweep circuits are reading 10volts. But the RF/IF poritions of the set still have the tubes connected in series, measuring as I did before they all read between 5 and 6 volts, including the 12AT7 (V2) oscillator/converter. At the moment I am assuming this to be the source of my troubles.
All the resistance measurements at V1, V2 are more or less good. But the voltage measurements are way off.
V1 =
pin1. 0 instead of -.1,
2. 1.5 instead of .5,
5. 160 instead of 90,
6. 160 instead of 90.
V2 =
pin1. 260 instead of 220,
2. 0 for 0,
3. 0 instead of 8 (for the first 30 seconds it reads negative no matter which way I connect the meter)
6. 260 instead of 220
7. -.1 instead of -.2 (for about 30 seconds it fluxuates from a few volts positive to a few volts negative before it settles down)
8. .15 instead of .1

All my measurements were taken without the crt or the speaker connected.
Could the source of all this trouble just be the low filament volatge?
And if so, as most of the original circuits are still in place, I am tempted to just remove the modifications and make a replacement for the ballast tube out of high wattage resistors. What is the general opinion here about this? I am not one of those guys that has to have his tv exactly original, it is more important to me that it works, but in order to continue with the modifications, I would have to find a 12 volt transformer, and remove all the original series circuits which are still about 90% in place. It seems a great deal easier, and more likely to work, if I just took the filament transformers out and went back to a series circuit (which it still has anyway). If I do this how many watts do I need in those resistors? I am assuming that 100 ohm choke coil is standing in place for that 200 ohm resistor from pin 4 to 3 of the ballast tube, should I change this as well, is this better or worse than the resistor? And what about the isolation transformer? Should I remove it, or should I try and work it in somehow?

bgadow · #10 11-13-2005, 10:53 PM

It would be great if you could find out if this set EVER worked with those mods. Sounds like they went to a lot of trouble for it not to. I would give it a good shot before tearing out & starting over.

Adam · #11 11-14-2005, 11:09 PM

I have to get the transformers out of the chassis, just to see behind them so I can see how they are connected. After looking at the set as much as I did I had assumed the modifications were unfinished, it was fixed to the point where the HV and sweep circuits worked, and he stopped there. It does not seem possible to me to have as much of this set wired in series as it is (at least the 6 rf/if tubes through the video amp), and have it connecting to either the 6v or the 24v transformer and even getting as much voltage at the filaments as it is. If it goes to the 120AC, then as not all of the tubes are still in a series string, and there is no replacement for the ballast tube, then the voltage at these tubes would be too high. I would think if one were to wire the set with filament transformers one would just put all the 6v tubes parallel to the 6v transformer, 12 volt tubes parallel to the 12v transformer, and dispense with the series strings entirely. I was also considering getting a 12v. transformer and doing this.
There should be a schematic of the original filament circuit and the rf section here, later I will try to see if I can draw in the modifications.

Adam · #12 11-15-2005, 12:12 AM

Here it is.

Adam · #13 11-15-2005, 09:48 PM

I stayed up late last night and figured out whats going on. Now I think this filament circuit should actually work, a quick drawing of what's currently in the set below. It is just a several sets of tubes in series, in parallel with the 24v transformer. Pin 4 and 5 of (V2) the 12AT7 I was talking about earlier have been soldered together, so the tube works just like two 6volt tubes in parallel.
I don't understand why one side of each transformer is connected to ground, as all those filaments which are connected to ground are connected together anyway by wires (Even the ones going to the 6v trans. are directly connected to the ones on the 24v trans. -- this is what confused me for so long). (The drawing simplifies it so you don't actually see all the wires, for example, v10 actually has a wire connecting it to v7 and one connecting it to v8, then there is a wire connecting v7 to v8, then one connecting v8 to the crt and the 24 volt trans, and from that point a wire finally goes to ground ... or something like that ... Once I realized most of the wires were just the equivalent of chassis ground, it all began to make sense.) I would think the extra 24v AC going through the chassis might even cause some problems. I would like to disconnect these from ground entirely and just wire them up one to the other, with no links between the output of 6v and 24v transformers.
Also, all those resistors and caps are just leftovers from the old circuit (with the exception of the one near V11 which was added later to keep those filaments from having 6v too much voltage). I would think they would just be an unwanted drain of power now.
And lastly, the 25L6 audio amp tube was exchanged for a 6K6 and placed on the 6v trans. along with the crt. Is a 6K6 a good replacement for a 25L6 if only the filament voltage is changed? I don't understand why they did not keep the 25L6 and just put it in parallel with the 24v trans. Or better yet, if the 6K6 is a good replacement, put it after V11 and take that resistor out of there. Then use the 6v trans. exclusively for the 7JP4.
Anyway, with the exception of those possible additional modifications, I've decided I like the way the set is (now that I understand it) and am not going to change it from the way it is now.
This also means that with the filaments wired as they are, they are probably not causing any major problems, and I need to think more about what is causing this not to work.

Adam · #14 11-17-2005, 08:57 PM

I got the audio to work on this last night, I just replaced a bad electrolytic cap, C3B, C3 looks like it had already been replaced at some time earlier, too. This fixed most of the strange voltages I was reading on the IF and RF tubes. Also, the antenna terminals were up against the side of the 24v transformer, grounding both of them, so after I replaced the cap I only got very weak audio, after I wedged a bit of paper between the antenna terminals and the transformer (I will come up with a better form of insulation later) the sound came in good with a bit of fooling with the fine tuning.
This must mean that the 6K6/25L6 transplant works. Still no video, but when messing with the fine tuning now I can get in all sorts of wavy lines, better than nothing I guess. Next, I just am going to change all the electrolytics and see what happens.

Adam · #15 11-21-2005, 08:24 PM

This tv almost works now. The main problem turned out to be a bad tube socket on the 3rd IF tube, I couldn't figure out why I had low voltages only on that tube and the vid. out, but nowhere else, until I shook the tube a bit while I had the meter connected and the voltage went right up. I bent the prongs and put the tube in a little crooked, to get it to work the best I could, but I am still loosing some voltage there, and am planning on just changing the socket.
After I got some picture, I found the horiz. was out of sync. This just turned out to be a bad cap C67 (originally .03 600v, was replaced some time back with a paper .047 600v that went bad), and C68 was changed with a more modern orange mylar cap, but it wasn't put back right, one side was hooked to R66, but the other side, instead of going to T1, went back to pin 3 of the horiz. osc. tube.
The picture is still not very good, low contrast, vert. retrace lines, I am hoping this is still due to the bad tube socket, and not the 7JP4.
After I put everything in properly and clean up things a bit it should improve, some of those caps I changed wern't quite the correct value, and most changes I made by just twisting wires or using wires with alligator clips rather than soldering stuff, because I knew I would take those caps out once I got the proper ones. Also one of the IF tubes is still the original one, and it reads a bit weak.