Another Westinghouse H840CK15

[trinescope]

In that case I'm probably OK. The threaded rods are in fact weakly magnetic. I thought it had to have an actual magnet on the inner end.

[trinescope]

So far I restuffed the FIVE electrolytic cans on the main chassis (I thought there was four, but then another B+ resistor started smoking and I found the fifth can). Then I powered up the set without horizontal output tubes and verified the sweep oscillators. I had to replace the coupling capacitors at both the vertical and horizontal output tube control grids since there was excessive positive voltage on the grids. Next I installed the horizontal output tubes and everything seemed OK. Lastly I tried installing the CRT assembly back in to see if I can get a raster, but no go. I found normal voltages on the CRT cathode, G1 and G2 and some semblance of high voltage (slight static cling holding paper to the CRT faceplate). No abnormal light show in the CRT neck, either, so that's still a really good sign. Next step is to get a proper HV probe so I can verify the HV, focus, and convergence voltages.

One of the candohm resistors decided to short to the chassis, so I need to find a solution for that. It feeds power to the sound section, so for now it is disconnected.

EDIT: Actually my G1 voltages are a bit high, but that should cause the CRT to go to maximum brightness anyway. Will have to replace coupling capacitors in the chroma adder section for this. Had to go to the Sams folder to find CRT electrode voltage readings.

[trinescope]

I got a HV probe so I could trouble shoot the low HV. One of the doorknob capacitors is bad (measured about 1MEG resistance leakage, no good). When I pulled the plate cap on the second 3A3 the HV went from 6kV to 14kV. Repairing this should get the HV where it needs to be. Also during testing the focus control started smoking and got destroyed. I found out I have the dreaded vertical convergence transformer problem which caused the focus control to burn out. So I have three things to take care of before I can move much further.

I'm going to experiment with graphite and epoxy and see if I can repair the focus control. It looks like it's going to take quite a few attempts before I can get something I can use to replace the resistance track. Next I will see if any more reproduction convergence transformers are available. The doorknob capacitor is a bit expensive but available, so no problem there.

[trinescope]

I have another idea for repairing the focus pot. The challenge is that the pot can dissipate up to a watt according to some rough calculations. The resistance track is just a bit over 4 inches long and about a quarter inch wide. I need something that can take the heat. I ordered a sheet of .020" phenolic material, now I need to find some way to get a resistive material to adhere to it. Maybe try graphite in a clear lacquer.

[dtvmcdonald]

Originally Posted by trinescope

In that case I'm probably OK. The threaded rods are in fact weakly magnetic. I thought it had to have an actual magnet on the inner end.

If you have a bad screw magnet for a CT-100 or similar, there is an easy fix, which I used.

Just find a small ceramic magnet and break it into various size pieces. Set the
defective screw at a convenient spot, then try pieces at the OUTER end
of the screw until it more or less converges. Then glue it on, and screw the screw until its exactly right. Easy.

[trinescope]

Well I have results and some more problems to overcome. I can get picture and sound. I have no color sync at this point. I need to repair the broken coil plug on the hue control and see if that helps. It seems to have normal signal flow in the chroma section when checking with a scope, it just won't sync up. I can get on both sides of the lock point with the color hold control, so that is likely OK.

Next is the HV supply. I was able to repair the damaged focus pot. I made a new resistance track using thin phenolic board and pencil lead. I applied the pencil lead to the roughed up surface of the phenolic strip until I got the correct resistance value. I then installed it in the potentiometer housing. It seems to hold up even when constantly adjusting the control. I had to replace two doorknob capacitors as well. Now I get 27KV and can't regulate it down to normal. Possibly one of the high value resistors is out of tolerance.

Last on the list is the vertical convergence transformer. The reproduction parts are no longer available so I will have to rewind it myself at some point. For now I have wired around it for testing. Of course the convergence is off, but at least I can see how the rest of the set is working.

The CRT may be weak or the kind of picture I'm getting is a result of the excessive HV. It's better than a completely dud tube at least.

[old_coot88]

I've used india ink for DIY high-value resistors. It would probably work for potentiometer elements too, depending on wattage rating.

[trinescope]

Yeah, that was one idea I tried early on for the focus pot repair, but the India ink I tried was a bit too conductive. I laid out a strip on paper and got less than 100K resistance. Now I was able to use it to repair the red screen pot since it arced and burnt at the end of its track where it was riveted. The resistors are available at Mouser, but they're about $20 a piece. I have only one meter (Keithley 196) capable of measuring resistor values this high, but it's in need of calibration for the resistance function and I'm not sure how accurate the readings really are. I calculated the voltages I should get from the schematic, so I'll try making measurements later to see where I'm losing voltage. The voltage at the 6BD4 grid is low (200V, needs to be closer to 300-400V) to get it to start conducting and lowering the HV to spec.

[etype2]

Originally Posted by trinescope

Well I have results and some more problems to overcome. I can get picture and sound. I have no color sync at this point. I need to repair the broken coil plug on the hue control and see if that helps. It seems to have normal signal flow in the chroma section when checking with a scope, it just won't sync up. I can get on both sides of the lock point with the color hold control, so that is likely OK.

Next is the HV supply. I was able to repair the damaged focus pot. I made a new resistance track using thin phenolic board and pencil lead. I applied the pencil lead to the roughed up surface of the phenolic strip until I got the correct resistance value. I then installed it in the potentiometer housing. It seems to hold up even when constantly adjusting the control. I had to replace two doorknob capacitors as well. Now I get 27KV and can't regulate it down to normal. Possibly one of the high value resistors is out of tolerance.

Last on the list is the vertical convergence transformer. The reproduction parts are no longer available so I will have to rewind it myself at some point. For now I have wired around it for testing. Of course the convergence is off, but at least I can see how the rest of the set is working.

The CRT may be weak or the kind of picture I'm getting is a result of the excessive HV. It's better than a completely dud tube at least.

Did you contact John Folsom? He may have a VCT. SEE THE BOTTOM OF THIS LINK: http://www.videokarma.org/showthread...=270450&page=5

[trinescope]

Yep, already tried emailing John Folsom, he replied that he has no more transformers available. The one in my set has either shorts or leakage from secondary to either ground or the primary. This one doesn't look fun to take apart or rebuild. Lots of varnish and many thousands of turns of fine gauge wire. I probably won't get an exact turn count out of this. Also looks like the laminations are stacked in alternating directions, so they'd have to be pried off one by one.

[kvflyer]

Sure is nice to see what certainly appears to be a good CRT! Major obstacle overcome. Thanks for sharing your progress and pictures.

[trinescope]

The apparently weak CRT turned out to be a wiring error on my part in the color video amplifier section. I had a coupling capacitor essentially shorting blue and green signals together as well as shunting the green signal to ground. Correcting that as well as going through a few adjustments in the chrome section allowed me to get a color picture at last. The set still does not like displaying very bright scenes, but these sets probably never got really bright pictures to begin with, so I will just keep the brightness down. Color lock is still a bit weak. Still need to come up with a solution for the vertical convergence transformer to get this working like it's supposed to.

[wkand]

Originally Posted by trinescope

Well I have results and some more problems to overcome. I can get picture and sound. I have no color sync at this point. I need to repair the broken coil plug on the hue control and see if that helps. It seems to have normal signal flow in the chroma section when checking with a scope, it just won't sync up. I can get on both sides of the lock point with the color hold control, so that is likely OK.

Next is the HV supply. I was able to repair the damaged focus pot. I made a new resistance track using thin phenolic board and pencil lead. I applied the pencil lead to the roughed up surface of the phenolic strip until I got the correct resistance value. I then installed it in the potentiometer housing. It seems to hold up even when constantly adjusting the control. I had to replace two doorknob capacitors as well. Now I get 27KV and can't regulate it down to normal. Possibly one of the high value resistors is out of tolerance.

.

Eric, you have made unbelievable progress in a very short time. Take a bow!!! You have used a ton of creativity in getting things to work.

Regarding the pots you repaired. How did you get the graphite from the pencil lead to adhere to the phenolic board? You had mentioned using a certain type of glue, but I couldn't find the reference when I reread your notes.

[trinescope]

Originally Posted by wkand

Eric, you have made unbelievable progress in a very short time. Take a bow!!! You have used a ton of creativity in getting things to work.

Regarding the pots you repaired. How did you get the graphite from the pencil lead to adhere to the phenolic board? You had mentioned using a certain type of glue, but I couldn't find the reference when I reread your notes.

Thanks for the kind words. I tried mixing India ink into epoxy but that didn't work, probably because the ink is water based and didn't mix well. Also, the epoxy didn't get hard enough to be able to be used as a resistance track, even without anything added. Next I tried adding ground up pencil lead to lacquer, but that didn't work either. I could have tried getting powdered graphite and adding that to lacquer again, but I didn't want to go through waiting for that to arrive, so I went back with my old idea of pencil lead rubbed onto a suitable surface. I initially used card stock for a trial and got a strip which measured almost exactly 5MEG, but since this part dissipates a bit of heat I didn't feel it was a good idea. That's when I started looking for a thin phenolic sheet. I got one that was .020" thickness and looks like thin PC board material. I roughed up the surface a bit with fine sandpaper and rubbed the pencil on it until I got close to the correct resistance. It really didn't take a whole lot of pencil rubbing to get there, and you could see light through it.