Predicta Holiday - First power up attempt

[madlabs]

Really shameful how poorly I think things through. Duh again. So Bob, do you agree that swapping the two pins (2&3) on the crt socket?

Still going to try some rejuve on the 21EPA4 and still looking for opinions on the best way to go about it.

Thanks!

JOnathan

[mrjukebox160]

I have an old Heathkit CRT tester/rejuvenator that works like the circuit in the PDF. It does a very good job. I agree you have nothing to loose so I would try it.

[madlabs]

So, built up a quick and dirty variable power supply based on a LM317. Driving the filament close to 3V provides a much brighter picture. However, it is really washed out and the contrast doesn't seem to be working although the brightness does. Is there anything other than a weak CRT that can cause the washout? Any ideas why the contrast isn't working when it was before? My DMM finally crapped out, bringing experimentation to an end for the evening. I ordered another but I am headed to work until Wednesday anyway.

I hope to have enough time to build a rejuvenator this week but it is going to be a busy one.

Thanks!

Jonathan

[madlabs]

Not getting much time in this week. Got my new meter (Fluke 117). I fired the set up and noticed that the video output tube (6AW8A) is getting hot with a dull red plate. I don't believe it was like that previously. Looking at the voltage:

Pin 9 = -30 (negative 30?!, SHOULD BE +190v)
pin 8 = 180 (should be 140)
Pin 6 = 1.5, should be 1.2V

Haven't had time to check all the resistances. Any quick suspects?

Jonathan

[madlabs]

Well. Things just took a sour turn.

As noted above, the video output tube looked like it was running hot and had a very wrong voltage on pin 9. Doing resistance tests showed a problem between the 280V source and the main PCB. I discovered that R33, a 3.9k resistor had very high resistance. I swear I checked and it passed when I was under the chassis. Replaced it, got good voltages and the best picture I have had. Not great by any means but better. While I was admiring it, about 10 minutes into the run, smoke and flames erupted from the flyback wires. Sigh. I had been playing with the focus wire that has two possible positions but that's it.

So, now I need a new flyback and to find out what blew the original. As it will be very difficult to find an exact replacement, any suggestions as to something that will work? This isn't a museum restoration, just a set for occasional use.

Argh!

Jonathan

[madlabs]

Hmm. Upon taking resistance measurements, the flyback appears to have survived. Could I really be that lucky?!?! I can see a nasty scorch between connection D (wire to the plate of the horiz. output tube) and the flyback frame. Carbon tracking that finally got bad enough? Or excessive plate voltage on the horiz. output tube?

Back to puzzling.

Jonathan

[N2IXK]

Flyback might have survived. Can you post pics of the damage?

If carbonized phenolic terminal boards, you can scrape it out to get rid of the conductive char layer, and then coat with insulating lacquer. If you need to replace/patch a piece, a sheet of FR-4 epoxy glass laminate works well.

[Eric H]

You have to handle that Fly with extreme care though, they are usually so cracked and decayed that the wire from the tire to the rectifier will break right off.

[madlabs]

OK, here is a pic. Is it possible that the plate voltage was too high or do y'all think it was a carbon track that built over time and today was it's day? I was getting a brighter, better picture than I had been after replacing the 3.9k. So maybe it was being asked to work for the first time in a long time.

I'm going to start digging and cleaning the board and see how deep it goes. But what can I check before I fire (haha) it back up? I got lucky once, don't want to push my luck.

Thanks for the help!! My blood pressure is returning to normal. :-)

Jonathan

[Eric H]

It just arced over, either from dirt or moisture and time.

I'd try to Dremel out the path between the terminal and the frame, you can leave enough of the Phenolic around the terminal to support it but remove all of the charred part.

Lots of hair fine wires on that board so you'll need a steady hand with the Dremel.

[N2IXK]

That should be repairable using reasonable care. I would consider carefully unsoldering the wires from the lug, removing the lug completely from the phenolic, cleaning up all the carbon, and remounting the lug (or a new eyelet/tie point of some type) on a bit of FR-4 epoxied over the original location.

If your B+ is higher than spec, then the pulse voltage at the HOT cap will be, as well. But that was probably an existing carbonized area that decided to arc over due to dust/contamination or whatever.

[Kevin Kuehn]

Maybe I'm seeing things, but it almost looks like there was a trail of some material(possibly flux) that ran from the terminal down to the fly frame. Whatever it was it finally carbonized.

[Eric H]

Dirt, grime 50 year old Cigarette soot, eventually it gets conductive enough to arc over and then it's too late.

It's often a good idea to clean the HV area on any set, a Q-Tip and some 91% Alcohol will do the trick.

[madlabs]

Yeah, I shoulda, woulda, coulda cleaned the HV area. I will next time, I'll tell you that.

Dug up a piece of phenolic and cut out the burnt area. It was pretty big, had to file a 3/16" groove to get rid of all the soot. Then I glued the new piece in on top with the terminal already installed. Thankfully I was able to remove and re-attach the flyback wires without breaking them. Whew! Fired it back up and to my great joy it worked!

Picture is indeed improved. Watched a "Tales of Tomorrow" on it. Pretty good one, actually. Still much improvement needed but it still lives!

Thanks a ton for the help, folks. Once I get a chance to work on it some more I'll have plenty of questions.

Jonathan

[N2IXK]

Nice repair!

I would shorten that screw a bit, though, to reduce the risk of corona/arcing to the donut in the future. Maybe cover the cut end with a blob of electronic-grade RTV.