Resistor or jumper at hv rectifer socket: Zenith 25mc30 - Page 2 - Videokarma.org TV - Video

Zenith6S321 · #16 02-16-2015, 06:28 PM

Quote:

Originally Posted by NewVista

Does the Goodman manual say anything about vert foldover?

I left the new 1.5k (2w) resistor in as it was reassembled.

Still have my H-synced artifact on screen which can be heard in speaker also, seems to be a micro arcing in some component getting into the RF then onto screen - more pronounced as you fine tune into chroma/sound

Here are the Goodman service tips that include the 25LC20. The fifth page describes an arcing issue. I will send the last page of this in another post.

Dave

Zenith6S321 · #17 02-16-2015, 06:35 PM

Last page.

dieseljeep · #18 02-16-2015, 07:04 PM

Quote:

Originally Posted by old_coot88

The filament loop wire is not resistance wire.
What manner of engineer would want to squander hard won deflection power as waste heat?

The hardened insulation is probably from ozone.

Probably the same engineer that specified the 1.5K resistor, in series with the CRT second anode.
According to Sams, it's stated as resistance wire. It's a small guage solid wire, instead of stranded wire, like used on practically all the flybacks, that I ever replaced.
I don't know, how much voltage drop is required, as Sams doesn't specify a resistance value.

old_coot88 · #19 02-16-2015, 08:31 PM

Quote:

Originally Posted by dieseljeep

Probably the same engineer that specified the 1.5K resistor, in series with the CRT second anode.
According to Sams, it's stated as resistance wire. It's a small guage solid wire, instead of stranded wire, like used on practically all the flybacks, that I ever replaced.
I don't know, how much voltage drop is required, as Sams doesn't specify a resistance value.

Dang. Learn somethin' new every day. Be interesting to know more background theory on da resistance wire thing. Maybe to de-stress the 3AT2 and exclusive to Zenith (?)
All the HV rect filamant loops I ever replaced were standard anode wire. Occasionally there was one (usually B&W) that had a small resistor of a few ohms in series with the fil. This was always in other brands than Zenith.

Tubejunke · #20 02-16-2015, 09:34 PM

Thanks everyone, and special thanks to Dave with the pages posted. This turned out to be a great thread that I think will be an excellent source in the future for others. I think I am far from the first to be hunting a corona or arching issue which just can't be seen; even in the dark.

As I have said before; I don't know if I would ever have found this if the damage from the arching hadn't burned my rectifier socket causing a pin to pull with the tube and leading me into further investigation.

Zenith6S321 · #21 02-16-2015, 10:41 PM

Quote:

Originally Posted by NewVista

Does the Goodman manual say anything about vert foldover?

I left the new 1.5k (2w) resistor in as it was reassembled.

Still have my H-synced artifact on screen which can be heard in speaker also, seems to be a micro arcing in some component getting into the RF then onto screen - more pronounced as you fine tune into chroma/sound

The Zenith 29JC20 service manual says:

"The most common cause of vertical fold-over is failure of the vertical oscillator output tube. Vertical output grid coupling capacitor being shorted or leaky can cause improper operation of the vertical output tube, causing vertical fold-over."

So maybe replace the grid cap and try some other vertical output tubes?

Dave

Zenith6S321 · #22 02-16-2015, 10:45 PM

Quote:

Originally Posted by Tubejunke

Thanks everyone, and special thanks to Dave with the pages posted. This turned out to be a great thread that I think will be an excellent source in the future for others. I think I am far from the first to be hunting a corona or arching issue which just can't be seen; even in the dark.

As I have said before; I don't know if I would ever have found this if the damage from the arching hadn't burned my rectifier socket causing a pin to pull with the tube and leading me into further investigation.

Hope the information helps. Its just guesses until you try it and find the cause. Please let us know how it turns out.

Dave

NewVista · #23 02-16-2015, 11:24 PM

Quote:

Originally Posted by Zenith6S321

Here are the Goodman service tips that include the 25LC20. The fifth page describes an arcing issue. I will send the last page of this in another post.

Dave

Thanks for extra pages and tips! Lot to try here.

Tubejunke · #24 02-16-2015, 11:45 PM

Quote:

Originally Posted by Zenith6S321

Hope the information helps. Its just guesses until you try it and find the cause. Please let us know how it turns out.

Dave

I am certain now of exactly what to do as far as proceeding with the repair of the burned socket which was without question. In my search for a replacement socket I was finding that some sets had the resistor and others (newer) didn't. I needed more information as my inclination was not to eliminate or alter something that was at least in my set. Now I can proceed with certainty and the information given is beyond helpful.

I still can't help but wonder how I had HV with the resistor burned in two. My only guess is that it was arching across the two burned open ends of the resistor. The burned out socket (pins 4 and 5) as well as same tube pins badly corroded may be further evidence backing this theory. Heck, pin 4 is a blank! Beyond a shadow of a doubt the resistor didn't burn any of the last number of times the set was in use.

Beyond the resistor topic my immediate concern will be that of HO cathode current being high at line potential. I doubt that this repair will correct that, but all of this is a learning process for me. Perhaps the issues mentioned and soon to be corrected do play a hand in that. I will know as soon as the socket arrives (hopefully later today). Lots of snow here for this part of the country which makes for a perfect day to work on electronics projects.

NewVista · #25 02-17-2015, 10:42 AM

Quote:

Originally Posted by Tubejunke

my immediate concern will be that of HO cathode current being high at line potential. .

I'm worried about that also given the melted wax on my 25LC . Perhaps a small cathode resistor could be added to Horiz Out and its voltage drop could be used as tuning indicator to optimize the Horiz Efficiency Coil? It would also reduce H.O.T. primary current.

I see the 25LC schematic AC input is designated 120v - within spec of modern utility voltage.

Was looking for "troublesome" degaussing thermistor and found the 25LC20's have no automatic degaussing coil

Tubejunke · #26 02-17-2015, 05:38 PM

This may sound ridiculous, but I have to ask if anyone can verify some 3AT2 pin outs for me. Simply what I need to know is whether the jumper replacing the 1.5K resistor joins at pin #4, or pin #5 along with the crt second anode lead.

As I have said, my sockets connectors 4 and 5 both fell out, but when I first pulled the tube and discovered this I thought that #4 came out with the tube and had nothing soldered to it. The new socket has the resistor and anode lead at pin #4 which again is a blank pin. I am probably just mistaken about which connector came out with the tube. The blank pin is probably just used as a junction for the "series" resistor. If it went to #5 then it looks like both ends of the resistor or jumper would be across or shunted to the cathode by looking through the glass. I just want to wire this up right the first time and need to be sure. The schematic only shows the heater connections and plate cap. Thanks!

Zenith6S321 · #27 02-17-2015, 07:27 PM

The 25MC30 schematic shows the 1.5K resistor connects to pin 1 and 9 of the 3AT2. The 3AT2 PDF shows that pin 4 is a no connect. So pin 4 could be used as a tie point for connecting the other end of the resistor to the anode lead. The schematic does not show this, so I'm guessing.

Dave

3AT2 PDF:
http://frank.pocnet.net/sheets/049/3/3AT2.pdf

Tubejunke · #28 02-17-2015, 07:59 PM

OK, I just figured out that I have been using Sams Set 722, Folder 4 which is for the 25LC30/U! Geez! It only specifies pins 1 &12 as heater connections. Mistake #1! Either way, like you I am assuming that the NC #4 is indeed a tie point as #5 certainly would not be right. I need to get a .pdf on my 25MC30 somewhere. However, it shows pins 1 and 9 according to what you are seeing and that would still have the resistor shunted to the cathode as the.pdf states, "pin 5 same as pin 1."

I hope I am correct in thinking like you that only one end of the resistor would be at the cathode, or pin 9 and the rest simply goes to second anode at CRT.

Electronic M · #29 02-17-2015, 11:07 PM

Quote:

Originally Posted by Tubejunke

OK, I just figured out that I have been using Sams Set 722, Folder 4 which is for the 25LC30/U! Geez! It only specifies pins 1 &12 as heater connections. Mistake #1! Either way, like you I am assuming that the NC #4 is indeed a tie point as #5 certainly would not be right. I need to get a .pdf on my 25MC30 somewhere. However, it shows pins 1 and 9 according to what you are seeing and that would still have the resistor shunted to the cathode as the.pdf states, "pin 5 same as pin 1."

I hope I am correct in thinking like you that only one end of the resistor would be at the cathode, or pin 9 and the rest simply goes to second anode at CRT.

I probably have a paper copy of the factory schematic, and possibly have the whole Zenith manual too. If you want I could check if I have it, and scan it for you if I do.

dieseljeep · #30 02-18-2015, 09:16 AM

Quote:

Originally Posted by old_coot88

Dang. Learn somethin' new every day. Be interesting to know more background theory on da resistance wire thing. Maybe to de-stress the 3AT2 and exclusive to Zenith (?)
All the HV rect filamant loops I ever replaced were standard anode wire. Occasionally there was one (usually B&W) that had a small resistor of a few ohms in series with the fil. This was always in other brands than Zenith.

Sams does specify 1.8ohms as the resistance of the wire. Didn't see that before.