Resistor or jumper at hv rectifer socket: Zenith 25mc30
 

I am replacing an arched out HV rectifier socket in my Zenith 5111. The rectifier is a 3AT2. The 1.5K Ohm resistor that once resided there across pins 5 and 9 was burned in two. A very kind gentleman and member pulled a socket assembly for me from a chassis 1 year newer in which no resistor was used. This could be from a prior repair or by a change in design.

In another thread I have questioned what this series resistor actually is doing in the circuit, but with that not being the primary topic I never found out. Someone with a lot of experience in TV repair from that period said that this resistor was known to burn up and that techs often simply jumped over it. I am thinking that it may be a bleeder resistor, but it's only speculation. That would at least explain it not being essential in the circuit for operation.

I have a 1.2K Ohm 2W on the way. Easy enough to pop in just for safety and security. However, the jumper is already in place in the part coming and I am going to be working in very cramped space. So if anyone has any specific information as to the need or lack thereof as well as function of this resistor, it would be greatly appreciated. I may even need to find the correct 1.5K if it were to be a matter of critical voltages or current being spot on, but so far the information given to me speaks that this will not be an issue.

Thanks

Tube

I found HV service tips covering early Zenith color TVs in Tab Books No. 502 by R.L. Goodman, Zenith Color TV Service Manual chapter 5 page 68. It says:

"In all chassis using the 3AT2 HV rectifier, the picture may have all sorts of vertical, horizontal, and diagonal lines across the screen, plus elaborate rainbow effects. Also, there may be a loss of color, even though RF interference is not evident in the picture.
The cause, in this case, is an isolation resistor, located under the HV rectifier socket that has burnt or deteriorated. This is caused by a shorted HV rectifier. The most effective solution is to eliminate the resistor completely. Solder a wire across the socket terminals where the resistor was wired in, and replace the 3AT2 HV rectifier."

" ... check the filter resistor under the HV rectifier socket for arcing. DO NOT replace the resistor. Solder a lead-wire between the socket terminals where the resistor was removed."

"Improved High Voltage Rectifiers
(X and N Series Chassis)
Improvements in 3AT2 high-voltage rectifier tubes has eliminated the possibility of vertical streaking known as diodehausen. Thus, the 1.5K resistor in series with the high-voltage anode lead is not needed and has been replaced with a jumper wire to keep the outer shell of the 3AT2 socket at high-voltage potential."

I bought my used copy on Amazon. It covers from chassis 27KC20 through 14Z8C50, which includes the 25MC30 chassis.

Dave

Dang, I was gonna suggest jumpering that resistor out, but thought the better of it, figuring there must be some arcane reason for its being there.

Wow! This information is absolutely amazing and thanks so much Dave! If my fly is not somehow damaged I am thinking that the socket replacement that I am about to do is not only going to take away the raster distortion, but also may give me 'for sure' color as that has been intermittent through all of this. I haven't talked about it so as not to throw too much out there and confuse the main issues.

VK once again and as always has been an invaluable tool in this matter! Really, it's the members, which are some of the best people that I can imagine gathered to one place.

I should have my socket in a few days and with a little luck may have one nice playing Zenith! I will keep everyone posted if for no other reason there are more Zenith people starting to come out from behind all of the CTC repairs. Not bashing RCA folks; they just need more repair data for more repair. With Zenith the quality really did go in before the name went on!

All 'cept for their plastic efficiency coil forms.:D

You are quite welcome, glad to help. Hope it fixes your color issue.
Dave

The best part about fixing a "golden age" Zenith is that the service manuals pretty much cover every damned failure possibility, due to the fact that on the rare occasion something did go wrong, it usually involved one of the voodoo component bits. Detail is everything in terms of maximizing your revenue per lineal service-bench foot. Short occupancy is good, hangers-on for head-scratching and goose-chasing are bad. :thmbsp: When your unfamiliar symptoms are described spot-on in the book it makes you just want to hug the engineers.

I guess I will go ahead and jump over what was going to be a new resistor. Although not needed for the set's operation, I was thinking that at least it was providing a short circuit protection of sorts as they go open due to shorted rectifiers. When way more experienced techs say "DO NOT", I listen!:thmbsp:

Thanks again!

I just replaced 1.5k resistor on my 25LC20 (before reading this) old one looked all right but measured low (1.26k). Low resistance is also a sign of heat stress/damage. Maybe it's there to protect rectifier and or Pic tube or people from 22kv!

The HV rect filament wire was hardened from heat and had 1/2" bare stripped connections to socket! Who did that? Not factory?

There was a lot of wax melted from flyback!

The 2,2k final Horiz Output screen resistor looked cooked - was very high 11k! but set worked.

Will soon see if this corrects my vertical lines (is this "diodehausen"? screen picture)

Here is a picture of the Goodman manual page on the 3AT2 and HV resistor issue. Also here are the two pages of 25MC30 service notes.

Dave

I am guessing that diodehausen must be an effect similar to barkhausen resulting in vertical lines. You might want to try the Goodman suggestions posted above. They mention a newer version of the 3AT2 solving the problem.

Have you checked the HO cathode current? You mention 22KV, shouldn't that be 25KV? A lower HV setting on my 29JC20 causes the cathode current to rise.

Dave

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

I'm the guy who's guilty of replacing the resistor on the socket I sent to tubejunke. I wanted to get it in the mail in a timely manner. I only had a 1.2K 2W res on hand. The socket I got had a jumper.
I recall, that I inquired about the resistor at the Zenith distributor. The head technician said, just to jumper it out. It's not needed anymore. That has to be 40+ years ago, before I bought the Goodman book.

Regarding the HV rect filament wire, that wire is resistance wire and probably gets warm in operation. The insulation probably receeded from the heat. That I've seen before. :scratch2:

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

The hardened insulation is probably from ozone.

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

Last page.

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. :scratch2:

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.

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.

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

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

Dave

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

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.

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:worried:

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!

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

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.

Sams does specify 1.8ohms as the resistance of the wire. Didn't see that before. :scratch2:

The way I found the socket, the filament went to pins 9 and 12. The regulator plate lead went to pin 9 and jumper went from 9 to 4. The 2nd anode lead went to pin 4, as well as the jumper going to the metal cup.
I, like a fool, removed the jumper and installed the resistor. It looked like the jumper was installed by a tech. Didn't look like factory issue.
Just remove the resistor, install the jumper and wire, per these instructions.

OK, so my thinking was right about it not making sense to connect both ends of the resistor or jumper to internal cathode connections. #4 being an NC (no connection) will just be a tie in as someone else has mentioned. I was just mixed up between 4 and 5 which in my socket are both pulled out and there is just a hole there. Anyway, I will give it a try hopefully this weekend.

Thanks!

[QUOTE=Zenith6S321;3126651]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

Hi Dave, is there any way that you can post something showing this resistor connected to pins 1 and 9. Reason being is that if it shows it there then both ends are interconnected to the cathode. According to the .pdf on the tube, pins 2, 5, 6, & 9 are the same as pin 1. Now the second anode lead does for sure connect to one end of the resistor, but I dare to ask is it at a no connect as a tie point or is it at 1 and 9. There is a big difference that presents obvious concerns. My lack of completely understanding the circuit may lend to this.

Sorry to keep going back to this, but what I am reading surely makes me wish that I could see a for sure Zenith reference to this.

[QUOTE=Tubejunke;3126823]Here is a picture of the HV area of the Goodman 25MC30 schematic. It does show pins 1 and 9 together going to one end of the resistor. The other end of the resistor could go to pin 4(NC) to anchor the anode lead. Looking at the 3AT2 pdf I thinks that's ok as pins 2 and 9 are the cathode so they are at the same potential and already internally connected to pin 1 (one end of the heater). The other end of the heater is pin 12 which the schematic shows going to through the 2.35 ohm resistance wire to the flyback heater winding. Goodman does credit Zenith with a good portion of the illustrations in his service manual but he does not specifically say that the schematics are from Zenith. Did you take any pictures of the socket before you disassembled as a cross-check of the schematics?

Dave

OK thanks! This diagram drives home the fact that everyone was right but me. I just wanted to see it in black and white as my Sams's schematic only shows the heater connection. Your schematic clearly shows the corona ring being bonded to pin 1 which is the same as pin 9 where that connection is made on mine with one end of the resistor and the plate cap of the regulator. Makes no difference. So the other end along with the second anode lead are at pin 4 of NC just as we all suspected and as DieselJeep has it.

I didn't get a picture because the initial connector pulled out with the tube clean of solder and somehow I had it stuck in my head that it was on pin 4. Duhhh! GeeZ! When I pulled the assembly apart the split resistor, connector and anode lead fell to the bottom of the cage leaving me with two adjacent open holes. In the name of not looking like a complete idiot; at least I suspected that what I had in mind would have caused some major issues and all of this might have been for nothing.

Thanks once again for advice and more helpful information. My 25LC30/U is not going to serve me well in this. I didn't realize that Zenith had that many design changes in only a few years. The set pictured on my folder looks like my set. Maybe next time I will have good news.

I thought I could get the HV cage off without removing or pulling back the chassis as I got all but one screw out. That one screw is mid way deep and close to the cabinet. Can't win em' all, but removal of the cage will make the job much easier than trying to work through the cover.

Is HV actually set to 25kv on these? What is the procedure for setting the efficiency coil?

25Kv is generally where most people seem to set them and many manufacturers recommended. The efficiency coil is used to set the cathode current of the horizontal output tube. You have to place a meter in series with the grounded cathode of the tube. There are a few opinions on the ins and outs of how the meter is set up, but I simply broke the cathode to ground connection and ran two leads out so I could check this current anytime I want. Some people believe in placing a capacitor across the two leads to protect the meter. You may want to look into that, but I didn't worry with it and had good results.

Other than that you operate the set normally and turn the slug slowly and find what is called the "dip" point or the point where the lowest current flows and that's about it. If you are like me, you won't be satisfied with that current as it may still be too high due to some other component causing that condition. I hope this helps. I think you can Google setting the cathode current on a vintage color television" and there is a pretty in depth article on this topic, but it is a bit scattered in nature. If you find it you will see what I mean.

You will not get an accurate current reading without that capacitor. The inductance of the meter will change the tuning of the output circuit, and you will get inaccurate readings.

Try it again with a .47uF cap (any voltage >10 should work). You should get different readings, and they may be lower....

Remember the goal of measuring equipment is to load or disturb the behavior of the circuit under test as little as possible. The meter will have very low DC resistance, but may have higher AC impedance, the cap neutralizes that impedance by giving AC an alternate path to follow.

Here are pictures of the whole schematic, if that helps.

Dave

Thanks Dave! That should help a lot.

Anyway, today I got everything reassembled and powered it up. Nothing bad happened, but nothing good either. No HV and only a blue glow in the top portion of the rectifier. I put it on my B&K tester and it is dead as a doornail like the heater is open.

Not sure where or why that would have happened. So, now I will hunt another 3AT2. A local supplier here has International Servicemaster brand which I don't want to get. If anyone has a supply of these and needs anything of that style base; I have a box of these type tubes which I have no use for including some pretty expensive sweep tubes. Just throwing it out there.