On going Zenith 25MC33 roundy restoration - Videokarma.org TV - Video

Electronic M · #1 05-27-2013, 01:21 PM

I got this set at the ETF meet last weekend and was working on it here and there during sanity breaks during finals last week. At the meet I and another collector saw the old orange drops in it and assumed it had been restored which turned out not to be the case.

I powered it up and got good sound but no HV. Well still assuming good caps I replaced nearly all the hroizontal/HV section tubes which were mostly bad anyway. The focus rect. tube was missing which I later found was for a reason. This landed me no net improvement. I then pulled the chassis, tested and replaced several caps in the horizontal section. This got me 11-15KV of HV, and a shrunken raster with pull-in on the sides.

What caused that?, what else I had to fix?, and what is still not working?...All that in my next exciting post.
Gotta run.

dieseljeep · #2 05-27-2013, 01:37 PM

Quote:

Originally Posted by Electronic M

I got this set at the ETF meet last weekend and was working on it here and there during sanity breaks during finals last week. At the meet I and another collector saw the old orange drops in it and assumed it had been restored which turned out not to be the case.

I powered it up and got good sound but no HV. Well still assuming good caps I replaced nearly all the hroizontal/HV section tubes which were mostly bad anyway. The focus rect. tube was missing which I later found was for a reason. This landed me no net improvement. I then pulled the chassis, tested and replaced several caps in the horizontal section. This got me 11-15KV of HV, and a shrunken raster with pull-in on the sides.

What caused that?, what else I had to fix?, and what is still not working?...All that in my next exciting post.
Gotta run.

Power supply, voltage doubler problem?

*zenithfan1* · #3 05-27-2013, 07:31 PM

That's where I'd start looking too. Check the cans, I bet the doubler is bad. Probably a 160 or so at 400 or so volts, been a long time since I've worked on any of my Zenith sets.

Big Dave · #4 05-27-2013, 07:33 PM

Replace the doubler. Also, check all resistors in the HV regulator section. An open one can also cause low HV.

compucat · #5 05-27-2013, 08:48 PM

Definitely start with the power supply. i too have a 25MC33 Zenith roundie and the electrolytic caps in the power supply are old enough now to cause trouble. This set should easily be able to produce 25 KV of high voltage which is what it should be adjusted for. These sets are full of brown drops which I'm not sure if they should be considered for replacement. My set still has most of them and I suppose one day it will need a full recap. Mine is in regular service and the only issue I have is the vertical acts like it is going to fail sometimes. I have not figured out the cause yet. The picture quality on these sets is fantastic, though, well worth the effort of a good restoration. Follow all the setup procedures in the Sams after restoration and you will be amazed at the performance.

Tom S · #6 05-27-2013, 09:16 PM

Just went throught that on my CTC10 160@450VDC went open. I'll put a buck on it.

Electronic M · #7 05-27-2013, 09:48 PM

Yup it was the doubler. One of the doubler cans was wide open I did not have a 160uF 250v on hand so I subbed a 250uf 250V in for the time being and it corrected the horizontal shrinkage.
I kind of suspected it since I had a NEW doubler cap in my Silvertone fail and the symptoms were the same. After replacing the doubler I brought it up on a variac with my typical over caution, and as I approached line voltage I started to hear BAD corona sounds from the HV cage. My over caution proved merited; there was perhaps the worst corona I've yet seen in any set coming off the HV line. It had to be over an inch long, that purple spark, and if I cranked the variac closer to line voltage it turned to an intermittent arc that caused the CRT neck to light in step.

My camera did not want to capture it, but if you can see the black spot in the red wire with a tiny purple dot in the middle where the red wire is in-front of the HV rect. plate cap that's is where the arcing was.

Since the wire had arced through I decided it was best to just replace the wire and HV connector suction cup with one salvaged from an abandoned SS set I scrapped. I also cleaned the rectifier cup, and removed the odd blob of clear silicon that someone put on the interior of the cup for reasons beyond my comprehension.
The cup did get better with a bath in the sink, but there was waxy dirt on the interior that soap and water refused to touch. To deal with that I broke out the big guns and used 'goof off' to remove it. That stuff can dissolve PVA and make the surface of those clear plastic covers on turntables turn rough and hazy. Since the HV cup is not clear and almost never seen I decided who cares if it messes with the finish if it gets the dirt off. Some pictures of this below.

I noticed while doing this that the focus rectifier tube socket was connected to nothing and that a 60's HV diode was installed. Not exactly correct but it is period and does seem to work decently so I think I'll leave it be for now.

Now I had a few problems remaining the set had not been given a proper gray scale, purity, or convergence setup in way too long. Also the vertical don't want to fill the bottom of the screen, and for some reason it seems the chassis hole marked 'vert. centering' looks to never been populated with a control...Ugh! That and I wanted to set it up before turning the color on.
It just don't want to be pure though, and the static convergence magnets don't have as much range as I'd like to see....I think I'll try a couple of spare purity rings I have stashed away next time I try to redo the set up. the results are below.

After giving up on that I tried the color, but that was barely functioning. Most of the color stage tubes were weak or shorted so I swapped them and got almost perfect color bars....Only which one is supposed to be the blue-green bar and which is the green bar?

There are also dirty tube sockets in the IF that I need to deal with, and for some reason it seems to tune channel three better with the tuner set to two.

I got this far without a schematic or any service data beyond the tube layout inside the cabinet, and I'm proud of that. But there are only a few tricks I've got left before I need to grab a reference to see exactly where it is off speck.

bgk283 · #8 05-28-2013, 01:42 PM

looks like its coming along nicely, if memory serves the HV rectifier is a 3at2? Ive got a bunch, being an RCA guy ,that I will never use, you are welcome to them, Bruce

Electronic M · #9 05-28-2013, 02:13 PM

Quote:

Originally Posted by bgk283

looks like its coming along nicely, if memory serves the HV rectifier is a 3at2? Ive got a bunch, being an RCA guy ,that I will never use, you are welcome to them, Bruce

If I ever need them I'll contact you. I have a fairly good stock of HV rectifier tubes already though. Whenever I go to a radio club meet and TV tubes appear in the donation auction they usually end up coming home with me.

bgk283 · #10 05-28-2013, 02:27 PM

I hear you, it didn't take me too long to have a stash like an old time electronics store, have fun

marty59 · #11 05-28-2013, 11:00 PM

With the Service/Normal switch in the service position, you should be able to verify vertical centering. If it's off-center there should be some nylon/fishline "loops" that are hanging out of the yoke. Pulling on one will affect the horzontal, the other loop the vertical. Most likely linearity and height will be difficult to achieve. See if the cathode capacitor is still the original and check B+ and load resistors.

You should have that set playing in no time!

Electronic M · #12 05-29-2013, 02:02 AM

Ohh, so that is what those strings are! I thought they were junk (like the pieces of HV wire some tech left) and tried to yank them out....Good thing I failed to yank them off. I probably messed up the settings when I pulled on them. If they are all the way back is there a way to get them forward?

Service switch kills ALL light on the screen so I did the screen adjustments by pulling one of the vertical wires to the yoke. It was off center(too high) when I did that so I guess I shouldn't have yanked those strings.

I'll have to look up the datasheet for the vertical output tube or get the sam's to find the cathode pin. By "B+ and load resistors" do you mean in the PS or vertical circuit?

marty59 · #13 05-29-2013, 01:01 PM

Quote: By "B+ and load resistors" do you mean in the PS or vertical circuit?

Just referring to the vertical circuit at the moment!

Username1 · #14 05-29-2013, 02:05 PM

Thanks for posting such detail on that set. I also have a 25MC33 Horiz output tube was cracked. I figured it had no osc, and overheated and cracked. I got a new tube but then didn't have the time to do any more this winter. Actually I still didn't turn it on, I found the bad tube just eyeballing around in the set before thinking about turning it on... I'm kinda inspired now to get busy on it....

dieseljeep · #15 05-29-2013, 06:40 PM

That was really a hack that worked on that set sometime in it's past.
I'm guessing that the HV rectifier filament winding arced through at one time, so some clod used the focus rectifier winding instead.
It took some ingenuity to do it in that manner, but still hack at the same time. Plus poor lead dress.
When I found the same problem, I would cut out the filament winding and use a solid state replacement for the HV rectifier tube.
I still prefer a focus rectifier tube instead of that lousy stick.