Are These Circuit Traces Bad - Videokarma.org TV - Video

TinCanAlley · #1 08-07-2013, 02:24 PM

I purchased an NOS horizontal output board for my Avanti and upon installing it, nothing. There's sound, but no image. Not even the sound of the degauss. So I swapped back the old module and it worked again. I check everything on the top side of the module to make sure they were in their sockets, etc. Everything looked fine. It's when I turned the board over. I noticed that a few of the silver traces had black sections that look like the old laserdiscs had when they got laser rot.

Anyway, I'm attaching a pic and would appreciate any input. I bypassed the main canister today and it didn't help with the jail bars. I don't feel I wasted my time as I had already replaced all electrolytic caps earlier. Besides, the 500uf sections were 35% and 40% on the plus side and the 400uf sections were about 25% and 30% on the plus side. Not sure this makes too much of a difference, but the ones I put in are only 10% when tested.

Thanks

TinCanAlley · #2 08-07-2013, 02:59 PM

Never mind. I decided to test the transistors on the board (figured one could be bad). I'm not an expert at testing, but found one that seemed odd, so I decided to take the one from the working module and swap it. When did that and installed the board, the set fired right up.

Now I need to get new transistor.

Thanks

zeno · #3 08-07-2013, 05:05 PM

The etch is fine. I have seen it before , I think its magic
marker.
On Zenith modules its common to have bad plug in IC &
transistor sockets. Especially on the upright ( 25EC45 etal )
chassis. They eliminated them I think on all rebuilt boards.
The sockets are less reliable than the parts !

73 Zeno

TinCanAlley · #4 08-07-2013, 05:52 PM

Quote:

Originally Posted by zeno

The etch is fine. I have seen it before , I think its magic
marker.
On Zenith modules its common to have bad plug in IC &
transistor sockets. Especially on the upright ( 25EC45 etal )
chassis. They eliminated them I think on all rebuilt boards.
The sockets are less reliable than the parts !

73 Zeno

I tried the original transistor again and the set wouldn't fire up. Putting in the one from the old board in again worked. So now I need to find me an NTE171 locally. Not gonna spend a couple of bucks for one and then $7 for shipping.

Interesting, though...The jail bars are still there after all this work. The image is better, though. I tried the video processing board I got and it was a disaster. It's an update board and NOS, but when I fired up the set, the horizontal was screwed and the control on the back of the set couldn't fully correct it. The color was also messed up. Some areas were black and white with flashing colors now and then and the brightness and contrast were messed up as well. I figured it was safer to put back the original board than try to adjust setting to see if the new board was working properly

zeno · #5 08-07-2013, 06:47 PM

You need to be careful with any brand boards in the box.
They are quite often the bad boards. Someone changes it &
never marks it as bad. Many shops were careless about
returning duds for core credit, often loosing track of where they
came from etc. No invoice -no credit.......
Also many put them aside to rebuild themself "when a chassis
comes in & I got time" Needless to say that usually was never &
bad boards piled up. Then someone comes along 30 years
later & liquidates them on E-Bay. No way in hell he knows.

CAVIAT EMPTOR

73 Zeno

Findm-Keepm · #6 08-07-2013, 07:19 PM

Quote:

Originally Posted by zeno

You need to be careful with any brand boards in the box.
They are quite often the bad boards. Someone changes it &
never marks it as bad. Many shops were careless about
returning duds for core credit, often loosing track of where they
came from etc. No invoice -no credit.......
Also many put them aside to rebuild themself "when a chassis
comes in & I got time" Needless to say that usually was never &
bad boards piled up. Then someone comes along 30 years
later & liquidates them on E-Bay. No way in hell he knows.

CAVIAT EMPTOR

73 Zeno

I quite agree - when my dad retired back in 2008, he had exactly 7 good Zenith Duramodules and about 30 boxes marked with the symptoms the module had. After the heyday, and after junking a few sets, shops ended up with lots of questionable stuff.

Quote:

Originally Posted by TinCanAlley

I tried the original transistor again and the set wouldn't fire up. Putting in the one from the old board in again worked. So now I need to find me an NTE171 locally. Not gonna spend a couple of bucks for one and then $7 for shipping.

As far as needing a transistor, post the 121- number, as many of us have stashes of original parts - yours for postage, in my case. Don't default to buying new stuff when we might have what you need, and are willing to give it away!

Cheers,

TinCanAlley · #7 08-07-2013, 07:43 PM

Quote:

Originally Posted by Findm-Keepm

I quite agree - when my dad retired back in 2008, he had exactly 7 good Zenith Duramodules and about 30 boxes marked with the symptoms the module had. After the heyday, and after junking a few sets, shops ended up with lots of questionable stuff.

As far as needing a transistor, post the 121- number, as many of us have stashes of original parts - yours for postage, in my case. Don't default to buying new stuff when we might have what you need, and are willing to give it away!

Cheers,

Thanks. The part # is: 121-755

I bought these off ebay and they were noted as being NOS. They sure don't look used and are a newer cream colored board instead of the old brownish (I'm so bad with any color other than primaries). The boards are the updated versions of the originals, but they can't be much newer as they also have plug in transistors.

rcaman · #8 08-07-2013, 07:47 PM

looks like a capacitor has leaked on the traces. steve

Findm-Keepm · #9 08-07-2013, 10:08 PM

Quote:

Originally Posted by TinCanAlley

Thanks. The part # is: 121-755

I bought these off ebay and they were noted as being NOS. They sure don't look used and are a newer cream colored board instead of the old brownish (I'm so bad with any color other than primaries). The boards are the updated versions of the originals, but they can't be much newer as they also have plug in transistors.

Zenith may have had a second production run, as many shops ordered modules but failed to turn in duds. I remember PTS and others would issue a hotlist if modules they would buy at the dud price, to develop sufficient stock. Zenith System 3 modules were that way in the early 80s - some modules were backordered to the factory to get replacements. We fixed all of ours, so we never swapped anything but tuners and tuner controls. Duramodules (the Zenith term for your module) were exchanged, as the modules were cheap, and Time-to-repair was greater.

I'll check in the morning and see if I have your transistor.

Cheers,

TinCanAlley · #10 08-07-2013, 10:40 PM

Okay, the new boards aren't cream color (I was going from memory). They're actually white and green. Here's a pic of the one I installed. The bad transistor is the one with the screw hole just behind large orange capacitor.

DaveWM · #11 08-08-2013, 07:30 AM

so you still did not look into the horz blanking? that would have been the 1st place I would have checked.

zeno · #12 08-08-2013, 09:47 AM

Quote:

Originally Posted by DaveWM

so you still did not look into the horz blanking? that would have been the 1st place I would have checked.

Yes lets get back to the jail bars ! The filter can was a good learning
experience but as I suspected unneeded..............
The Sams is in front of me & I am redy to rock & roll !

73 Zeno

DaveWM · #13 08-08-2013, 09:57 AM

is this the 25dc56? I assume so,

I would look at the diode and blanking transistor, as well as the voltage divider in front of that.

Scope the fly pulse all the way thru the two blanking transistors.

TinCanAlley · #14 08-08-2013, 11:12 AM

I'm doing the best I can. I'm new to this and am doing the things I can do/know how to do. If those don't work, then I can start to tackle the harder things. I don't own a scope, so I'll have to borrow one. I've read through the SM and don't fully understand how to test the blanking circuit, etc.

I'm willing to go all the way in testing an fixing you you're willing to help and understand I'm pretty new to all this.

My first fix is to undo something I just did. If this problem sounds familiar, yell. After changing the horizontal board and caps, I now have a strange effect on the screen. It's hard to explain, but I think if you've seen it before, you'll understand. There are invisible waves rolling diagonally across the screen towards the upper left corner for a moment. They then stop and reverse direction rolling down towards the lower left. They stop again and reverse. The closest I can give an example of is when you're adjusting the APC to get the movement out of the color bars, except these aren't colors rolling, just invisible waves. I'm not sure if it's the what I did, or some other interference that's being introduced.

The chassis is the 25EC58 and if you want to point to things to test/replace, please fire away!

Thanks!

DaveWM · #15 08-08-2013, 01:00 PM

you need to deal with one problem at a time.

sounds like some kind of interference, make sure you are not setting a RF source near the set (like a DVD player on top of the tv) try a different signal source, double check that you have everything hooked up right (300ohm to 75 ohm balun) make sure you have not messed up lead dress inside the tv (crt wires, HV wires,tuner IF link). All grounds are in place.

Shot gun repairs often introduce new problems making diagnosis a more difficult problem. don't take it personally, a lot of peeps seem to think replacing every cap in sight will fix everything. It has been my exp that most sets from the mid sixties on do NOT require a lot of cap replacement, and doing so just introduces possible new problems.

So on to the existing problem without a scope its just a guess that the horz blanking is a problem causing the jail bars. if you have the schematic you should be able to find the diode that feeds the pulse to the blanking transistor. I do not know if its on a modual or chassis mounted. On the 25DC56 its NOT on the modual.