Are These Circuit Traces Bad
 

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

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

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:smoke:

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

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:smoke:

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.

looks like a capacitor has leaked on the traces. steve

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,

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.

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:smoke:

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.

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!

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.

What do you mean by "invisible" waves?

Okay, how about transparent waves. You can see the waves as they run upwards and then downwards, but they are clear. It's just the slight distortion they case to the image that lets you know they're there.

I tried to capture them via video, but it's harder than it would seem. I'm still trying, though.

I see a few things marked with "blanking." I'm attaching some pics of a basic circuit layout that might be what you're referring to. If it's there, point it out and I should then know where to look on the schematic. Also attaching a pic of a pin with a label regarding raster blanking. Not sure what that is, but the lead is headed over towards the tuner wiring.

Q202 is the one, you could try checking it, but I doubt its the problem. under the chassis you should see diode that goes to the base of that, then a couple resistors 33k then a couple 100k (5%). from there it goes to the flyback thru a cap.

I would check the transistor, diode,and resistors. Pretty sure you will need to take them out of the circuit to check, which will be a PITA and of course they may be ok. With out a scope you are handicapped into doing work to check things rather than just looking at the signal.

Again this is only a guess, so don't expect it to fix things its just an educated guess.

If I were you I would start looking into buying a scope, I know there will be some that say its a waste of money, true you can fix a lot of stuff without one, but when you run into tough dogs, it will save time, "don't be a dope, use that scope"

oh in case you do not know, its always wise to use heat sinks when soldering semiconductors, esp old ones that may be germainium. I use hemostats. clamp near the body of the part to sink out heat from the leads when soldering our un soldering.

My brother has a scope I could borrow on Sunday. If I got that, could testing be something I could be stepped though (where to hook up, what to set it for and what to expect)?

not really, using a scope takes some experience, and knowing what to look for.
Besides just knowing how to set it up takes practice. It would be a good learning exp, but may not be enough to point to the problem. This is why you need to get used to using one a lot, the more exp you have with it the better.

Okay, I found the transistor socket on the bottom side of the chassis. I found a 1K resistor from the socket. I'm trying to trace where it goes from there. I think I'll need a more powerful soldering gun. The hollow end of the pins are filled with solder and the ends are stuck inside. My 35w can't melt the solder as the pin and such are pulling the heat away.

Time to make a list of things to purchase.

Oh, my brother knows how to use the scope (he fixes old B&W sets), so I'll try and borrow him along with the scope.

I've been using a pair of needle nose. They act as a heat sink and allow me to wiggle the lead loose and such.

don't forget the heat sink, a lot of heat can damage a part before you get to check it. Those zenith solder pots can be difficult to deal with, I could get things out but getting back in was not so easy. Try cleaning the outside of the pot, then a little flux to get the heat to transfer well. use the sink I use it even on resistors, no reason to get them so hot.

Hopefully the schematic will have some scope patterns for that transistor to look at. without that it may not be much help. My DC56 has a pattern showing a pretty strong vert pulse with smaller horz pulses at the output from the blanker (emitter i think).

Sorry, I don't have one. I go from 121-500 to 121-758, nothing in between.

Cheers,

So that's what those are on the schematics. I have them from Sams Photofacts, but they're scanned to PDF and not very clear. I can see the scope signal, but the other information makes no sense and on many can't be read.

So with those on the schematics I should be able to connect a scope to that point of the circuit and compare mine to theirs? If so, it sounds simple, but I'm sure the part I can't read must be the settings or such.

I'm about done with the work. Once I get rid of the jail bars, all will be perfect and I can relax and enjoy the picture.

No problem. Just found out there's an old electronics shop a few miles from me that has lots of older stuff sitting around. I'm going to make a list of parts and call to see if they have them.

With those Zenith terminal strips, if that connection point is not only filled with solder but has a bunch of leads attached sometimes the best thing to do is cut the component out and "J" hook the new one in as opposed to the possibility of heat damage by using a bigger iron.

A scope can be handy as mentioned. Years ago I had a Zenith 24MC32 that had three "very transparent" vertical bars running through it and they were not always noticable. There was no sweep distortion so I believed it had to be in the video somewhere. I was still quite the novice back then so I lived with 'em...

Sounds like your brother may be handy too!

Thing is, I have to lift one end to remove it from the circuit for testing. I think I'll go the scope route and if that doesn't work, I'll live with the bars. I can see myself doing more damage if I start poking around with a big soldering gun.

Not to worry. I only used a gun to do filter cans & the grouned
lug of terminal strips. A pencil with the hottest tip will work on
the other lugs. We used the common weller station.
Alternative is snip off one end to test then tack back on
if OK. I always replace into the lug to keep it looking
neat & save time but thats just me.
BTW I dont know if its come up but the ground lugs on
Zeniths break off sometimes & go int. Poke each one with a
small screwdriver & resolder any that crack open.

73 Zeno:smoke:

ditto those ground connections on zenith term strips, esp happens after you have been tugging and flexing them a lot.

you can check that diode in circuit somewhat. both sides go to the power supply, but thru some rather high resistance so you should be able to get a front to back measurement to at least see some change when testing.

CR 201 on the schematic

I wish the Photfacts scans were clearer. If I'm counting correctly, I believe CR201 is across pins 11 and 12. I'm attaching a pic of what I think it is. I can also post a pic of the entire section of the underside if you like.

Does anyone else think it looks like it was either replaced before, or someone already cut one lead, tested it and then soldered it back together?

Looks like it was snipped but not changed.

73 Zeno

I am looking at the zenith factory schematic, so the numbers could be different from the sams, but that looks right. you can check that diode, the 33k and if you look further back there should be a couple 100k 5% resistors. Interesting that its already been snipped and soldered back.

the sams should show a 20v pp blanking pulse at the base AND emitter of the blanking amp transistor at the horz rate.

if bad this looks like the replacement

http://www.nteinc.com/specs/500to599/pdf/nte506.pdf

you could also check the bias voltages at the blanking transistor (if the blanking pulse is incorrect per the scope). If the diode is good and the fly pulse is getting to the base of the transistor BUT the bias is incorrect, that would effect the gain of the transistor and upset the output at the emitter. The sams shows the pulses at both the base and emitter (which look the same) I presume to check that the transistor is working properly (if not it would be a defective transistor OR incorrect bias).

Forgive my ignorance, but the transistor (Q202) is clear on the other side of the chassis from the circuits we're looking at. Is that because the pulse is coming off the horizontal module, going through these circuits and then traveling cross to the other side of the chassis to the transistor? I'm getting better at reading schematics and wish they were to scale. LOL

Anyway, I've managed to locate just about all the pieces and will talk with my brother tomorrow about setting up a day for him to come by with his equipment so we can do some serious testing.

Last question... Where are you seeing the pulse from that circuit? I'm looking at Photofacts and there isn't one for the emitter, but is one for the base which is shared with the collector of the vertical blanking. Strange thing is, there are two different patterns for the same test point. Both are 18V and then they differ with numbers I can't figure out. One graph is 30 and the other is 7875 (both numbers followed with a little curved line that looks like an oversized tilde). They both are different wave forms. No clue as to what those numbers are trying to tell me.

Thanks