View Full Version : Help sorting out schematic discrepancy? (Zenith 17D20 chassis)


jmetal88
01-17-2016, 08:50 PM
I was doing my initial re-cap (based on info from Sams Photofact Set 469, Folder 2) for a set I've had for a while based on the Zenith 17D20 chassis, and after I was done, I realized I had three unaccounted-for paper capacitors left in the chassis. After tracing where each one was connected, I was able to account for one of those (a 0.1uF 600V capacitor in a location not marked on the identification chart turned out to be C55, installed to a different physical location but equivalent electrical location to where it should have been, and I had just ordered the wrong number of 0.1uF capacitors), but the other two are at odds with what I'm reading on the schematic.

One of the other two turned out to be just a different value paper capacitor substituted for what was probably a ceramic capacitor in the schematic, C67. C67 in the schematic is 4700pF, and on my chassis it is 0.01uF (10000pF). It connects between ground and two resistors, one of which is a 10k resistor connected to the 260V source, and the other of which is a 100-Ohm resistor connected to Pin 4 (one of the grids) of the Horizontal Output 6DQ6B tube. I am unsure whether I should replace it with another 0.01uF capacitor or if I should revert to what it shown on the schematic and put a 4700pF capacitor in there.

The second one is a bit more complicated to describe, and I may end up needing to post a picture of the relevant portion of the schematic versus a drawing of what's actually happening to illustrate my point. This capacitor is connected between one pin of the Brightness potentiometer and one pin of the Vertical Size potentiometer, and the schematic shows a completely different circuit. In the schematic, the 260V source is connected to a 330k resistor, which is connected in series with a neon lamp, which is in turn connected to the end of the Brightness potentiometer opposite ground. On my actual chassis, a 0.22uF paper capacitor is connected to the end of the Brightness potentiometer opposite ground in parallel with a 1.5M resistor to what looks like the wiper of the Vertical Size potentiometer, which would be the '580V Boost' source (according to the schematic). It appears that there is no neon lamp on my chassis. I can't imagine that kind of modification to the circuitry would be done without a very good reason, so my instinct here is to just replace the 0.22uF capacitor with a newer one of the same voltage rating and leave it at that, but does anyone have any clue as to why they would've made that change?

jmetal88
01-17-2016, 08:54 PM
Oh, wait, after I posted this, I got to thinking it might be more appropriate for the Rectangular Screen Tube Televisions section. Any mods reading this, feel free to move it if so.

Electronic M
01-17-2016, 09:24 PM
There are 3 reasons for schematic discrepancies: 1)production changes, 2)schematic typos, 3) hack repair/modification.

Unless the differences look to be the work of a hack attempting to repair the set ALWAYS go with the parts/wiring in the set instead of what is on paper. If you make the set match a typo or a pre-production change variant at best performance could change slightly, at worst the set could stop working and or be damaged.

In the case of there being evidence of hack repairs in the affected area it is normally best to follow the schematic.

I've lost count of the many times I've discussed this point....Perhaps I should make it into a sticky? :scratch2:

Oh, wait, after I posted this, I got to thinking it might be more appropriate for the Rectangular Screen Tube Televisions section. Any mods reading this, feel free to move it if so.

Done.

Adam
01-17-2016, 10:04 PM
The schematic in the Zenith manual for the 17D20 agrees with the sams. However, both the changes described appear in the schematics for the just slightly newer chassis: 16E21 and 16D21, so it is likely this set came that way.

jmetal88
01-17-2016, 10:17 PM
The schematic in the Zenith manual for the 17D20 agrees with the sams. However, both the changes described appear in the schematics for the just slightly newer chassis: 16E21 and 16D21, so it is likely this set came that way.

Okay, that's good to know. The *major* change, I definitely didn't want to mess with, but knowing that substituted value also appears in later sets gives me more confidence that it's the value that's supposed to be there.

Adam
01-17-2016, 10:45 PM
The 16D21/16E21 does a .22uF cap in parallel with a 1.5M resistor going to the boost voltage, rather than that circuit with the Ne lamp going to the 265V B+.

And it does use a .01uF where the 17D20 has a .0047, but the resistors there are 100 and 8.2K rather than 100 and 10K.

jmetal88
01-17-2016, 11:45 PM
The 16D21/16E21 does a .22uF cap in parallel with a 1.5M resistor going to the boost voltage, rather than that circuit with the Ne lamp going to the 265V B+.

And it does use a .01uF where the 17D20 has a .0047, but the resistors there are 100 and 8.2K rather than 100 and 10K.

Oh, you know what? It is an 8.2k on my chassis. I didn't think to look and see if that was a different value as well. It's one of those high-power resistors with the value printed on, and the printing was facing in where I couldn't see it.

jmetal88
01-17-2016, 11:51 PM
I am a little bit worried about what the picture tube is going to look like once I get this up and running. Some previous owner had a brightener in the back of the set. But the odd thing is the brightener wasn't actually installed -- it was literally sitting in its retail box, closed up inside the back of the set.

zeno
01-19-2016, 09:40 AM
The customer may have bought it & it didnt fix it. So
put it where it wont get lost. OR the CRT may have got changed.
Usually easy to tell from the tags, EIA number & date code.
It was not unusual to find such things in an old set.

As for part differences learn how to spot reworked soldering. You
only have to see it a few times. Always more solder, not as neat
& a different shade of grey. Nipped wires also a give away.
Always use the factory in set value UNLESS you have an apropriate
factory change bulletin.

The brightener question is why you should always do the miminum
work to get something on the screen. That way no unpleasant
surprizes after you have spent time & $$ like a bad CRT.

73 Zeno:smoke:


I am a little bit worried about what the picture tube is going to look like once I get this up and running. Some previous owner had a brightener in the back of the set. But the odd thing is the brightener wasn't actually installed -- it was literally sitting in its retail box, closed up inside the back of the set.

jmetal88
01-27-2016, 07:38 PM
Well, I finished putting the rest of the caps in tonight, and we have a raster! The picture tube is a little bit dimmer than the smaller Zenith that I got mostly working a few years ago, but it doesn't look like it'll be hard to see at all. Now I need to hook the tuner back up and get it loaded in the cabinet to see what the picture actually looks like.

jmetal88
01-27-2016, 08:29 PM
Okay, after hooking up the tuner and looking at actual video on the display, I think I can see why the previous owner had purchased a brightener. But it seems to be more of a contrast problem than a brightness problem. A brightly-lit scene looks fine on this set, but a dimly-lit scene is very difficult to distinguish.

EDIT: Aw, something's wrong with the audio circuitry, though. I can barely hear anything even with the volume turned all the way up. Guess I'll have some troubleshooting to do.

jmetal88
01-28-2016, 07:44 PM
Okay, I got the audio problem solved. I have a resistor that's supposed to be connecting 260V to the audio transformer that's open. I just hooked up an equivalent resistance (had to parallel a few resistors because I didn't have one at the proper wattage) across the open resistor via a couple of clip leads and got reasonably clear audio as soon as the set warmed up. Now I'll have to either take the chassis back out of the cabinet to solder or attempt soldering upside down just outside the back edge of the cabinet, haha.

Electronic M
01-28-2016, 09:22 PM
AGC adjustment might brighten the picture.

jmetal88
01-29-2016, 06:24 PM
AGC adjustment might brighten the picture.

Boy, your diagnosis there was spot on! My pictures have much deeper blacks after adjusting it, and my audio has a lot less buzz as well!

Electronic M
01-30-2016, 01:45 AM
Happy to be of help to ya.