How do these waveforms look - Videokarma.org TV - Video

TinCanAlley · #1 08-29-2013, 12:38 PM

I am still learning how to use this vintage scope, but I think with the new probe I'm getting there. Attached are two waveforms I got off the horizontal and vertical blanking test point (their connected base to collector). The 7875 waveform has some misc lines in it. I don't know if they're supposed to be there or not (scan of waveform from SM isn't very clear). The other wave from looks close except for spacing. I'm sure I haven't gotten the exact setup of the scope correct, but it's working much better with the new 10x probe I got.

Anyway, let me know what you think. Next step is to find the next test point in the horizontal circuit. I really need to get rid of these jail bars.

TinCanAlley · #2 08-29-2013, 01:27 PM

Couple more. These are the horizontal sweep and the horizontal output transistor. I can see the sweep pattern being correct, but the output transistor doesn't. Of course, again, it could be my setup of the scope.

DaveWM · #3 08-29-2013, 01:42 PM

Quote:

Originally Posted by TinCanAlley

I am still learning how to use this vintage scope, but I think with the new probe I'm getting there. Attached are two waveforms I got off the horizontal and vertical blanking test point (their connected base to collector). The 7875 waveform has some misc lines in it. I don't know if they're supposed to be there or not (scan of waveform from SM isn't very clear). The other wave from looks close except for spacing. I'm sure I haven't gotten the exact setup of the scope correct, but it's working much better with the new 10x probe I got.

Anyway, let me know what you think. Next step is to find the next test point in the horizontal circuit. I really need to get rid of these jail bars.

looks pretty good if you can confirm the PP

DaveWM · #4 08-29-2013, 02:04 PM

from the looks of it the blanking pulses are ok, this is where a scope come in handy, knowing this ahead of time would have saved you doing all that other checking, but that's ok you are learning a valuable lesson, use that scope.

So back to the jail bars, before going there I think you need to figure out why the setup switch is not working. The reason I say that is IF there is a problem in the crt or video out transistors bias (seems to be in the setup mode anyway) THEN this could effect the cut off voltages of the CRT when it gets the bias to cut off during retrace.

Example you should be able to get rid of the jail bars if you just turn down the brightness (cut off the CRT more). Its the only thing I can think of (CRT bias).

I did notice there is a zener diode int the EC of the 3rd video out. Not sure if you checked that emitter voltage already.

TinCanAlley · #5 08-29-2013, 02:28 PM

Should I put the CRT on the Tester again? I am not sure I did it correctly the first time because I wasn't sure what the manual meant by (one divider on the meter). It's a B&K 470.

DaveWM · #6 08-29-2013, 02:49 PM

Quote:

Originally Posted by TinCanAlley

Should I put the CRT on the Tester again? I am not sure I did it correctly the first time because I wasn't sure what the manual meant by (one divider on the meter). It's a B&K 470.

If you can get a decently bright raster, I doubt the CRT is at fault. But you should learn how to use the tester to if you think you got it figured out, by all means check it. those Zenith CRTs of the era were quite good (long lasting).

you should not have to turn up the screen drives very far to get a bright raster (no signal just snow). If you turn them up and get a normal white brightness raster, then switch to service mode and DO NOT see the horz lines (RGB they will be faint but should be there, close together if not on top of each other), then there is something wrong that should be corrected before the jail bars.

DaveWM · #7 08-29-2013, 03:01 PM

152v at 2,11,6
47.5 at 7,12,3
540v,436v,501v at 5,13,4

the key numbers are the diff in 2,11,6 and 7,12,3 this is the bias on this case the cathode is higher than the G1 105v the larger this diff the more cut off the CRT will be.

is what I get from my sams on a 25DC56 (dont have the sams for yours and my zenith manual does not have them for your chassis

TinCanAlley · #8 08-29-2013, 03:20 PM

Quote:

Originally Posted by DaveWM

152v at 2,11,6
47.5 at 7,12,3
540v,436v,501v at 5,13,4

the key numbers are the diff in 2,11,6 and 7,12,3 this is the bias on this case the cathode is higher than the G1 105v the larger this diff the more cut off the CRT will be.

is what I get from my sams on a 25DC56 (dont have the sams for yours and my zenith manual does not have them for your chassis

We did all these in this link:

http://videokarma.org/showpost.php?p...3&postcount=38

TinCanAlley · #9 08-29-2013, 03:23 PM

Quote:

Originally Posted by DaveWM

If you can get a decently bright raster, I doubt the CRT is at fault. But you should learn how to use the tester to if you think you got it figured out, by all means check it. those Zenith CRTs of the era were quite good (long lasting).

you should not have to turn up the screen drives very far to get a bright raster (no signal just snow). If you turn them up and get a normal white brightness raster, then switch to service mode and DO NOT see the horz lines (RGB they will be faint but should be there, close together if not on top of each other), then there is something wrong that should be corrected before the jail bars.

Well maybe I wasn't using the CRT tester wrong. It was telling me that I had H-K leakage and need a isolation transformer. It also said my G1s were leaking and I needed to do a rejuvenation. I didn't think this was right, so I opened up the unit and found the following (see pic). So much for quality control.

I've fixed it and am about to try it again.

DaveWM · #10 08-29-2013, 03:36 PM

well the voltage seem ok, so i am out of ideas. Maybe some one else can come in with something I have over looked. But I would not stop checking stuff until that setup routine is working like it should.

you may have done this already but you did follow the sams guide for brightness limiter and greyscale setup? I see its more involved than I thought (as far as moving the taps around using a BW picture etc...

TinCanAlley · #11 08-29-2013, 03:40 PM

Well not sure what's wrong with the CRT tester beyond the capacitor that came free. I put the cap back in place and tried again. I set the heater voltage to 6.3V and then turn it to HTR Leakage. It says it's good if it is anywhere in the yellow area of the meter scale. When I try it, the needle drops all the way back and that's outside the yellow range. I then switch it to G1 Leakage and it's the same thing for each gun. However, when I set the G1 voltage and G2 cutoffs, each gun tests good.

I think there are more issues with this tester. I'm pretty sure I wouldn't get the picture I do if the heater wasn't working properly as well as the G1s.

TinCanAlley · #12 08-29-2013, 03:43 PM

Quote:

Originally Posted by DaveWM

If you can get a decently bright raster, I doubt the CRT is at fault. But you should learn how to use the tester to if you think you got it figured out, by all means check it. those Zenith CRTs of the era were quite good (long lasting).

you should not have to turn up the screen drives very far to get a bright raster (no signal just snow). If you turn them up and get a normal white brightness raster, then switch to service mode and DO NOT see the horz lines (RGB they will be faint but should be there, close together if not on top of each other), then there is something wrong that should be corrected before the jail bars.

If I crank up the G2s, I get a very bright raster. Switching the setup takes away the raster and all I see is a very faint blue line. If I turn down the blue, there are no lines at all.

DaveWM · #13 08-29-2013, 03:45 PM

Quote:

Originally Posted by TinCanAlley

If I crank up the G2s, I get a very bright raster. Switching the setup takes away the raster and all I see is a very faint blue line. If I turn down the blue, there are no lines at all.

you were posting this while I was editing. not sure if i gave you the correct procedure.

TinCanAlley · #14 08-29-2013, 04:13 PM

Quote:

Originally Posted by DaveWM

you were posting this while I was editing. not sure if i gave you the correct procedure.

I followed the Sams to the letter for everything except the greyscale. I had to modify it by adjusting with a B&W program and then adjusting the taps. I double checked G2 voltages to make sure they weren't too high or low.

The brightness limiter adjustment is at .6v DC.

There's no way it could be the switch, could it? If not, do I need to follow down the schematic and start looking at things along the setup side of the circuit? I did notice that the 100K and 33K resistors we were looking at (just behind the diode) is feeding into the setup side of the switch (I changed both those 100Ks and the 33K as well as the diode). There's also a 1.8K and an 18K resistor (didn't change them). After those it looks like it goes directly into the convergence section and getting 170V from T207 (green lead). It is going directly to pin four of J201/P201 which is then feeding the coil and pot for the blue lines giving me problems (Changed the coil, but not the pot).

I wish I were better at reading the schematics, but I think I got that all correct.

DaveWM · #15 08-29-2013, 08:03 PM

I would suspect the 1.8k 1st since it looks like a return path to ground for the emitter and is 5%