|
|
|
#16
04-23-2011, 03:35 AM
|
||||
|
||||
|
Thanks! I may wind up making a new one.
Measuring voltages, everything comes up normal, with the exception of the plate of the vertical oscillator is way too low: I'm getting 45 volts on my digital multimeter, when my Rider's schematic puts it at 74 volts. The plate is only two resistors away from the B+ feed, so I replaced them as they were out of tolerance, but still get the same low voltage. Swapping tubes has no effect, either. As it is, the picture I get "floats" vertically, it does a half-hearted attempt to lock, before drifting away again. The vertical flipping doesn't increase in frequency, there just doesn't seem to be any lock. Interlace is also poor. The effect is the same whether injecting signal at the video grid or through the antenna. 
|
|
#17
04-23-2011, 09:06 AM
|
||||
|
||||
|
Check the grid of the vertical tube, it's probably high. When that happens the tube will conduct harder than it should, which drags down plate voltage causing more drop through the plate resistors. Fix the vert tube bias, and try again.
__________________
Evolution... 
|
|
#18
04-23-2011, 01:35 PM
|
||||
|
||||
|
Riders provided no voltage values for the input grid of the vertical oscillator. The other half of the 6sn7 does have the values given, and they rate normally (59 volts for the plate, -15 volts for the grid).
But, looks like we're getting closer to the problem, so I'll be digging into the grid circuit here shortly. BTW, the Riders is now available at the ETF site: http://www.earlytelevision.org/tv_sc..._diagrams.html Glad to see many more schematics up at that site, maybe someday I won't need all my Rider's binders...
|
|
#19
04-23-2011, 02:12 PM
|
||||
|
||||
|
Well, the schematic does show 2v at the cathode so I'd expect the first grid to sit somewhere around 0 or something negative. If the cathode is low, I'd suspect the cathode resistor and bypass cap. If the grid is high, look at that integrator or the 15k resistor next to it. Also check the voltage at the sync clipper plate and the resistors connecting it to the vert tube.
__________________
Evolution...
|
|
#20
04-23-2011, 06:37 PM
|
||||
|
||||
|
Well, checked all the resistors between the clipper plate (which is at normal voltage) and the vert osc grid and found a few out of tolerance ones. Voltage went up to 50v on the osc plate as a result.
Lifted the grid lead to see how much it was loading down the plate voltage, and it went up to 67 volts, so you're right, Miniman, the grid is improperly biased and loading down plate voltage. Grid voltage sits at about .1 volts, cathode voltage is dead on at 2 volts. If I get time tonight, I will work on making Wa2ise's integrator and see what happens.
|
| Audiokarma |
|
#22
04-24-2011, 08:29 AM
|
||||
|
||||
|
|
|
#23
04-24-2011, 08:45 AM
|
||||
|
||||
|
Quote:
|
|
#24
04-24-2011, 01:48 PM
|
||||
|
||||
|
Well, I found a resistor that had gone low, and replacing that one bumped the voltage up to about 60 volts. But just then, the width coil flashed over and caused the flaming 5U4 effect again.
So....completely disassembled the width coil this time. It is toast. The coil is in two sections, with a center tap, I believe, and one section is just destroyed. This may have been what took this set out of service so long ago, as I found a few other attempts at trying to shore up this coil before I used that cocktail straw. I'll try to give more details later, but now I have to see if I have anything close to this coil on my parts sets...
|
|
#25
04-25-2011, 05:49 AM
|
||||
|
||||
|
Quote:
It worked for my oscillator coil (my Fleetwood thread). The big thing is the number of turns and the direction in which you wind that need to match. In the case of my Fleetwood, I couldn't get the number of turns, so I opted to get the total length of wire, and wound it in as close to the same winding pattern as I could - and it worked!
|
| Audiokarma |
|
#26
12-13-2011, 01:23 AM
|
||||
|
||||
|
FINALLY had some time tonight and rewound the width coil. From the closeup picture earlier in the thread, you can see that it is two spools. When I pulled the coil, the top spool crumbled in my hands. The center shaft on the spool burned up, exposing bare coil wires to the adjustable inductor, shorting the windings to ground, causing the 5u4 to flash over.
As the spool was gone, I could only make note of the winding direction of the coil, and strip it. The spool ends were still intact, if a little cooked (no conductive carbon, thought). So I made a properly sized cardboard tube for a new center shaft, and began winding magnet wire onto it (#36, seemed pretty close to the original) by hand. Once I got a good base laid down, I chucked the coil into my drill, and spun the rest on. The whole operation took about an hour and a half. I hooked it up, and the set powered up. The coil didn't heat up, like the old one did, so I hooked up a video source and the crt, and was rewarded with a decent picture. Vertical hold is still twitchy, but it you leave it alone, it stays put. Sound is extremely poor. I suspect low bandwidth getting through, but adjusting the various sound and tuning controls still produces the same result with fuzzy highs and lows. BTW, the old width coil measured 15 ohms, and the newly wound one checks out at 15.6 ohms! Not bad! Otherwise, the picture looks decent, and the set is much easier to operate. I suspect that the bad width coil was heating up and loading things down a bit. Will take some measurements next.
|
|
#27
12-13-2011, 08:45 AM
|
||||
|
||||
|
If memory serves correct, the circuit RC network was called a "gimmick". The purpose is to cascade RC time constants, thereby re-shaping the sync pulse to improve vertical lock. In the old days, getting any watchable picture from weak signal was tough and Zenith seemed to excel. Has anyone ever seen this method used on the Horizontal pulse?
|
|
#28
12-13-2011, 10:11 PM
|
||||
|
||||
|
Getting closer-I can't quite get the image to fill the screen, and focus is a tad soft at the very fringes, but overall acceptable viewing from a few feet away. As the tube is a rebuild, it just may not be possible to get a perfect image from it. Also has a purple stain burned into the center phosphor, about the size of a half dollar.
Cured the hum in the audio (bad brand new lytic), and then started testing tubes to see if one was weak, and found that a 12AV7 had been used to replace the 12AT7 Noise Limiter/Audio takeoff stage. This was largely responsible for causing the poor audio, as well as some low frequency video smeering, and probably didn't help with the Hor/Vert locks, as they both function much better now. I still am getting some static in the image, cleaning tube pins helped, but I will have to pull the tuner apart and scrub down the ch. 3&4 strips. I kept it on for 2 hours, and other than adjusting the vert twice in the first 30 minutes, it behaved nicely: adjsladjs.jpg Just curious: This set uses some odd tubes. I know Zenith used different tubes in their radios and tvs during the 60s, had they started this practice earlier? I see a 6C4, 6BN6s, 6CB6s, and a 6BF5 audio output. I replaced several of these from my stash, and the only ones I have are all Zenith branded replacements.
|
|
#29
12-14-2011, 12:15 PM
|
||||
|
||||
|
They did use those horible 6X5 rectifiers on their pre-war radios when most other home sets used 5Y3.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4
|
|
#30
12-15-2011, 02:03 AM
|
||||
|
||||
|
I rechecked voltages tonight, and yup, with that new coil in my weird loading issues stopped.
I also noticed why my screen adjustments weren't quite perfect: the focus electromagnet was replaced with a pm RCA assembly! The yoke also appears to have been replaced as well. I cleaned up the pins of the channel strips, but you really can't get anything into the other set of contacts under the tube deck. So, I applied some strips of sandpaper to a tuner strip, and then racked the tuner back and forth over that channel, cleaning those hidden contacts. This operation cleared up most of the static I was getting in the picture. Started in on the cabinet today as well. It had been stripped as found, so I applied filler and will try to lacquer this weekend. GEDC1006.jpg GEDC1007.jpg Last edited by leadlike; 12-15-2011 at 01:22 PM.
|
| Audiokarma |
 |
|
|
My audio output transformers looked the same, and until I tested the continuity, my main thought was "gee, I hope those are still OK".
Linear Mode
