721-TS Problems - Videokarma.org TV - Video

bgadow · #1 02-01-2005, 08:16 AM

While waiting for parts/schematics for some other projects I pulled this set out after a long hiatus. It has been recapped & many resistors have been checked. It has 2 major problems:

1) No vertical sweep. A capacitor between the 6SN7 grid & filament fills the screen nicely. The osc.xsfmr. checked okay when I last had it out but I was reading somewhere else about leakage across the windings (?). Quick & dirty substitution of some others I have (not the correct xsfmrs) changed nothing.

2) C4 is a 2-section low voltage cap that I replaced previously. I sometimes leave those types alone but this one was beginning to "puke" out the bottom. In running the set the other day I started to smell something and looked down to see C4A bulging just like the can of soup on the stove in some old bw short I've seen. (Stooges?) Anyway, I shut it down, that cap was an older stock so I swapped it, the newer one did the same thing, got very hot and finally popped. Closer examination showed that C4B may have already died so I changed them both, now it is C4B that gets hot. I have triple-checked the connections, C3A, 80mfd, had been replaced but I tried subbing it again just for the heck of it, no help. I pulled the big power resistors on the back out of circuit and they all checked okay. (more to come...)

Chad Hauris · #2 02-01-2005, 09:03 AM

What is the voltage on the electrolytic cap like...can you put a scope on the DC lead to the cap and see if somehow significant AC is getting in for some reason?

If the vertical osc is amplifiying but not oscillating there is a problem in the feedback loop...did you double check all resistors and caps in that portion of the circuit?

bgadow · #3 02-01-2005, 11:47 AM

Sorry, mid-post things got busy here...I've been through the vert circuit pretty well but I will check again. Kinda tight in that corner, especially with all the new electrolytics buried under the chassis right there.

The + side of C4A read, IIRC, around 250v which seemed in line with Sams. I don't think I measured the other side. I'll follow up on these tips, and try to get a scope on there, too. I'm still not real good with scopes but I'm trying. I should add that back the last time I worked on this the horiz.ctr. control fried. In its place right now is a pair of 10ohm wirerounds. (I didn't want to fry another control)

Chad Hauris · #4 02-01-2005, 06:16 PM

Seems like you may be getting 15,750 hz ac pulses from the yoke coming back into the cap somehow...you will have to use the scope to confirm this though.

bgadow · #5 02-01-2005, 09:52 PM

The positive side of those caps read 295, the negative side read 255. I swapped in the highest volt (50v) caps I had for these values but I'm still getting overheating. I really need to practice with the scope; with both the it and the multimeter I'm showing negligible AC. I'll keep working on that. This problem is more important then the vert since it keeps me from running the set for very long.

On the vert end, I retraced everything again looking for bad connections but everything looked okay; checked the transformer out of circuit and it seemed in line. One reference I have says the .0047 in the circuit is usually a ceramic disc and that tolerance must be tight. I have a new "paper" type cap in there-I've heard of circuits where a disc can't substitute for a paper, but not vice-versa.

wa2ise · #6 02-02-2005, 12:44 AM

Quote:

Originally Posted by bgadow

2) C4 is a 2-section low voltage cap that I replaced previously. I sometimes leave those types alone but this one was beginning to "puke" out the bottom. In running the set the other day I started to smell something and looked down to see C4A bulging just like the can of soup on the stove in some old bw short I've seen. (Stooges?) Anyway, I shut it down, that cap was an older stock so I swapped it, the newer one did the same thing, got very hot and finally popped.

Looking at that circuit it looks like you'd need caps with very low ESR (effective series resistance). Switching power supplies use low ESR caps, so maybe try salvaging caps from a no longer needed PC power supply. Or use several smaller caps wired in parallel. That would split up the AC current load and each will not get as hot.

bgadow · #7 02-05-2005, 11:07 PM

I've been studying this in depth, trying to learn something. My "progress" so far:
I paralleled the low voltage caps. The 2-470mfd @50v caps are doing fine but the pair of 100 @ 50v got very hot. Well, when faced with brute force I offer brute force right back! In went a pair of 100 @ 450v! Take THAT! I also found that the vert centering control had been cooked at some point so in went another pair of wirerounds for now. At this point nothing is cooking so I can take my time trying to diagnose the lack of vertical sweep.

I undid the vert osc xsfmr leads and I measure 182 ohms/1500 ohms. Sams sez 165/1310, so I guess thats close enough. No reading across the two. The grid of the vert osc. shows nothing; there is a .0047 cap to the xsfmr and the xsfmr side reads 95vdc. The grid also goes through a 1 meg resistor to the vert hold-95vdc also on the wiper there. Wiper of the height control shows about 400v. Tried jumpering the resistors in the circuit. Putting ac (cap from filament) gets sweep on both osc. and output grids. With a lousy antenna hooked up I even got video-good enough to tell the guy was playing golf! Now, if it sounds the least bit like I know what I'm doing, don't give me that much credit! I'm trying though. To show how I think, looking at the Sams I see that the osc grid should show -6vdc so I tried hooking in a 6v lantern battery to see what would happen. Well, the sweep expands, but just for a flash.

At this point I guess either the xsfmr really is dead-any way to tell short of substitution? or there is something else I'm overlooking. I'll keep at it, any ideas welcome.