Who's got a 27" K7400 chassis handy?

modessitt

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Otherwise known as a WGM2774.

Anyway, I'm trying to diagnose one that has raster but no picture. You can turnup each color until you get a full screen of that color, but no game picture. Since the signals basically go straight to the neckboard, I swapped the neckboard and got the same problem.

I'm thinking the issue lies with the L550, L551, and L552 that are the only part in each color path between the video input connector and the connector to the neckboard. On the 25", these are straight jumpers, but on the 27" they are populated. Now, the "L" designator would indicate an inductor, but there are 470uf 10v caps in these locations. WG's support site doesn't have a parts list for the 7400, nor show the differences between the 25" and 27" models. I've seen pics online that show these spots populated, but I need someone to look at one (preferably working) and tell me what part (and value) is there. Do you also have caps? Or do you have inductors?

BTW - these parts are right on the edge facing you if you have it mounted horizontally in a cabinet, just to the right of the connector to the neckboard. No need to pull it out of the chassis to check. Just get a flashlight...
 
I'd have to dig through some of mine to see if there is an actual 27" version around. (I don't recall seeing one)

I'm thinking the issue lies with the L550, L551, and L552 that are the only part in each color path between the video input connector and the connector to the neckboard.

Since you are getting "colors", I wouldn't think this was the issue.
I would be looking into maybe a loss of sync/deflection signal somewhere.
Have you checked other voltages going to the neckboard like the 15v and Vid B+ ?

Been a while since I touched any 7400, so I can't really be of much help.
 
Since you are getting "colors", I wouldn't think this was the issue.
I would be looking into maybe a loss of sync/deflection signal somewhere.
Have you checked other voltages going to the neckboard like the 15v and Vid B+ ?

I'm not getting colors from the game. I have the normal black screen like no video is connected. If I turn up the color pots on the neckboard, I can make the screen all red, all green, or all blue - just like I could with no video connector attached.

So, it seems I have a working monitor (somewhat) that won't show the game image, and I'm trying to find out if these caps are blocking it...
 
I only got a 27K7500, and it's a scalped/junk chassis. what about that ribbon cable going from the chassis to the neckboard? I've seen those break too, but I highly doubt all 3 color signal wires would break all at once.
 
It was late last night, and I was thinking about this today while driving. I'l do some voltage checks on the color input, then the pins to the cable then at the neckboard. If I have just one break - like on the video ground wire - that would cause no picture, too...
 
Well I got a K7400 that is hooked up to a 27" tube P763 on the chassis..

I don' have L550, 551 552 but I do have L501, 502, 503.. and R545, R550, R551 in series before the Inductors.. the color code for the resistors are brown teal brown gold.. and the inductors are all brown red black silver.

You can use the K7500 service manual under replacement parts list for electrolytics. I just used that to verify my K7400 electrolytic caps the other day and they all matched up except for one which was at a 50V rating in the spec sheet instead of 35V rating on board.

Let me know if you need a picture of the chassis or not ;) maybe then you guys can help me with the slight waviness of the picture i am getting with this chassis heheh
 
I know what caps are used, and I checked the K7500 parts listing to see if it's 27" version used something like this, and it doesn't. I've seen some pics online of other 2774's, and they show the same parts.

Here is what I am talking about (circled). I need to know if they are supposed to be there, and if not, what should be:

IMG_20110721_215901.jpg
 
Ah wow, that's weird... mine are just wire jumpers.. Ill check my other dead k7400.. i am pretty sure that one was hooked up to a 27" as well (beast busters)

Yep both of my K7400's have wire jumpers there and no components.. but I guess its also possible these chassi of mine are improperly matched with 27" tubes.. bought these a while ago coupled with the tubes.

other markings on both boards: 025A1450-001
 
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The 2574's I have just have the jumpers. Do yours have the pincushion board? What about the sticker in the back left corner telling the model number?
 
Yeah, the little rectangular board that mounts vertically with two pots? If so then both of mine have them
 
Hadn't had a chance to dig any of mine out to look, but I did notice the three in question are a different brand from the others. "Wincap" is pretty typical of originals, but "Nichicon" are usually replacements. And like you said, why the "L" designation? Weird. I'll try to get out and look at some of mine today.
 
Well, not sure if this will help or not......

I have 2 P763 chassis WITH those caps. One is not labeled to distinguish if it is 27" or 25" but it DOES have the pincushion board. The other is clearly labeled 2774.
I also have 2 other P763's that do NOT have the caps, both have pincushion boards, neither have the basic "2774", 2574" labels though.

None of the P793 K7500 chassis I have have any of those caps.
2 are for 27", 1 is for 25".

P763 without caps 054A7401-002
P763 without caps 025A1450-0011 "Betson SF 275002"
P763 with caps ---------
P763 with caps WGM2774 054A7401-010
P793 without caps WGM2775 054A7502-001
P793 without caps WGM2775 054A7501-003
P793 without caps WGM2575 054A7502-001

The only other thing I can say is that the ones that DO have the caps all look like they were added by someone, like a mod or "fix" for something.
I'd say it's safe to test with and without those caps. (install jumpers instead of caps)

PS: I only added the 7500 chassis info because they are so similar....so kind of a future reference thing.
 

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Well, thanks for the info. looks like they are what SHOULD be there, if anything should be there besides a jumper. I'll try a jumper on one to see if allows anything through....
 
I forgot to mention that the ones with caps were all 470uf 10v caps like yours.
So they seem to be the correct value.......for whatever reason they are there.
 
Well, it's fixed. Had color voltages proper to the neckboard and still no picture, and swapping neckboards didn't help.

While doing some reading on the theory of the K7500 (similar design), I was reading about Auto-Beam Limiting and how C812 can cause contrast to drop low. Since the 800 circuit seems to deal with contrast and brightness, I inspected it. Looks like only a partial cap kit had been done on this chassis, and none of the 800 caps were part of that partial replacement. So, I replaced the 800 caps - and now have a nice picture.

Looks like some other caps weren't replaced either, so I'm going to do the rest of those...
 
Well, it's fixed. Had color voltages proper to the neckboard and still no picture, and swapping neckboards didn't help.

While doing some reading on the theory of the K7500 (similar design), I was reading about Auto-Beam Limiting and how C812 can cause contrast to drop low. Since the 800 circuit seems to deal with contrast and brightness, I inspected it. Looks like only a partial cap kit had been done on this chassis, and none of the 800 caps were part of that partial replacement. So, I replaced the 800 caps - and now have a nice picture.

Looks like some other caps weren't replaced either, so I'm going to do the rest of those...

OT, I removed the little resistor off the neck socket on a U5000, you may have seen this done to those neckboards before, but I have a chassis where the contrast is either still extremely weak or it'll just keep changing to a blue tint. this has happened on 2 different neckboards. did I get a bad batch of caps with that one maybe? it's done this on 2 different tubes as well.
 
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