Missing Diode?

[Jaydon]

Hello everyone. I have a WG k4900 with a vertical collapse. I've done a recap, reflow both the main and neck boards, even changed out the B+ cap and flyback. Nothing has changed. I did notice that there is jumper at D301 instead of a diode. Is this normal or am I missing something?

Thanks!!

 

[andrewb]

The schematic doesn't show a part for D301, so that jumper is most likely factory.

Also FYI, you can see from the pic that that board has heat damage. Any place where the PCB is discolored, means it got a lot of heat. And those are often the places where you can get things like cracked traces, cracked resistors, bad transistors, broken solder joints, etc. So it's always good to wash any monitor chassis, and then really focus on those areas with a fine-toothed comb.

 

[Jaydon]

Will do. Thanks again!!!

 

[Jaydon]

I have another random question. The adjustments pots are suppose to be 5K, right? Mine only measure to a little over 3K when maxed. Is that normal?

 

[andrewb]

How are you measuring them?

 

[Jaydon]

I'm pulling them out of the board and using a multi meter. With the 2 leg side on top and the 1 leg side on bottom, I put one end of the probe on one of the top legs and one end of the probe on the bottom leg. I did try every combination. And this way resulted in the 3K that was closest to the 5K specs.

 

[andrewb]

It isn't uncommon to find adjustment pots that are slightly different from what they should be. Depends on the case, but in most cases they don't need to be exact. If it matches the other pots, it's likely fine. A pot being slightly different isn't going to cause collapse like you're seeing.

 

[AlexZander]

Values listed in the manual. K4900 manual
Main Board;
VR201, VR351, VR352 are 2k ohm
VR301 5K
VR303 500 ohm

Neck Board;
VR401, VR402 500 ohm
VR403, VR404, VR405 5K ohm

 

[Alup]

I see pot mods alot on these old chassis, old ops would increase or decrease their value to "push" the monitor a bit further to get it perfect.

Also, @andrewb its a 4900, have you auctually had the luck of finding one not burnt to a crisp? Lol.

Seriously though, replace q302 qnd q303 make sure to reapply thermal paste. You can get them from here - https://www.arcadepartsandrepair.com/store/pinball-kits-parts/pinball-parts/2sc2073/

 

[andrewb]

I see pot mods alot on these old chassis, old ops would increase or decrease their value to "push" the monitor a bit further to get it perfect.

Also, @andrewb its a 4900, have you auctually had the luck of finding one not burnt to a crisp? Lol.

You mean the PCB's? I'm not a raster guru by any means (I haven't done a million of them), but most of the WG's I've worked on have heat-damaged spots. I think that phenolic PCB material that WG used tends to show it more than fiberglass.

But it's also a testament to how many miles these monitors all have on them, which reflects how well they were made. People love to shit on this model or that model as being hard to fix, but that's partly because these monitors all have thousands of hours on them. You can't really fault any of them, IMO.

 

[Alup]

You mean the PCB's? I'm not a raster guru by any means (I haven't done a million of them), but most of the WG's I've worked on have heat-damaged spots. I think that phenolic PCB material that WG used tends to show it more than fiberglass.

But it's also a testament to how many miles these monitors all have on them, which reflects how well they were made. People love to shit on this model or that model as being hard to fix, but that's partly because these monitors all have thousands of hours on them. You can't really fault any of them, IMO.

I love the g07 and 4900 however just about every 4900 and g07 ive worked on has been burnt like that. Also, some models are indeed pretty hard to repair partly because you cant get parts for them, im looking at you kortek.

 

[Jaydon]

Hi Alup. Thanks for the response. When I got the board, one of the legs on one of those transistors was completely broken. So I replaced both of the transistors (with thermal paste) when I did the recap. Yesterday, I did pull and test all the resistors, diodes, and caps in the burnt area of the board and everything checked out fine. Some of the resistors were reading a little lower than I would like (37.8K instead of 39K). But they weren't open. I also rechecked the transistors.

When checking for broken traces, I did find one. But I still have the vertical collapse. Back to the hunt. There must be more.

 

[rod90]

Hi Alup. Thanks for the response. When I got the board, one of the legs on one of those transistors was completely broken. So I replaced both of the transistors (with thermal paste) when I did the recap. Yesterday, I did pull and test all the resistors, diodes, and caps in the burnt area of the board and everything checked out fine. Some of the resistors were reading a little lower than I would like (37.8K instead of 39K). But they weren't open. I also rechecked the transistors.

When checking for broken traces, I did find one. But I still have the vertical collapse. Back to the hunt. There must be more.

If that is a broken trace, the next one to it has the same scratch.

 

[Jaydon]

I thought the same thing. But it checked out ok. Strange

 

[rod90]

Did you scrape the mask back?

 

[Jaydon]

No. I didn't. But after looking under magnification, it looks like that jumper was a mistake. I hope I didn't fry anything else.

But here is another head scratcher. Hopefully someone smarter that I can figure this out.

Riddle me this: I have a short between these two pins on the board. When I remove the Horizontal coil, the short on the board goes away. I thought it was faulty coil. But when I test the pins on the coil itself, there is no continuity. Then the short comes back when the coil is reinstalled. How is that possible?

 

[AlexZander]

I have a K4900 chassis sitting here. I just metered those traces out. Those two width coil legs are connected through R381>trace pad 30> and a jumper.

 

[rod90]

Your coil is just a big piece of wire. Inductance is your friend on that, not resistance.

 

[Jaydon]

Thanks guy!!! You all are awesome.

I'm still going to have to wrap my head around why it only shows continuity when the coil is soldered onto the board. But neither the coil doesn't nor the pads show continuity individually. It will come to me eventually.

Back to the hunt.

 

[Jaydon]

Just to give an update. I did find that the 2sd898b transistor on the side of the chassis looks like it's bad. I ordered a new one and will install it when it comes in. But something just didn't seen right. I couldn't see how that would cause a vertical collapse. So I started doing some more research and discovered the testing of the yoke. Why not give that a shot?!?! Well, it turns out that when testing the yellow and green wires on the yoke (for vertical), I have no resistance.



So do I try to replace the yoke? Any advice?