Recapped SE/30 - Occasional Horizontal Screen Shake

[asphaise]

Yesterday I was playing on my recapped SE/30 and noticed the screen has a very minor horizontal shake or wobble. Just a couple of pixels wide. It is sporadic and not constant. I let the machine run for a while and it didn't seem to affect it one way or the other. The main board and analog boards were professionally recapped and the problem is pretty recent. I didn't notice it the last time I had the computer running a week or two ago. Are there any things I can check, as an electronically inept layman? My tools and talent are limited but I'm willing to give it a shot as long as it doesn't involve precision soldering.

Is it possible this isn't an internal issue and is a result of some sort of electrical interference? Nothing in the immediate area of the computer has changed, so that is probably a long shot.

Thanks for your advice.

 

[Garrett]

My bet is you'll need a new HOT (horizontal output transistor). I've replaced ~5 over the years. They fail in the manner you describe. Should still be available from sellers like DigiKey!

 

[MeTheMac]

I would have to be @Garrett is correct.
If that doesn't work, another thing you could look at is the fan. Sometimes they can start to create some interference causing somewhat similar results. They get old and really loud. It's a long shot, but you never know

 

[asphaise]

My bet is you'll need a new HOT (horizontal output transistor). I've replaced ~5 over the years. They fail in the manner you describe. Should still be available from sellers like DigiKey!

Thanks for the tip. I'm doing some searching on the internet to try and figure out what a horizontal output transistor is and which is the right one for the SE/30. I found a schematic for the Mac Classic online. The document identifies the HOT as Q3, which shows Q3 as BU406. Digikey tells me that transistor is obsolete but can be substituted by BU406G. Does any of that sound right?

 

[asphaise]

I would have to be @Garrett is correct.
If that doesn't work, another thing you could look at is the fan. Sometimes they can start to create some interference causing somewhat similar results. They get old and really loud. It's a long shot, but you never know

Thanks. I've actually replaced the fan with the JDW Noctua swap. It's possible I did a bad job. Maybe I should briefly remove it and see if the screen wobbles at all with it gone.

 

[This Does Not Compute]

This is the part I used for BU406 when I built my SMC recreation analog board: https://www.mouser.com/ProductDetail/onsemi/BU406G?qs=vNc2DXHODiKv4UObSEVN0A==

 

[asphaise]

This is the part I used for BU406 when I built my SMC recreation analog board: https://www.mouser.com/ProductDetail/onsemi/BU406G?qs=vNc2DXHODiKv4UObSEVN0A==

Thanks for the information. I'll take the computer apart this weekend and make sure I can find the transistor.

 

[asphaise]

My curiosity got the best of me and I went looking for the HOT. I don't see a Q3 or a BU4906 on the analog board. I guess that means the Mac Classic is different than the SE/30. The only thing I see on the analog board that bears any similarity to a BU4906 is the item I've circled. If I read it correctly, it is a Motorola 7905CT. Reading online, it doesn't seem to function the same as BU4906 at all.

Could someone describe to me where I might find the the HOT in the photo of the business portion of my analog board? I've tried to keep the photo as small as possible. Thanks again for the guidance.

 

[S. Pupp]

BU406 is the vertically mounted transistor bolted to the metal shield.

 

[asphaise]

Well, that's embarrassing. You are 100% correct. Thanks for setting me straight. I'm going to go ahead and order some replacements and hope I can replace it without damaging anything. There are three solder connections and they're the poke-through kind. That and the attachment point to hold the transistor in place will hopefully help me out.

Thanks again.

 

[ScutBoy]

Following, I have a few machines with the same issue.

 

[asphaise]

The replacement BU406 transistor has finally arrived and I've made the swap. The change isn't difficult, provided you have some basic level of soldering skills. If you're like me, you'll struggle for quite a while to get the old transistor to desolder and then build up some ugly blobs around the new transistor. Getting the nut, washer, and spacer back on the inside of the sheet metal bracket required a long pair of needle nose pliers.

In the end replacing the transistor seems to have fixed the issue. I left the machine on for over an hour to make sure it wasn't time or heat dependent and everything works great. Thank you so much @Garrett for correctly identifying the trouble's cause, @This Does Not Compute for the replacement source, and @S. Pupp for helping me find the transistor on the board.

 

[_nec6678]

I have an SE FDHD with a similar problem to what is being described in this thread, except that the shake is a more intense, larger twitch. Also, there seems to be a faint line constantly scanning down the screen. Should I look into this transistor?

 

[JDW]

I have an SE FDHD with a similar problem to what is being described in this thread, except that the shake is a more intense, larger twitch. Also, there seems to be a faint line constantly scanning down the screen. Should I look into this transistor?

There's no harm in buying the BU406G replacement from Mouser and doing the swap. Probably best to order other things like capacitors, etc. to make the cost of shipping more worthwhile. I may buy one of those parts too because I have an analog board that causes screen issues, and until now, I always thought it was the flyback transformer. Who knows... Maybe it is the BU406.

I would also recommend posting a photo of the "faint line" you mention, or better, a link to a short video of it.

 

[_nec6678]

The faint scanning line is practically impossible to see in a video, due to the refresh rate of these CRTs. I don't have quick access to a video camera with an adjustable frame rate, so a picture is the best I can do. You can barely see it, but it is on the axis the arrows are pointing to. (Ignore the brighter lines on the top left and bottom right of the picture - those are from the bad frame rate.) The line does move down to the bottom of the screen, but once it does, it starts somewhat below the middle instead of the top.

Thank you very much!