Macintosh SE unusual screen pattern

[Alister]

So I acquired my Macintosh SE back in early 2017, when it was fully functional and usable. Since then there have been numerous problems with the machine, the hard drive failed first and it took until recently to replace it with a 50 pin Quantum LPS drive, the floppy drive failed after shortly after the hard drive and was replaced promptly and then the screen began displaying an unusual pattern out of the blue. The images, albeit of relatively poor quality due to my lack of a good digital camera- I am a strong believer in photographic film, attached show the patterns. The startup bong occurs and the computer does boot, but the screen is very bright and it is hard to see the desktop. One can faintly see the boot disk icon in the centre of the screen, this can only be seen when the brightness is turned down. In an attempt to solve this I turned to the 'Dead Mac Scrolls', this provided several paths for repair. The guide first suggested replacing the SN74LS38N chip, page 168. Looking back I this was probably a waste of time, being that turning down the screen brightness reveals how the screen has both video and raster. This didn't work and so my next step was to adjust the cut off on the side of the analogue board, as suggested on page 167. This also didn't work and thus I elected to replace the 2N3904 transistor on the CRT board, suggested on page 169. This also had no effect. The most recent attempt, as suggested on page 166, was to replace the R20 and R21 resistors on the analogue board. This too has resulted in failure, and I have run out of ideas as to what could be wrong. Looking through some other threads, it appears some others have had success applying the aforementioned methods but not with me. Could anyone hypothesise what is wrong and what parts should be replaced to repair it? Being a student in a remote part of England precludes swapping parts with other 68K Macs and importing a complete replacement of the analogue board due to remoteness and excessive cost. I simply must try and replace parts until it works, I don't have a community to swap boards with in my local area!

Thank you!

 

[maceffects]

These re-scan lines could indicate a few things.  I notice it looks like the CRT itself is out of adjustment (too wide), the first thing I would start with next would be (if you haven't already) replace capacitors (on the analog board) as those can/do cause strange things that are hard to diagnose. 

 

[Alister]

That seems to be what could be causing it. In due course I'll replace them, I tried reflowing the solder in an attempt to correct the problem but that yielded no result. I might have another look at the analogue board too, looking at another post on these forums it appears this problem also affects Mac Pluses too. Thanks for the advice.

 

[BadGoldEagle]

I remember there being a part on the CRT neck board that caused this when it failed. Don't remember which one though...

 

[LaPorta]

There's a whole section in the Dead Mac Scrolls on this. Not that I remember what the issue may be off my head...

 

[Alister]

I replaced the 2N3904 transistor on the neck board, as per the Dead Mac Scrolls. Looking at a screenshot I believe I took from ccadams website a while ago, the r25 resistor might be at fault so I'll replace it. The same problem seemed to occur on the Macintosh Plus too, often caused by the R5 resistor breaking down. The section in the Dead Mac Scrolls might have been pages c.160 onwards, the SE and SE/30 section. I found that and tried everything it recommended there- I might have a look through again and see if I missed something or if there are any more details in the Mac Plus section.

 

[LaPorta]

Could the actual cutoff trim pot be bad?

 

[Alister]

That's an interesting hypothesis, it does still adjust but the possibility that it could have failed is certainly one worth investigating if all of the resistors turn out to be good. I can't imagine where to source one of those nowadays, perhaps I could substitute a modern replacement variable pot for it?

 

[Alister]

Ah, unfortunately, replacing the r25 resistor- which I believed to be the same as the r5 resistor in the Mac plus- has achieved nothing. I might need to source a new analogue board to replace it with. The flyback transformer certainly needs replacing, rust seems to have accumulated at the bottom of it and the anode cap has seen better days. I doubt that a new old stock replacement could be sources so this board probably needs to go. Flickering on the screen warned me about the flyback transformer a few weeks ago and it seems prudent to replace it before it malfunctions- which Larry Pina suggests could damage other parts of the computer.

 

[tt]

This screen pattern looks familiar, but I don't recall doing anything other maybe fixing a loose connection for one of the wiring harnesses. I guess I have been fairly lucky in not needing to do elaborate fixes other than replacing capacitors. I would carefully try to jiggle wire bundles near the connectors (be safe) and tapping the sides of the case to see if it changes the screen pattern. Did you check the voltages for the power supplies?