Mac SE CRT smearing to the right problem

[ivannudem]

Hi everybody,

I have a Mac SE which has a troublesome CRT, the symptom is that there are bright streaking/smearings/bleedings to the right of long black lines or large black area as shown in the following photos.

It also have some slight black shadows/smearning to the right of a think vertical line such as the About Mac Finder window (as shown in the 2nd photo), the brighter I tune the display (by the tuning pot under the CRT, not the one on analog board), the more severe it is.

It's very like the situation mentioned in this post https://68kmla.org/forums/topic/60642-se30-crt-ghosting-bleeding-to-the-right/ [1] but not that severe.

I also noticed this smearing phenomen on other people's mac such as this one https://jonthysell.files.wordpress.com/2021/03/2020_09_18_19_37_41.jpg, to the right of the window, there are some black streaking shadows

I recapped the analog board last weekend (all electrolytic caps including the 3.9uf non polar one, I replaced it with the film cap recommended in this forum), but no improvement.

I swapped a known-good video driver board / neck board (which connects to the crt neck), but the symptoms were still present.

i consulted some people on vcfed, they said that this is mostly due to video driver amplifier's frequency response issue, but i found that the black&white video signal goes from the mother board directly to the video/neck board, and I swapped a good neck board it didn't solve the problem.

they said "

In the case of the high frequency end being limited, the image becomes smeared and after what should be a sharp edge, there is a blur or streak trailing after it from the left to the right. Primarily loss of HF response results in an equivalent of de-focus and loss of high frequency detail in the image.

But, one interesting this is, what happens if the low frequency response of the video amplifier is corrupted ? This is a less discussed problem. It results in the defect you are seeing with the "smearing" over broad sections of the video signal, after a significant video voltage level transition.

So how does the low frequency phase distortion come about ?

Invariably in most video amplifiers there is R-C coupling between the stages. As the capacitors age and become leaky, in this case it can result in excessive low frequency response causing the broad areas of image smearing.

So short smearing is loss of video HF response, long smearing to call it that is low video frequency phase distortion, typically due to faulty coupling capacitors in the video amplifier circuitry.
"

As the analog board schematic indicated (in attachment), the AB is maily responsible to provide HV, and Horizontal, vertical deflection signals, could that part go wrong, but I have recapped all electrolytic caps on the AB, and there doesn't seem to have video signal (the 1bit black and white signal) amplification on the AB, the amplification of video signal is done on the video neck board, which I have swapped with a known good one but didn't solve the problem.

Is this possible that the issue is caused by the deflection circuitry on the AB?

I don't have a scope, only a multimeter, could someone help me point out possible area which might cause this issue?

Thanks

 

[MacTech68]

After reading much of the material you've posted links to, and finding your thread on vcfed, I'm a little stumped.

If you've swapped to a known working neck board, then the issue is either the Analog Board, PSU, Logic Board, or CRT. Accordingly, this rules out a poor 2N3904 video amp and associated components on the Neck Board.

If you can, a process of elimination is probably your only option (sans suitable Oscilloscope and training/experience).

Try swapping the logic board first, followed by the Analog Board (Including PSU). You will likely need to make at least one adjustment; Screen voltage - the method for which can be found on page 120 here:

https://tim.id.au/laptops/apple/legacy/macintosh_se.pdf

NOTE 1 - Screen voltage adjustment it is referred to "brightness" in these docs

NOTE 2 - remember to set the external "contrast" control BEFORE making the internal "brightness" adjustment.

IF, by the end, you've swapped all but the CRT itself, and the problem persists, then I think you can conclude the problem is the CRT.

Also note that the 30V supply rail for the video amp is derived directly from the Flyback - could that mean the Flyback and/or associated horizontal circuits might cause this? I'm at a loss to say.

Finally, if you would like a circuit to the SE Neck Board, see page 304 here:

https://vintageapple.org/macbooks/pdf/Macintosh_Repair_&_Upgrade_Secrets_1990.pdf

NOTE 3 - there are two physically different neck boards for the SE. Brought about by the original barrel fan requiring replacement with a standard box fan, making the original Neck Board too wide to safely fit. Replacement boards (same circuit AFAIK) were taller than they were wide, leaving clearance for the larger fan.

Good Luck !

 

[ivannudem]

Hi MacTech 68, thanks a lot, you're probably right, yesterday I searched some online forum they said that if the 30V supply to the video amp is bad it could also cause this kind of smearing... I already swapped a spare logic board (I bought from local craigslist as spare part, the seller claimed to be good) but the problem is still there.
I will try to swap the AB and then CRT if necessary.

Thanks for your advice.
Ivan