SE/30 horizontal sweep lines....still!

[SEthirty]

Just checked the outputs of the muxes, and... it looks like they work fine. I probed the mux output pins (7 and 9 of the 74F253's) on UC7 pins 9, 10, 11, 13, 14, 15, 16 and 17, but they all seemed to display serial data. Let's look further from there.

 

[techknight]

So you can rule everything out from there on back.. now you need to look forward.

You can start by cutting the video signal wire going from the analog board to the main logicboard. This will tell you where to start looking. Im thinking shorted capacitor somewhere, as you have no signal but bad dc-offset biasing your tube on...

 

[uniserver]

And you already tried swapping the video board with a known working one?

 

[SEthirty]

Made some progress: there is no input serial video signal on pin 9 of the UG6, which should have come from pin 13 of the 74LS166... need to do a continuity test, but otherwise I think that 74LS166 is our culprit!

 

[SEthirty]

Continuity is fine, but I get no output directly on pin 13 of the 74LS166, so I think I'll order a new one, and hope the Mac will work again! :beige:

 

[techknight]

Pull the datasheet of that ic and make sure all your truth table conditions are met first...

 

[uniserver]

because i get that same screen , when the video board goes bad with the se.

 

[SEthirty]

because i get that same screen , when the video board goes bad with the se.

I assume you mean the neck board; I already checked every single component on that one, even replaced the transistor, but that didn't help. I also haven't got another one, so it is quite difficult to swap them. ;-)

 

[SEthirty]

Pull the datasheet of that ic and make sure all your truth table conditions are met first...

Found a nice datasheet here: http://pdf.datasheetcatalog.com/datasheets/270/334520_DS.pdf

On the first page you can see the function table; I could see parallel data on pin 4 and 5 with the scope, clock is running and power is OK.

Pin 1 (serial) is tied to pin 9 (clear) and constantly held high.

Pin 15 is high, but "pulsed" low at a rate of approx. 2MHz.

...But I can't see on myself if this is all good or not.

 

[SEthirty]

The journey continues... I've just replaced the 74LS166A with a 74HC166A, and it seems to work again! ...But only partially.

We finally have video on the monitor again, but is is overlayed with a striped "jail bar" pattern:

I've checked for continuity on all the connections the 74HC166A has to make and for shorts, but everything measures fine. What could this be?

 

[techknight]

There is either bad VRAM, or one of your muxes that loads up the VRAM is got a broken trace. your VRAM is missing information. So either the data coming from it, or going to it, is missing databits.

Reason? Bad VRAM, or more likely, bad trace.

 

[SEthirty]

After an our of probing I found that UK6, the video ROM, was missing a connection on pin 10 with UD8 on pin 4. I made a direct connection with a piece of thin wire, and... wait for it... it finally works!

Here you can see it is displaying the "you did not properly turn the computer off last time" message in Dutch:

I'm glad I finally fixed it!

Thank you very much techknight for pointing me in the right direction! You deserve a pat on the back. :I

 

[uniserver]

hey man good find.. so pin 10 on the Vrom to pin 4 on the UD8, man your cap goo must have been extra bad!

so you think this and the I/C you replaced and this bridge wire from vrom was the fix here?

 

[SEthirty]

hey man good find.. so pin 10 on the Vrom to pin 4 on the UD8, man your cap goo must have been extra bad!
so you think this and the I/C you replaced and this bridge wire from vrom was the fix here?

Yeah, aside from a lot of probing and measuring those two things finally did it.

The cap goo was indeed very hungry! :-x

 

[uniserver]

pretty nice man!

 

[techknight]

Well i knew something was up soon as I seen that scope shot.

Glad you got it working.

 

[genie_mac]

Congrats! Another one saved

 

[uniserver]

i just fixed SE/30 just now with a dead 74LS166

the cap goo seems can short them out… and they go bad.

 

[SEthirty]

i just fixed SE/30 just now with a dead 74LS166
the cap goo seems can short them out… and they go bad.

That's great to hear!

 

[uniserver]

i need to add "10 on the Vrom to pin 4 on the UD8" another variant of SE/30 screen diarrhea. shot gun fixes are cool!