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

[Tekk]

Hey everybody.

I picked up a SE/30 the other day. When powered on, it wouldn't chime, but I did get some horizontal lines on the monitor. Great, this problem is covered on this awesome website, so it should be pretty easy to fix, right? It wasn't.

I first turned to the analog board, and found an open resistor (none of the ones listed as potential culprits on the website, though). Replaced that. Same issue.

I then ordered a capacitor set (thanks trag!) and recapped the logic board, because I figured that might be the problem, and it would have to be done anyway. Woo hoo, it now chimes! Same video problem though.

I moved down the list of culprits on the site. I pulled transistor Q1 from the video board and tested it using my multimeter's hFE function, which showed it as bad. Replaced it, same problem. I then removed IC U1 from the analog board (a 74LS38 quad 2-input NAND trigger) and tested it on a breadboard. Three out of the four gates worked properly, one was stuck at a high logic level. According to this schematic, the broken gate is driven by the video signal from the logic board. Very promising. So, since there's one unused gate on the chip, I added a couple of wires to reroute the signal to/from the unused gate instead of the broken gate. Same issue!!

Here's a video I made (sad mac chimes are because I removed a SIMM; with all 4 SIMMs installed, it does the happy chime).

I'm now out of ideas. Does anyone see anything I'm missing? :-/

 

[uniserver]

Yeah its probably a rotted via or trace. Personally its issues like this that make me hate the se/30.

 

[genie_mac]

It's a bit hard to see what's happening on the screen in the video; I'm wondering if it is an analogue or digital issue. Any way you can check the logic board with another SE / SE30? I'm sure if you could you would have done that already. I would guess it's an analogue problem as the screen doesn't seem to show a static faulty image like vertical bars etc but more like some kind of sweep issue. Seeing that there were problems on the analogue board may mean that there is more bad news Sorry can be of any help, troubleshooting can be tedious and frustrating alright...

 

[uniserver]

yeah i need a better video buddy. i was thinking you were getting this.

 

[James1095]

Sounds like he's talking about retrace lines. You get these when for any of a number of reasons the electron gun isn't cut off during the horizontal retrace period. It shows up as a bunch of sloped horizontal lines across the screen from top to bottom. You can see the same thing by turning the Screen control up too far.

 

[techknight]

it almost looks as if the electron gun is stuck on. But, if it were, the raster would be insanely bright, and not barely visible. its almost if the brightness is cranked all the way up to see that.

 

[Tekk]

Sorry the video is so bad. I recorded it on my phone to send to a friend via MMS, so I was trying to keep the file size low. I was also holding together my precariously put-together SE/30 with the other hand.

The screen looks like this:

(image swiped from here)

Since I've found 3 bad components in the analog circuitry so far, I'm leaning toward an analog problem. Will keep everyone updated with developments.

Thanks for the help so far.

 

[uniserver]

i had that same issue with me Macintosh SE... i found the issue was with in the video board that connects to the back of the CRT...

do you have another se, that you could swap the video board and see if that makes any progress?

 

[nvdeynde]

What does it give with the logic board disconnected ?

If it's the same then the issue is on the analog or video board, otherwise it's a logic board problem.

 

[Tekk]

What does it give with the logic board disconnected ? If it's the same then the issue is on the analog or video board, otherwise it's a logic board problem.

With the logic board disconnected, the display is completely black. Time to get out the old oscilloscope, I guess.

 

[techknight]

That is to be expected, because the Horiz/Vertical frequency is generated by the logic board. It is not sync pulses, it is the scan-drive pulses. The analog board doesn't have any PLL locked oscillators, so you will get no HV without the horiz drive from the logic board.

 

[James1095]

it almost looks as if the electron gun is stuck on. But, if it were, the raster would be insanely bright, and not barely visible. its almost if the brightness is cranked all the way up to see that.

Not necessarily. If the cathode is stuck low, the gun is "stuck on" in that it isn't blanking during the retrace period, but the brightness can still be controlled by adjusting the G2 voltage, which is what the "Screen" control does.

The cathode emits a cloud of electrons, and the more negative the cathode is, the more electrons it will emit. G1 is the first control grid immediately after the cathode and is held a bit more negative, repelling the electrons back to the cathode. IIRC the Mac ties G1 to ground, and the cathode is modulated between about 25-50V by the video signal. G2 is next in line, also called the first accelerating anode. This is a few hundred volts positive relative to the cathode and serves to yank electrons past G1 where they head toward the screen. The higher the voltage on G2, the more electrons are pulled from the cathode and brighter the image but the faster the cathode wears out.

Less relevant here but useful for those wishing to better understand CRTs. Next electrode is focus, sometimes called G3. This is normally several hundred volts to a kilovolt or so and serves to shape the beam. Lastly is the second anode, this is connected to the big red wire and is typically 8-25kV depending on the size of the screen. This gives the electrons the final big acceleration push toward the screen.

 

[Tekk]

The SE/30 lives!

I got my hands on another video board, swapped them out, and it now works perfectly. Still haven't figured out what was wrong with the original video board; that's for another day.

One interesting side note: capacitor goo had eaten through a tiny surface-mount transistor in the ADB circuit, so I dutifully soldered another transistor (a through-hole one) to the back of the PCB, connecting to the correct vias. The ADB didn't work. After I desoldered my new replacement transistor, it worked. Huh....

I discovered that the original Quantum ProDrive no longer works, so the next step is finding a replacement HDD. In the meantime, here's 6.0.8 running from a floppy:

(notice the sweet retr0brighted case)

Thanks everyone.

 

[uniserver]

your welcome for suggesting that

 

[onlyonemac]

The picture's too wide.

 

[Tekk]

Meh, you're right, I'll fix it next time I crack it open to replace the PRAM battery. :b&w:

 

[uniserver]

Did you swap out the video board per my suggestion?

or was that something you decided to do on your own, Process of elimination type thing?

Also, I need your help. i want to figure out what goes on these video boards, Its not a huge problem, but i feel in the next 5 to 10 years it will be a problem. I think there are some of those Paper Caps on there, and they fail giving you that bad video result...

 

[James1095]

Is this the neck board you're talking about? If so, I would look first at the resistors, and test the cathode drive transistor on there. The circuit on those boards is simple enough that it could be traced out fairly easily. IIRC it's a very simple single transistor amplifier with a resistor pulling the cathode voltage up beyond the cutoff point and a transistor pulling it down towards ground to turn it on. If the pullup resistor opens or goes up in value, or the transistor shorts, the cathode will be stuck on.

 

[Tekk]

Did you swap out the video board per my suggestion? or was that something you decided to do on your own, Process of elimination type thing?

Also, I need your help. i want to figure out what goes on these video boards, Its not a huge problem, but i feel in the next 5 to 10 years it will be a problem. I think there are some of those Paper Caps on there, and they fail giving you that bad video result...

Honestly, I didn't see your suggestion until after I had swapped it out. Oops. The local computer shop gave me a broken SE with good video, and I swapped the video and analog boards in my SE/30 with the boards from that one. It worked, so I put my analog board back in. It still worked, revealing the problem as something on the video board.

Is this the neck board you're talking about? If so, I would look first at the resistors, and test the cathode drive transistor on there. The circuit on those boards is simple enough that it could be traced out fairly easily. IIRC it's a very simple single transistor amplifier with a resistor pulling the cathode voltage up beyond the cutoff point and a transistor pulling it down towards ground to turn it on. If the pullup resistor opens or goes up in value, or the transistor shorts, the cathode will be stuck on.

Yup, the neck board. I replaced the transistor as part of my initial troubleshooting but never tested the resistors. I lent the board to a friend so I'll have to get it back and do some further analysis.