LC 475 Chimes But No Display, Floppy, HDD Activity

[DannyDorito]

Moving this from https://68kmla.org/bb/index.php?threads/wtb-lc-475-logic-board.46276/ to here. Originally wanted to just buy a new one but was given hope that this one is fixable. See trading post thread for more details, but in short I have a 475 with no activity beyond the chime. Board has been recapped, main chips reflowed, cleaned many times with alcohol and distilled water, and a new PRAM battery. All voltages are good off the PSU, and power appears to get to all parts of the board. I'm waiting on some eBay VRAM, as I can't verify that's the issue, but I'm somewhat doubtful. Got the oscilloscope out to mess around, but without knowing what to look for beyond clock signals it's not super helpful

I need to get decent board images, I'll add those too when I can.

 

[Phipli]

Moving this from https://68kmla.org/bb/index.php?threads/wtb-lc-475-logic-board.46276/ to here. Originally wanted to just buy a new one but was given hope that this one is fixable. See trading post thread for more details, but in short I have a 475 with no activity beyond the chime. Board has been recapped, main chips reflowed, cleaned many times with alcohol and distilled water, and a new PRAM battery. All voltages are good off the PSU, and power appears to get to all parts of the board. I'm waiting on some eBay VRAM, as I can't verify that's the issue, but I'm somewhat doubtful. Got the oscilloscope out to mess around, but without knowing what to look for beyond clock signals it's not super helpful

I need to get decent board images, I'll add those too when I can.

Old oscilloscopes tend to struggle with clocks in the tens of MHz, so you might find a cheap frequency counter helpful.

On the 475, if I remember, all clocks are derived from a single one, so if the computer chimes, then the clock works.

I'd use the schematic to work backwards from the video port to consider what is needed.

If you plug a VGA adapter in (this tells the computer a monitor is connected, which makes sure it is outputting), then put your scope on the VGA output pins - do you have a sync signal at all? Do you have colour signals?

Are the ID sense wires getting to the memory controller chip? Is the RAMDAC getting the sync, blank, LD and clock signals?

You can also work forward. How far is the boot process going? Does it read a floppy if you put one in the drive? Even a not bootable floppy should be read and ejected if just the video isn't working.

If it doesn't read a floppy, chances are the computer isn't actuall booting far enough for video rather than the video not working. Your problem is likely somewhere else if that is the case - passing basic ROM tests, but failing shortly afterwards.

Have you tried another CPU? Is the board overclocked?

 

[DannyDorito]

This scope claims to be 40 MHz, but I may try to pick up a frequency counter. I don't have a VGA Adapter at the moment, I may have to pick one up. I've been using the Apple monitor I have to test. I'm pretty sure it works, it did a few months ago as I recall.

Are the ID sense wires getting to the memory controller chip? Is the RAMDAC getting the sync, blank, LD and clock signals?

That went over my head a bit

I'll check again if it reads a floppy but I don't believe it does.

You're probably right that it's failing later tests as you say, since no reads from the hard drive happen, it just does its own basic initialization on getting power. I don't have another '040 to try at the moment, I know it's not overclocked though.

Somewhat on that point, the board is completely stock, it came from a university surplus sale, I got quite a few mostly working and some nonworking Macs out of it. Most of them I've gotten fully working through recapping, but I've ended up with an SE/30 with ADB issues, this LC, a battery bombed IIci (beyond repair), and a 512k that doesn't play nice with the mouse (and no holy grail keyboard). Fully working Classic (with an 030 accelerator!), 20SC, and DB-15 monitor though. All in all not the best haul ever it would seem

Anyway, assuming the case that it's not getting past the initial POST, where do I go from there?

 

[DannyDorito]

Board images if it helps somehow. I've got no idea where to go from here other than VRAM which I'm still waiting on. :/

 

[zefrenchtoon]

hi !
I will try to send you pictures of mine. It did need of many trace repair because of caps liquid.
Did you try removing ram stick ?
Did you change the battery ? Is there any battery in the holder ?
Without battery, you have to double start the computer to enable video circuit. Without one, video does not initialize at cold boot

 

[DannyDorito]

I think I've tried about everything that doesn't require sourcing parts or board-level repair; both of which I'm willing to try. Phipli walked me through a ton of troubleshooting in my other thread.

I don't have a ram stick in mine, just vram, so nothing to remove
Battery has been changed, I've tried many.

 

[zefrenchtoon]

Here the main repair that has to be done. Maybe there is another one on the other side of the board.

 

[joshc]

Board images if it helps somehow. I've got no idea where to go from here other than VRAM which I'm still waiting on. :/

Can you convert these to normal sized JPG images? They’re over 100MB each

The legs of the Egret don’t look too healthy (several are dull) and the solder joints in your caps look dull or blobby in places.

Do you own any flux and do you have a hot air gun or any experience using it?

 

[DannyDorito]

Sorry about the images, took them in raw on my phone and didn’t convert them down at all, just to png.

I do have a hot air gun and flux, although I have to say I’m not particularly skilled with it. I did all of the caps and reflowing with a soldering iron and flux. Too worried about melting the plastics or blowing a tiny resistor off the board.

 

[zefrenchtoon]

Sorry about the images, took them in raw on my phone and didn’t convert them down at all, just to png.

I do have a hot air gun and flux, although I have to say I’m not particularly skilled with it. I did all of the caps and reflowing with a soldering iron and flux. Too worried about melting the plastics or blowing a tiny resistor off the board.

Did you add the ROM slot or was it "factory" soldered ?
I would be curious about a dump of its ROM

 

[Bolle]

Did you add the ROM slot or was it "factory" soldered ?
I would be curious about a dump of its ROM

Early boards had them installed at the factory. The onboard ROMs on those boards are identical to later boards.

 

[DannyDorito]

Did you add the ROM slot or was it "factory" soldered ?
I would be curious about a dump of its ROM

Came from factory. I always wondered why none of the boards I saw online had it. Were there even rom SIMMs made for the 475?

 

[zefrenchtoon]

Came from factory. I always wondered why none of the boards I saw online had it. Were there even rom SIMMs made for the 475?

I don't know but as you can see in this thread, it can be used to program ROM sticks.

 

[zefrenchtoon]

Came from factory. I always wondered why none of the boards I saw online had it. Were there even rom SIMMs made for the 475?

Sorry, the thread I had in mind was this one.

 

[DannyDorito]

Sorry, the thread I had in mind was this one.

Interesting! Wonder if the ROMs are to blame, that would be an excuse to buy one of the cool reprogrammable SIMMs

Can you convert these to normal sized JPG images? They’re over 100MB each

The legs of the Egret don’t look too healthy (several are dull) and the solder joints in your caps look dull or blobby in places.

Do you own any flux and do you have a hot air gun or any experience using it?

I reflowed the egret again to no avail (maybe a better cleaning is in order, flux residue is annoying), but I couldn't get the caps to look much better, although I'm pretty sure they're making contact as the meter seems to say that they all have power.

 

[Phipli]

Interesting! Wonder if the ROMs are to blame, that would be an excuse to buy one of the cool reprogrammable SIMMs

You normally get nothing, corruption or a sad chime if the ROM or ROM traces are bad. The ROM has a checksum so if address or data lines are bad, the checksum ... Erm... Check, fails.

 

[DannyDorito]

That's what I assumed