Macintosh LC 475 chimes, but no video

[techgeek]

Hi guys! I need help with a Macintosh LC 475 that I've been trying to revive for quite some time. It had a badly corroded battery which I removed and cleaned up the rust - no traces or anything else on the motherboard has been damaged except for two capacitors which have since been replaced. This is not the first motherboard I have worked on, so I am sure that the remaining capacitors are OK. The power cycling does the trick, but after the LC chimes nothing shows up on my Apple CRT monitor. However, if I connect one of my LCD monitors it detects a signal, but what I find very strange are the H and V frequencies (see the pictures below). The horizontal frequency is 91.7KHz and the vertical one is 113Hz. Does anyone have any idea what could be causing that? I have never seen any such behavior before. Any ideas would be greatly appreciated.

View attachment IMG_20200324_173138204.jpg

 

[techgeek]

Comparing my mobo to pictures posted on here, it seems that mine is missing a diode??? In the picture it is labeled as D1 (immediately below the battery), and it has 2 color bands. Does anyone have any idea if this is really a diode or perhaps something else?

 

[Juror22]

It looks like you are missing C194, C100 and D1 from the picture that you posted - it still looks like there is a little more repair/cleaning needed, before I would start hooking it up and testing it.  I'm sure that someone will post what D1 is (I'm pretty sure its not a resistor).

 

[bibilit]

The Lc 475 won’t display anything without a good battery, so if the battery has been removed...

 

[techgeek]

It looks like you are missing C194, C100 and D1 from the picture that you posted - it still looks like there is a little more repair/cleaning needed, before I would start hooking it up and testing it.  I'm sure that someone will post what D1 is (I'm pretty sure its not a resistor).

I feel like an idiot for not noticing the absence of the other components. The soldering pads are gone and now I have to think how to solder these through holes.

 

[Cory5412]

Aside from other missing components, a way to work-around not having a PRAM battery installed, or having a bad one in place, is to rapidly turn the machine on, off, and back on.

It's "not well researched" what impact this has on the power supply so my personal recommendation if you do this is to leave the machine running to the extent practical, unless you plan on not using it for a long time. (Or: install a battery.)

 

[glay78]

The Lc 475 won’t display anything without a good battery, so if the battery has been removed...

Hi can I also check is that the same for LC575? I’ve got a board recapped that chimes but no display too.

 

[MrFahrenheit]

Hi can I also check is that the same for LC575? I’ve got a board recapped that chimes but no display too.

Generally speaking, an LC that chimes but has no video often means it needs a new PRAM battery to initialize the video. Often, if there's a red pushbutton on the logic board, it means it might require a battery to display. You could very quickly turn on, wait a second, turn off for about 1/4 second and back on again. If that works, it's likely needing a battery.

 

[glay78]

Generally speaking, an LC that chimes but has no video often means it needs a new PRAM battery to initialize the video. Often, if there's a red pushbutton on the logic board, it means it might require a battery to display. You could very quickly turn on, wait a second, turn off for about 1/4 second and back on again. If that works, it's likely needing a battery.

Ok I’ll try and yea I think I have the red button on my LC575

 

[MrFahrenheit]

Ok I’ll try and yea I think I have the red button on my LC575

It just occurred to me that the 575 is soft power on only. So you likely can’t properly do the on/off/on trick. I would suggest resetting PRAM 10 times from power up to see if that clears the issue. Also try a new battery.

 

[glay78]

It just occurred to me that the 575 is soft power on only. So you likely can’t properly do the on/off/on trick. I would suggest resetting PRAM 10 times from power up to see if that clears the issue. Also try a new battery.

Tried and doesn’t work. It should be some broken trace somewhere. Unlikely is the battery or PRAM fault

 

[pedroskw]

Hi, I have a similar problem, lC475 with startup chimes but no video, the floppy doesn't start and the ADB port seems not responding (pram reset not working). Where can I find the schematics for this model or what do you advise me to check?? thank you

 

[MacKilRoy]

Hi, I have a similar problem, lC475 with startup chimes but no video, the floppy doesn't start and the ADB port seems not responding (pram reset not working). Where can I find the schematics for this model or what do you advise me to check?? thank you

Has the logic board been recapped yet ?

 

[pedroskw]

Has the logic board been recapped yet ?

obviously

 

[MacKilRoy]

obviously

I’m sorry, but it’s not obvious from your paragraph that you wrote.

It’s a valid question considering many believe cleaning with IPA and a toothbrush amounts to significant maintenance and prevention.

I was merely trying to glean details from your problem machine in an attempt to assist you in diagnosing your issue.

 

[pedroskw]

I’m sorry, but it’s not obvious from your paragraph that you wrote.

It’s a valid question considering many believe cleaning with IPA and a toothbrush amounts to significant maintenance and prevention.

I was merely trying to glean details from your problem machine in an attempt to assist you in diagnosing your issue.

any suggestions?

 

[joshc]

any suggestions?

Bad Egret chip is possible if there are no signs of startup at all. Hard to diagnose without a scope other than just swapping a different Egret chip in.

Your 3.6v battery is definitely good? PSU also recapped and giving good voltages?

 

[pedroskw]

Bad Egret chip is possible if there are no signs of startup at all. Hard to diagnose without a scope other than just swapping a different Egret chip in.

Your 3.6v battery is definitely good? PSU also recapped and giving good voltages?

I read that the 475 doesn't show video without vram simm installed, that could be the problem, I'll do other tests when I have the vram.
However, it does not initialize the floppy either.

 

[pedroskw]

nothing, even with the vram the mac doesn't start, i tested simms and CPU from another working LC. No video out and no read from devices.
where and what could I try to check with the oscilloscope? thanks

 

[alectrona2988]

It could potentially be bad traces. Sometimes the leaky caps like to eat away at those.