Jump to content
Sign in to follow this  
techgeek

Macintosh LC 475 chimes, but no video

Recommended Posts

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.

IMG_20200324_173138204.jpg

IMG_20200324_173127411-01.jpeg

IMG_20200324_173122566-01.jpeg

Share this post


Link to post
Share on other sites

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?

Missing.png

Edited by techgeek
Figured out tha it must be a diode not a resistor.

Share this post


Link to post
Share on other sites

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).

Share this post


Link to post
Share on other sites
On 3/24/2020 at 7:57 PM, Juror22 said:

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.

Share this post


Link to post
Share on other sites

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.)

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×