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LC apparently booting OK but with grey screen

ironborn65

Well-known member
Dear all,

I was able to fix a couple of 68k Macs (LC III, IISI), I was on my way for a LC. The board was not in bad conditions, a couple of capacitors had sign of early leaking.

I replaced all of them but with some differences respect the original spec

- all the 47uf 16v were replaced with tantalum 47uf 20v

- the 100uf 6.3v were replaced with a 100uf 15v non SMD electrolyte

- all the 10uf 16v were replaced with tantalum 10uf 20v

- 1uf 50v with 1uf 50v non SMD electrolyte

I did the same kind of placement with my LC III and it was just fine, increasing the voltage shall not be an issue, on the contrary AFAIK

This is what I got at boot:

- happy chime

- yellow LED blink on the SCSI2SD

- the screen becomes uniformly grey, a quick off-on and grey again

- the SCSI2SD blinks irregularly as it does for any regular boot

- the LED stop blinking with a System typical error beep sound, as it does when an extension is wrong, and this shall be the case here

- the screen remains grey

Apparently it boots just fine, there is an issue with the video.

The configuration is the following:

- the PSU provides a stable voltage

- brand new battery

- The motherboard has no additional VRAM

- I tried with additional RAM SIMM (same issue)

- No other cards are present

- The monitor is working, so do the cable, no adapted is used, but just in case I tried with another cable and monitor (same issue)

See the picture of the motherboard (some capacitors are not perfectly inlined but the connections are fine).

I see the following options:

- a faulty VRAM (I do not have an oscilloscope)

- the capacitor not being 100% of the right spec

- something I do not know

Can someone provide directions, ideas, help in general?

thanks you all for reading all this :)

I wish you all a better 2020 (it should be easy)

PF

IMG_20201228_000530.jpg

 
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cheesestraws

Well-known member
- a faulty VRAM (I do not have an oscilloscope)


Is this a whole machine you picked up or just the board?  At least my LC IIs won't work properly without a VRAM SIMM in them, I don't know if this holds for the original LC as well.  So I'd suggest putting a VRAM SIMM in it, that would be my first port of call.

 

ironborn65

Well-known member
H, thanks for that.

I thought about it, in the tech spec in low end macs, it says the VRAM is optional ... in this case the 4 RAM SIMMs on board must share the memory.

I can invest in a VRAM though, it could be handy for other macs in case. :)

Oh yes, it came with the entire pizza box

PF

 

cheesestraws

Well-known member
In which case, I may be wrong.  Would probably still be my first troubleshooting step, anyway, but I offer that for what it is worth, with no great degree of certainty :) .

 

bibilit

Well-known member
tech spec in low end macs, it says the VRAM is optional


Not on the Original LC, the Vram stick is mandatory to get a picture, IIRC optional for sure on the LC III, don't remember about the LC II

 

bibilit

Well-known member
Original Apple 512KB VRAM module for the Macintosh LC range.
This is the maximum video memory on the LC and LC II (there is no build in VRAM) and offers support for display resolutions of 512x384 at 16-bit colour and 640x480 at 8-bit colour. As standard the LC and LCII came with a 256KB VRAM module installed which offered display resolutions of 512x384 with 8-bit colour or 640x480 with 4-bit colour.

The Macintosh LC III (and Performa equivalents) has 512KB of VRAM built onto the logic board plus a single VRAM slot but supports a maximum of 768KB VRAM and so will not recognise this module. The smaller 256KB module should be added to upgrade age VRAM.

The Macintosh LC 475 (and Performa equivalents) has two VRAM slots and no built-in VRAM. It came as standard with two x 256KB modules offering support for display resolutions of 512x384 at 16-bit colour, 640x480 and 832x642 at 8-bit colour and 1024x768 and 1152x870 at 4-bit colour. The VRAM could be upgraded to 1MB by replacing the two 256K modules with two of these 512KB modules to offer display resolutions of 832x624 at 16-bit colour and 1024x768 and 1152x870 at 8-bit colour

 

cheesestraws

Well-known member
Not on the Original LC, the Vram stick is mandatory to get a picture, IIRC optional for sure on the LC III, don't remember about the LC II


It's definitely mandatory on the LC II as well, because I keep forgetting it's mandatory and freaking out when freshly-recapped boards don't boot before I remember.

Sigh.

 

MrFahrenheit

Well-known member
I can tell you I had issues using some “out of spec” tantalum caps on various Macs. I have 47uf 20 or 25v caps but they’re not the same ohm rating as the 47uf 16v ones. When I tried them in a Quadra 610, it failed to boot. Turning it on I got no mouse movement, and flashing question mark that was 1/5 the speed of normal (ie it was very much in slow motion), and no SCSI boot. I desoldered them and replaced with my standard caps and the machine booted just fine. 
 

I tested the caps on an LC and was greeted with similar effects. But additionally a grey screen. 
 

I do see you mention booting without VRAM, which is necessary to boot these. As is a new fresh PRAM battery for reliable video on boot. If after you locate a VRAM simm it does not boot check your caps. Tantalum caps are good replacements but they have different specifications than the factory electrolytic ones. There’s a kHz wobble of frequency and a resistance rating, neither of which I’ve seen on other types of caps. It seems installing caps that are too far out of range on these specs cause issues in the Macs. 
 

At least that’s my experience.  I’ve also run into issues others swear they’ve never seen and can’t be possible. 

 

ironborn65

Well-known member
I was mislead by the following statement in "low end macs"

Apple introduced a new color video standard (512 x 384 pixel) and a cheap 12″ color monitor to match it. With a VRAM upgrade, the LC supports 16-bit video (65,536 colors) on the 12″ monitor or 8-bit video (256 colors) on a standard 640 x 480 screen. .

I read "VRAM upgrade" as required only for getting 16 bits

VRAM SIMM in it's way

thank you all

best

PF

 
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