Quadra 700 good chime, no video .. thinking on RAMDAC (357S0010-A)?

[naruse]

Hey Everyone ,

I am working on a Quadra 700 that was battery bombed -- I managed to fix it so it chimes (good chime) but still I cannot get any video out from it. I already checked several similar posts (https://68kmla.org/bb/index.php?threads/quadra-700with-no-video-output.46895/) and went ahead and checked all connections against the schematics and everything seems fine.

I checked most of the internal connections on the DAFBII, vram, RAMDAC, etc and everything checks out.

I went ahead and checked all the clocks and found that the VIDCLK that should come out from the RAMDAC (357S0010-A) and goes in into the DAFBII is basically dead.

From the DAFBII_ACDC doc:


ACDC Video CLUT/DAC Schematics (357S0010-A) --> VIDCLK is an output here and is dead.


Schematics of the DAFBII:


I am assuming the ramdac is dead as when I turn on the machine it gets kind of warm and as mentioned above there is no VIDCLK sitnal.. YET, I tried out an external video card attached to the NUBUS and still I dont get any video ...

Does anyone know if the RAMDAC is necessary for the video overall to work? or with an external card it should just work.

Also, does anyone also has a technical document for the RAMDAC similar to the DAFBII?

I checked connections 5V, gnd, etc and everything is connected where it should, any other thoughts ?

 

[Arbee]

Because a NuBus video card doesn't work either, I think it's likely the RAMDAC is fine, but the system isn't doing video initialization and so the pixel clock isn't running (it comes from a software-programmable PLL chip). The RAMDAC on board has nothing to do with the operation of NuBus video cards.

 

[naruse]

interesting, thanks for the insight! -- do you happen to know where would this video initialization happen / any suggestions where should I look into for the video initialization?
BTW -- I dont have any battery attached to the machine (but I dont think this matters in the quadra 700)

 

[naruse]

Alright, I went ahead and checked on PIXCLK -- According to the DAFBII the PIXCLK input should be 57mhz -- yet I only get 35Mhz as shown in the diagram -- I am not too familiar with measuring clocks but could it be that U30 is bad?

I went ahead and checked the resistors for the crystal (Y3) and both are correct 4.7k (R156) and 511ohm (R155).

Turns out that U30 is a DP8531 which is a clock generator.

 

[Arbee]

Video init happens relatively late. After the chime it does the RAM test, and then it hits PrimaryInit where it starts up all the video devices, and then ADB init to scan for keyboards and mice. I'm actually not sure what could hang up the system between the initial chime and video startup without causing a death chime, but of course all bets are off with a battery bombed board.

The PIXCLK depends on which video mode you have selected, there's not one good value for it.

 

[dougg3]

Last week I fixed an LC III that had this same symptom. Chimed and then nothing, no death chime and no video.

The culprit ended up being one of the data lines on the onboard RAM. The ROM was fully connected to the CPU, but the RAM had one broken trace. I think it was somehow hanging up during the RAM test before it had a chance to play the death chime.

 

[naruse]

The PIXCLK depends on which video mode you have selected, there's not one good value for it.

ha!, alright alright -- I was checking and rechecking and rechecking the video clock generator circuit! -- I almost memorized it, exchanged resistors allover the place and everything checks out!

@dougg3 interesting, thanks for chipping in, I will be checking all the on board ram data lines on all the connections and will be reporting back.

Thanks everyone!

 

[naruse]

hey everyone, (happy new year!)

alright I'm back -- I went ahead and checked all the on board ram lines on _all_ the connections on all the pages (3, 4, 6, 8, 10 ,20) are correct.

I did noticed that on the PDS socket there was one missing connection -- I fixed it but still I get the same issue, no video.

I did notice that there is some resistance on the connections ~0.1 - 7/8ohm (but I dont think this matters).

Any other suggestions on what to look into? I'm quite puzzled and not sure where to check on next