Apple III video issues

A good thing to do is to check the ram chips to make sure these are working. If its a 5v ram board (4164's), then maybe try them in a IIe to verify they are working ok.

/Rob

ahh, ok. That makes it a little harder. Can you share a photo of your memory board?

It looks like the first row can be either 32k or 16k drams and I think also the second row can be either 32k drams with the 3rd row empty, or second and third rows can be 16k drams. So maybe you can remove the 32k rams and replace with 16k rams from a known good source for testing (AppleII+). You just need to be careful to insert the 4116's in the correct position in the socket as they have two less pins than the 32k rams. Have a google for 'apple iii 12v memory board' and have a look at the images to see this.

3 rows of 16k drams will yield a 96k Apple///. Not sure you can do much with that, but it should at least run the monitor rom ok.

Hi there, 

I have a 12v memory board apple III and I have a similar problem with the video output. Ive been chasing this problem for about 3 years and have not been able to solve. My III works fine as far as it boots and is pretty stable, the problem is the character set is garbled. The screen fills with a distorted lower case g and text is messed up. 2 weeks ago I got an oscilliscope and went around trying to track down the problem. I've sunk an estimated 50 hours into it now and still cannot figure it out. 

I can verify that, at least, the first bank and possibly the second bank of memory on the 12V board is functional as the system boots. I have a donor board with broken bus bars and a good memory board. Both memory boards result in the same video output and swapping memory chips doesnt make a difference.

Ive followed all the steps mentioned for the original poster, including replacing B10, B11, B12, B13, and replacing all the PROMs from the donor board, but there is no change. I went through the service manual and looked through the logic flow for the 40 column text mode (though all video is garbled), and specifically checked continuity on the A bus (DA0 to DA7) and where they connect to on E2 and d2 and D3. I have also swapped out both SRAMs in E4 and E5. Doing some more troubleshooting, I found that E5 was not getting power and bodged a wire from pin 18 on e4 to pin 18 on E5. Doing so didnt fix the video but did resolve the board being unstable due to sensitivity to touch. 

Ive checked continuity on most of the timing circuits as well, including C1m C1m* and PRE1M as well as the AX line. One thing I see is that the clock for C1m, C1M* Pre1m and AX all have a slight jitter to them, according to my scope. I'm not sure if this is normal. I thought that it may not be normal and may be causing timing issues with the flip flops loading the character srams, so I swapped out D11 (74s74), with another S74s74, and the jitter remains and the problem is there. I then swapped out D11 with a 74LS74, which makes the jitter go away, but then the video COMPLETELY stops working (just starts flashing garbage) and the system goes unstable. I figured the PROMs in B may have something to do with the issue, but even if i remove all 4 PROMs on the B column, the clock jitter seems to remain, so the problem seems elsewhere. 

At this point im not even sure if the clock jitter might be the problem. It shows up as a faint pulse just like the clock signal but it is slightly out of phase with the actual clock signal.

I also followed the truth table for different video modes (checking which pins are high on F7) in different video modes, and every video mode i tried, seemed to work. I've swapped out all the ROMs as well from the donor board, this doesnt help.

I'd really like to try to figure out what the issue is but with this much time sunk in, im at a loss... 

Thanks for your help!

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Check for cold joints on the ram exp connector and other ICs, including the CGR, or other continuity errors.

So I unsoldered and resodlered the ram connection on the ram board and the header pins from the board, it didnt make a difference. 

I also checked continuity on pretty much every single address and bus pin and every single one was fine.

specifically I checked these lines. Some of these are 0-7 like DA0-DA7, DB0-DB7, in those cases I checked every single one of them.DA, DB, DC, DX, VA, C1M, C1M*, OE374, BT

Checked for H* (h0-h5 and a few others)checked v* (all V lines)checked bksw1-3 and abk1-4checked A from 0-15 and a few others if you type A* in the searchchecked for AY*checked for D0-7

So it's finally fixed!!!

I was checking lines DV0-DV7. I got to DV4 which checked out. The probe accidentally slipped at hit pin 19 on e2 and it beeped. I checked again. Beeeeep!

turns out there was a short between pins 4 and pins 15 on the 2114 srams. I verified against the donor board and it didn’t have the same issue. It looked like DV4 and DV6 were shorter together.

After removing the sockets for e2, f2, e3, f4 and both 2114s, i found a whisker shorting two adjacent pins. Cleaned the through holes, soldered sockets back and the first pic was from the first power on!!!

Well done!