Motherboard Repair Issues

[Motormouth]

I am having issues with two motherboards.

One is an A3000.  The screen jumps around in interlace modes.  non-interlace modes work fine.  The same symptoms exist whether or not I use the built in flicker fixer.

Is this a capacitor issue?  or other.  I find it interesting that all interlace modes are affected but non-interlace modes are ok.


Second motherboard is an A4000.  I am only able to address a total upto 4 meg of fast ram on the motherboard (only 1 meg per simm slot) no matter what I do even if I put 4 x 4meg simms (16 total).  I tried multiple simms same issue.


J852 is set to 4mb (not 1 mb) +5 v
I get a +5V on center pin.  I checked the Ramsey.  I am getting +5 volts at pin 12.

Does the ramkey need other signals (maybe from the simm slots themselves).

I did remove the battery and put a coin battery with diode replacement.  The battery area is very clean, but is it possible that an inter trace got cut??

Anyone had the problem before?

Does anyone outthere still do motherboard repair???

[Motormouth]

Interlaced video: the video syncs are generated by Agnus - so possibly it's bad. You've ruled out the monitor I guess? When using the flicker fixer, do you mean the 31 kHz signal also gets jumpy?? Very strange...

It is a bit hard to describe what is happening.... It is like the 15.75 (or 31.5 khz) signal are timed correctly.  It is like the screen jumps around, to several astable multivariable modes (sorry for the science eeez), 7 or 8 of these modes.  individually the modes look to be stable.

I am going to replace the denise to see what happens.......   I don't have a spare 2meg agnus  (funny how I have a bunch of 512k agnuses "lying around")

RAM: the sizing works through pin 12 only. On the 3000, there's J853 required at 1-2 (2-3 for early Ramseys) but it's not present (hardwired) on the 4000 afaik. What happens if you set the jumper to 256Kx4?

if the jumper is set to 256kx4  I get 0V on pin 12 of ramsey.  I still get on 1 meg of ram per slot. very odd.

I'd trace/measure the CAS and RAS lines and all address lines from Ramsey to the first slot to make sure they work (if they didn't the machine shouldn't run but the problem must be somewhere). There are no sense lines from the RAM slots, you can rule that out. Other than a bad line (contact? solderpoint?) pretty much all there's left is Ramsey itself.

Good Idea.   I pulled the A4000 wiring diagram.  You are correct.   if anything is a problem it will show up in the first slot.