68kMLA Classic Interface

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Classic screen garbage after recap and ultrasonic clean

Posted by: elemenoh on 2020-10-09 14:27:25 Today I recapped a Classic I analog board and MLB. The MLB got an ultrasonic clean after recapping. Without the RAM expansion installed, it boots to a screen with thin horizontal lines, then a repeating pattern that looks a bit like columns of - — ) The fan and HD runs, but no other noises can be heard. With the RAM expansion installed, it boots to a screen with thick vertical bars, then the same repeating pattern as mentioned above. The fan and HD runs and eventually a "tick tick tick tick..." sound is heard from the speaker followed by a fast repetitive noise from the floppy drive which eventually stops. 5v and 12v are okay. A known-good ROM swap made no change All pins from UH6 were checked and it was replaced, but the symptom didn't change. I'm guessing something is flaky with the RAM or its controllers, but not sure where to focus next. Any suggestions? View attachment w:ram_instaled.mov View attachment IMG_3390.MOV

Posted by: 360alaska on 2020-10-09 15:07:51 Maybe UA5? The sound chip generates the reset signal, also, make sure one of the buttons isn't stuck down. Look here: https://drive.google.com/file/d/1arROEuXR8cY_02zv6NjKSsXKv1A3WtYE/view?usp=sharing

Posted by: elemenoh on 2020-10-10 02:05:06 UA5’s connections tested okay. I also sprayed the buttons with contact cleaner and exercised them but the symptom didn’t change.

Posted by: elemenoh on 2020-10-10 02:07:44 Also, If I press the reset button when the screen with repeating columns is up, the speaker will buzz for a moment but the pattern does not change and the screen does not flicker off.

Posted by: elemenoh on 2020-10-10 03:55:10 Confirmed pin 7 of UA5 goes to ground when the button is pressed.    Checked UH6 again, UL1, ram expansion pins, RAM address and data lines.    if the traces are okay, is this likely to be Abad RAM chip or something else like UL1? UH6 is the only one close to the leaked caps and that was already replaced.

Posted by: 360alaska on 2020-10-12 01:48:05 I think one time I had a bad trace going to ul1 with similar symptoms. Also, what's going on with UJ4 pin 33?

Posted by: elemenoh on 2020-10-23 11:39:12 The thing on Pin 33 of UI4 was just some fluff from a cotton swab. I removed it and will re-check the connections to UL1. Thanks!

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