Classic II (early 4-ROM) black screen - RESET held low, no 15.7 MHz clock from EAGLE

[JeffC]

I am trying to troubleshoot a Classic II early four-ROM logic board. I picked the machine up 5 years ago, at that time it would only display a checkerboard screen. It had leaking caps on the logic board, so I replaced all the caps and repaired a couple corroded/broken traces. I also recapped the analog board. No change in behavior, I still only got a checkerboard on boot. I recently took the machine out to work on it again, now it only produces a black screen when powered on.

Current observations:

• Black screen
• No chime
• CRT neck glows
• Solid +5V/+12V/-12V measured at floppy connector
• RESET is held low, no address/data bus activity, and the CPU never appears to leave reset.
• The main oscillator produces 31.33 MHz.
• EAGLE receives main oscillator 31.33 MHz on pin 48, but the 15.7 MHz divided clock signal is currently not present on EAGLE pin 11.
• EGRET oscillator Y1 produces 32.768 kHz.
• EGRET pin 15 (/RESET output) remains low, and pin 23 remains low.

Steps I have taken:

• Removed EGRET, U12, and U15 to clean underneath and inspect for corrosion. No visual damage found. There is one repaired trace between U12 and C13 which has been confirmed good by continuity testing. With EGRET removed, the machine consistently displays the checkerboard pattern and RESET is no longer held low.
• I verified continuity from every EGRET pin to all of its schematic destinations. I have not found any open traces. Note this board has the factory bodge from EGRET pin 23 to EAGLE pin 49, it is in good shape and has continuity.

At this point I'm trying to determine what conditions EGRET requires before it releases RESET and enables EAGLE to generate the 15.7 MHz clock signal. Does anyone have documentation describing the Classic II startup sequence in detail, or experience with a board stuck in this state?

I also cross-posted this on 68kMLA, if I make progress with either thread I will update both so the info is available for future searches.

 

[Mark_W]

If you've already checked voltage and clock signal on Egret, then it could be a dead EGRET chip. iirc on classic ii Egret chip is surrounded by several electrolytic capacitors and it's prong to electrolyte corrosion. You can try to swap the Egret chip into another working board or compatible Mac II series computer, see if it releases /rst signal and if the mac boots up.

Had similar issue on a few LC575 boards, one won't respond to keyboard power button, one will power on automatically but won't generate any display, they all had bad CUDA chips (on LC575 CUDA chip has 2 leaky capacitors nearby). I got some LC580 scrap boards later and recycled their CUDA chips, which work fine on LC575 boards despite having a different part number.

There's also a project that aims to recreate CUDA/EGRET by flashing empty HC05 micro controllers. I think they have positive results but I stopped following for a while, the setup to flash HC05 chip is a bit complicated so I just bought scrap boards from ebay instead..

 

[JeffC]

Thanks for the suggestion, I had forgotten the EGRET is shared by other machines. I swapped the EGRET from a known-good IIsi a few minutes ago, powered on the Classic II, and still only got a black screen. Unfortunately I don't have time to test any further at the moment, later today I will take some measurements and see if there was any change in behavior.