https://digilander.libero.it/rar2k/TRS80/Images/M100-Pcb.gif
Do you have a scope? You could check that the timer interrupt input RST7.5 is toggling every 4msec. Timer interrupt TP is generated at M18, which is the RTC chip. It is fed from 32.768kHz crystal X1. Not saying for sure this is the problem but it looks similar to other machines I have fixed where that crystal was dead. On Fri, Jul 4, 2025 at 9:06 AM Thomas Morehouse <[email protected]> wrote: > Thanks Steve. Can someone point me to appropriate schematics or diagrams, > finding a replacement crystal, and how to perform the surgery? > TM > > > On Fri, Jul 4, 2025 at 9:01 AM Stephen Adolph <[email protected]> > wrote: > >> it is suffering from excessive interrupt-itis. >> >> On Fri, Jul 4, 2025 at 8:58 AM Stephen Adolph <[email protected]> >> wrote: >> >>> looks to me like the 32768 hz crystal is dead. >>> >>> On Fri, Jul 4, 2025 at 8:56 AM Thomas Morehouse <[email protected]> >>> wrote: >>> >>>> Friday update: >>>> >>>> Batteries replaced with fresh AA alkalines yesterday (Thursday). Left >>>> powered off overnight to recharge backup battery. Powered up this morning, >>>> but no change. See attached screen photo. >>>> >>>> The screen clock seems to be working, but date never changes from >>>> 1/1/1900. >>>> Top four lines of screen appear normal, but below that, definitely not >>>> normal. >>>> >>>> The "center" screen line consists of constantly changing characters. >>>> Lowest screen line appears to be reiterations of "Bytes free" - without the >>>> "Byt". >>>> >>>> None of the keyboard keys have any effect; when "cursor" is over BASIC, >>>> hitting Enter changes nothing - screen remains unchanged. >>>> >>>> I've done several Cold Starts (CTRL+Pause/Break+Reset) and >>>> (CTRL+Pause/Break+Power switch), but no apparent change. >>>> >>>> Note: when I do a Power On, the screen quickly shows the correct >>>> original default M100 screen, then immediately changes to the garbled >>>> screen. >>>> >>>> Onward through the fog! >>>> Thanks. >>>> Tom M. >>>> >>>
