The quality of the information shared on this listserv is off the charts.I
joined many moons ago when the focus was on simply using the Model T platform.
The transformative innovation, while often over my head, has been remarkable to
watch, support and use.
On Friday, July 17, 2020, 03:17:23 PM EDT, Jeffrey Birt
<[email protected]> wrote:
M28 has to do with the low voltage reset. If pin 13 of M28 is low, then you are
not in low voltage reset so T20 will not be switched on which kills the power
supply.
Generally having a screen with all dark elements with these Hitachi type LCD
controllers means the LCD is not being initialized properly. After the driver
chips are initialized, they will turn off until told to turn on.
Given that ‘BEEP’ does not work it would seem like the LCD is not being
initialized and the boot code in ROM is stuck in a loop waiting for it. The
following text is from an email conversation I had with group member Ian (who
developed the M100/T102 test harness). It was nice of him to pass on this
information about the LCD boot process and I hope it helps you with your
troubleshooting.
Jeff
-------------------------------
LCD Initialization – From Ian
The boot routines check the LCD is in "up mode" and the busy and reset bits are
correct. If this is not the case it will keep looping forever, i.e. the
machine will not boot. If the screen is blank, blind type in ‘beep’ and press
return. If this works you know the machine is booting and that the LCD is
returning the correct status.
The LCD is split into 10 RAM like ICs that deal with the columns and then two
row chips. The CS20-29 lines are the individual column chip selects and CS1 is
the common select that allows all the ICs to be written in one operation
usually for refresh/clearing or individually with the relevant CS20-29 line.
If the screen is remaining dark this is the post reset condition and none of
the chips are being correctly set up and/or cleared with a screen bit data
write. If one of the CS20-29 lines had failed I would expect to see blocks of
the screen with garbage., in fact it would be blank as part of the
initialization turns on the ICs for display. I was surmising that the CS1
line might have failed which is preventing the screen from being correctly
initialized.
But thinking further if the D/I line has failed low then it would allow the
screen status to be read back and of course set up for the correct direction
mode – so the status would be correct. However, when writing screen bit data,
it would fail to write anything to the screen. So, it could be D/I- is stuck
low and only allowing Instructions to be written but not allowing screen bit
data to be written. This could mean the A8 track is broken and not connected to
the LCD.
If WR/RD had failed, then it would not initialize, and the screen would not
work - it would hang. I think the LCD ICs reset to down mode and not up mode
which the boot sequence wants to see.
-----------------------------------------------
From: M100 <[email protected]> On Behalf Of [email protected]
Sent: Friday, July 17, 2020 1:15 PM
To: [email protected]
Subject: [M100] Troubleshooting M102 (Was: M102, Black Screen, Help!)
Thanks Jeff. I have revisited all the documented diagnostics in the service
manual, and checked/re-checked all the voltages, reset circuit and LCD.
The current state is that the system has correct voltages in both
power-on/memory-on and power-off/memory-on modes. The LCD works, in as much as
it can be adjusted from light to dark contrast (full black screen) but displays
nothing otherwise.
I have checked the reset signal on the ICs, and see only one anomaly in M28
(Flip-flop) where:
- Pin #10 (RESET) - Low/0v (reset button out), Low/0v (reset button
in/active)
- Pin #13 (Q) - 0v (reset button out), 0v (reset button in/active). Neither
state shows either high or low.
- I will note that since I don't have a logic chart, I'm not sure if this is
normal or an anomaly.
In addition, I have checked T9, T10, T11 and T25 and with the exception of T9
all show what I assume is correct behavior on the Emitter -- H/4.9v (reset
out), L/0v (reset in/active).
- T9 however shows L/0.3v (reset out), H/2.7v (reset in/active).
- A check of all the transistors via my multimeter (diode test) shows they
all appear to be OK (0.7v from B to E, and B to C). So T9's behavior might be
good, but without anything to compare I don't know.
I also checked the keyboard, in case it was not working and thus the
<shift><ctl><break><reset> signal would never be seen -- as best as I can tell
it works.
I have checked/rechecked all flat cables connecting LCD + Keyboard, and they
all appear to have no bad connectors or breaks along the cable (checked in
various positions to ensure not an intermittent break).
Additionally, I have fully charged the memory battery, turned the system off,
removed the AAAs (I have no external power supply), turned memory off for 15+
mins and repowered the unit. No change in behaviour. Blind <enter> followed by
typing B E E P doesn't work, and if I leave the unit powered on, power-save
does not turn the unit off.
If the CPU is waiting for the LCD, and stuck in a loop how do I resolve this?
If it is related to M28 and T11 then I can start with replacing T11. However,
I'm reluctant to replace more parts, as full recap, new X1 and X2 and D13 have
not changed the behavior so far.
At this point, I'm at a loss and would welcome advice - I really do want to get
this fully functional. Thanks in advance. FYI I'm more than happy to talk real
time with anyone offline too.
Cheers,
Alan
On Fri, Jul 3, 2020 at 5:10 PM Jeffrey Birt <[email protected]> wrote:
One other thing I forgot to mention. I started a video series on general
troubleshooting methodology for vintage computers. The idea is to cover the
troubleshooting process rather than the repair of a specific computer. The
first video is about evaluation of the computer and getting yourself started
down the right path.
https://www.youtube.com/playlist?list=PLegscpHOheJt_EpuPs9hfBpOJ5elRhYvw
Jeff Birt
From: M100 <[email protected]> On Behalf Of [email protected]
Sent: Friday, July 3, 2020 6:28 PM
To: [email protected]
Subject: Re: [M100] M102, Black Screen, Help!
Thanks Jeff.
On Fri, Jul 3, 2020 at 5:27 AM Jeffrey Birt <[email protected]> wrote:
Have you done a hard reset? Ctrl+Break+Reset button. Have you checked adjust
the screen contrast pot?
