Model 1 Startup Screen

[Jannie]

TRS80 virgin here (so please be gentle! )

Started up my first Model I and it does boot with the following symptoms;

1. It turns on with a screen of garbled characters.
2. It then starts on the bottom right and start filling the screen up with the character 'A", from bottom right to top left, one character at a time, i.e. it runs upwards.
3. It then stays there, no response to keys or anything else.







Any suggestions where to start looking / testing?

 

[daver2]

This won't hurt a bit...

The 'garbage' screen is the same as the PET. The CPU is held in a reset for a short while on start-up and requires the CPU to be running to clear the screen.

On the basis that this is the schematic for your actual machine:

http://www.1000bit.it/support/manuali/trs/TRS-80 Technical Manual (1978)(Radio Shack)(pdf).pdf

I would start looking at a keyboard array fault (the 'A' key first...) or a driver/receiver fault with Z5 (74LS05) or Z3 (74LS368).

You can see the keyboard matrix in figure 17 on page 37 of the manual.

Just double-check that the chips on the board are the same as the schematic before you start...

I assume you can disconnect the keyboard from the logic board?

If you do this - and the problem persists - it must be elsewhere.

Dave

 

[Jannie]

This won't hurt a bit...

The 'garbage' screen is the same as the PET. The CPU is held in a reset for a short while on start-up and requires the CPU to be running to clear the screen.

On the basis that this is the schematic for your actual machine:

http://www.1000bit.it/support/manuali/trs/TRS-80 Technical Manual (1978)(Radio Shack)(pdf).pdf

I would start looking at a keyboard array fault (the 'A' key first...) or a driver/receiver fault with Z5 (74LS05) or Z3 (74LS368).

You can see the keyboard matrix in figure 17 on page 37 of the manual.

Just double-check that the chips on the board are the same as the schematic before you start...

I assume you can disconnect the keyboard from the logic board?

If you do this - and the problem persists - it must be elsewhere.

Dave

Hey Dave!, here we go again

The keyboard is connected to the main board with a horrible, rather inflexible, cable that is notorious for going faulty. I'm busy investigating how best to replace it with a more robust setup. It seems most people end up doing this.









I've been uhm-ing and ah-ing to desolder the cable to test..........

 

[daver2]

Looking at those photographs - I wouldn't desolder anything just yet...

Is this a photograph of your machine or someone else though?

If it is yours (or yours looks like this) - get some flux cleaner and some hard cotton buds and clean up the brown mess I can see around the connections. Over time, this mess is likely to become conductive and would cause havoc (my initial guess to start with).

Dave

 

[Jannie]

Yup, my own machine.

Busy cleaning as we speak

/Added: All squeaky clean now, but same symptoms

 

[daver2]

Worth a go!

Remind me, do you have an oscilloscope?

If you do, monitor Z3 pin 2 (KC0 line). This line should always be high with no key pressed.

Dave

 

[Jannie]

Worth a go!

Remind me, do you have an oscilloscope?

If you do, monitor Z3 pin 2. This line should always be high with no key pressed.

Dave

Yup, got a scope. Will definitely check, thanks Dave.

 

[Jannie]

Worth a go!

Remind me, do you have an oscilloscope?

If you do, monitor Z3 pin 2 (KC0 line). This line should always be high with no key pressed.

Dave

Saw there are two '368 chips on the keyboard circuit. Could not yet figure out which one is Z3 (the other one is Z4) but on both of them Pin-2 stays high, irrespective if a key is pressed or not.

 

[daver2]

Yes, there are.

In fact, pins 2, 4, 6 and 10 of both of them should all be high. Probably worth checking while you are at it.

Can you also check pin 1 of both chips please. This enables the internal buffers and allows the keyboard to be read.

If you have a dual channel oscilloscope, can you use one trace to monitor pin 1 and the other to monitor pin 3. Yes both chips one at a time (if you don't know which is Z3 and which is Z4).

Trigger on the low going edge of pin 1.

See what you get. I would be expecting to see a logic '0' on pin 3 when pin 1 is a '0'.

Dave

 

[Jannie]

Eventually got back to this system.

The keyboard cable was faulty and, wanting to keep the machine as original as possible, I ordered a modern replacement. With that in place the error (screen filling up with "A" from bottom right to top left) persisted.

Looking at the data bus, a few bits were at a significantly lower levels (and "look") than the others. While poking around to try and find the source, I looked at D-in and D-out on the 2102 VRAM chips. Touching D-out of Z63 brought the machine back to life and this was repeatable. So either a flaky chip being affected by the scope's impedance or a dry joint.

Resoldering the chip seems to have stabilised the machine and she's been booting correctly for two days now. Holding thumbs

The "3" key was non-functional but it also turned out to be a dry joint.

Lastly, one of the 4116 DRAMs failed consistently in my RAM tester (the other 7 pass consistently) but when I put it in the machine, it boots no problem.

Can I assume that the Model 1 does not do an exhaustive RAM test at boot and I should run a proper memory test on the machine?

If so, any simple test I could run? something simple enough to type in on a keyboard that loves to bounce

 

[ldkraemer]

There is a good RAM test that will locate a bad RAM. It's name is Babyroot, and I have it
available with documentation. Just send me a PM with your Email address. You can
type it in in basic or the hex bytes.

The first code locates bad memory address, and the second code locates the bit.

Looks like your ROM is a Version 1.1, which is not the Latest. ROM 1.1 had some problems
with keyboard Bounce. I upgraded mine to ROM Ver 1.3. I've also got that documentation
if you want it.

Also look to see what the number is on your Character Generator. If it's the correct IC, you
may be able to get Lowercase Characters easily. Otherwise you will need to do a bit more work
to get Lowercase Characters. I purchased a Lowercase kit at the Dayton Hamfest 45 years ago.

Larry