View Full Version : Trouble with model II

February 7th, 2009, 01:18 AM
Hi :),

i've bought an "untested" model II in an online auction house.
Now i donīt bring my troubleshooting attempt forward.
The power supply is repaired, all voltages are o.k. but i have no reaction on the screen and the FDC . On the scope i can see adress and databus activities. System clock and Reset-Logic on z-80-CPU-Board is o.k.

The system is equipped with a video board extension, ram expansion and a 68000 CPU card. For easier error search i want to bring back the system to basic configuration..Reducing the cards to original config will not be the problem...the system is still dead. I donīt have documentation for the 68k Upgrade, so iīm not sure if there are further modifications or modified settings or components on the basic boards. The model II technical reference (1000xThanks to marcus B. for providing the tech manual) is not very detailled and has no informations about the upgrade. Does anybody have some further informations ?

Iīve read out the Z80-CPU-board ROM..... it looks very emty from 0x1FF....Is this normal ?....my knowledge is bordered in such things...
Here it is: http://www.mister-freeze.net/bilder/trs80-II_rom2716.BIN

I would be glad to get your support ...thanks, Christian

Terry Yager
February 7th, 2009, 12:34 PM
I can"t help much, except that the 68K board is a drop-in. No mods to the mainboard, etc are necessary.


February 7th, 2009, 02:33 PM
when first switched on the model does its own checks like many other machines, any failures will result in an error code appearing on screen, before
it gets to the " insert disk" screen. The basic model 2 has 8 card edge connectors, on the cage, numbered from the power supply end ,1 to 8,
The standard system has the cpu card in 1, the floppy controller in 2, the memory in 3,and video card in 4, memory card may be 32k or 64k , you should be able to see the crt heaters working, and you seem to have a useable psu,
when switched on the rom on the cpu card initializes the boot sequence, and system checks, if this rom is bad, nothing will progress, try carefully removing
this rom and all other socketed ic's, clean all pins and reseat, check if you have any response? try screen adjustments, like brightness etc, if you can not get any screen errors or any thing at all, then try some cleaning and connections on the video board etc. best of luck, I had similar issues with my own model 2, I have now got to the stage of needing the operating disk, Just dont give up !

February 7th, 2009, 08:51 PM
Basically, your Model II has been upgraded to the equivalent of a Model 16A. Sounds like it has a 256K memory board for the 68K board and, possibly, a Hi-Res graphics board.

You can pull these boards to bring it back to a standard Model II configuration for trouble shooting.

Does the CRT heater light up when the computer is on? You may want to play with the contrast and brightness controls which, depending when the unit was made, are either on the front, below the CRT or under the "lip" on the left hand side.

There should be a cable running from the stock video card to the CRT cntroller board and, sometimes, this slips off the pins at the video card end.

You also might want to pull each card and reseat all the chips on them.

In an upgraded Model II, some of the cards had to be in a specific slot for the system to function correctly.

I know the Z-80 CPU had to be in the first slot (nearest the CRT) and the standard video card went in slot 3. I THINK the Hi-Res had to go in slot 4 and the 68K CPU card and memory card went in slots 5 (CPU) and 6 (68K memory) just because that was the first opening (in your setup) where there were two adjacent slots.

An external HDC card could go in any open slot, the same as a multi-terminal card and an ArcNet card.

I'll check in my 16A tomorrow to confirm the slot allocations though.

February 8th, 2009, 04:31 AM
Hi, Thanks for your replies,:)

i got the system in following conf. sorted by edge connector number:

1. z80-CPU-Board
2. FDC
3. RAM (64K)
4. Video
5. Video Extension
6. RAM (64K)
7. 68K
8. 68K Extension (RAM?)

i have left cards 1,2,3,4 connected and jumpered the RAM board as described in the tech reference. The CPU, DMA, CTC, PIO, CRTC the 4 Videorams and the first row of the 4116 DRAMs are changed with spare chips. If i increase the crt brightness i can see
the glowing lines. If i disconnect the video connector i can see that the sync fails and the lines are leaving the display range. So i think the Videocard and the CRT are working. The character-ROM is filled with data . That seems to be o.k. All chips in sockets are now reseated. ... I still get no letters on the screen.:( I expect an "insert disk" message, like described by Harry...
The ROM-dump ends with INSERT DISKETTE DIAG...at 01EC :confused: Does anybody have a ROM-Dump to compare ?

February 8th, 2009, 09:06 AM
you seem to be getting some sort of output to the crt, but the z 80 on the cpu
is not activating the start up rom, or the rom is bad, as you have now removed
all the upgrade cards, and all the originals are in their correct slots, when you power on do you have the red led on the 8" drive come on just after the power on led between the front switches? is the 8" drive running? one other simple check to be made is the reset switch, just pull the small 3pin plug off of the cpu board, and try to power up, if still nothing,remove the ram card from its slot
and try to power up , this should show an error on screen, if you have nothing
showing, I would suspect the cpu board z80, or its start up rom, best of luck

February 8th, 2009, 08:39 PM

yes, the error is located on the cpu card :D. The system will only give message with removed DMA controller. :confused:. (original and spare chip defect ?) Inserting Disk is followed by boot error (due to missing DMAC ?) .
I will report when i get a new DMAC.


February 9th, 2009, 03:11 PM
IIRC, not all the chips on all the board were oriented in the same direction.

Check the socket notch on anything you're swapping out.

February 10th, 2009, 08:38 AM

With a new DMAC the system is working fine now.
Fortunately the TRSDOS and the CP/M Disk came with the machine.
Itīs worthy of mention that imd-diskimages are available at dave dunfields site. Great !

i will now try to upgrade the system back with the 68 K card.

thanks for Help.

February 10th, 2009, 12:24 PM
Glad to hear it's up and running.