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TRS-80 Model 1 NTSC vs PAL

simmiv

Member
Joined
Nov 2, 2017
Messages
37
Location
Australia, NSW
Hi,
I'm in the process of restoring a TRS-80 Model 1 unit I've had for over 20 years here in Australia. It was donated to me and I've had it in storage along with a few other goodies from the 80's. When I finally got to opening it up and checking it out, it of course has the curse of faulty video rams. Other than that it appears to be in a pristine condition.
In the process of diagnosing the problems with it, I read the technical reference manual from start to finish so I could better understand how it works and how to diagnose it. One part of that was to use my oscilloscope to probe various signals. As I was checking out the Video Divider chain, I discovered that the Vertical pulse is 50 Hz, not 60 Hz as the technial reference manual depcits. Further to that, I checked out the crystal driving the CPU. In the technical reference manual and schematics, it's said to be 10.6445MHz crystal. On my TRS-80 Model 1 it's marked with 10.483MHz. This is reflected in my measurements throughout the video divider chain. i.e. according to the tech reference manual, the input frequencey to the video divider chain should be 887.0416kHz, wheras I measure 873.150kHz with the oscilloscope. The rest of the video divider chain is the same i.e.
Input 887.0416kHz, mine 873.150kHz..
The next stage is divide by 4, it's output should be 221.760kHz, mine is 218.828kHz..
Next stage is divide by 14, it should be 15.840kHz, mine is 15.5919kHz..
Next stage is divide by 12, it should be 1.320kHz, mine is 1.2993kHz
Next stage is divide by 2, it should be 660Hz, mine is 649.6Hz
Last stage is divide by 11, it should be 60.0Hz, mine is 49.97Hz

The last stage can't be dividing by 11, because if it was the output would be 59.05Hz.. Instead it seems to be dividing by 13 instead.

I wasn't aware that the TRS-80 Model 1 came with different composite video outputs i.e. 60Hz vs 50Hz. Is that a well known fact?

You can see my video on the restoration here
 

Eudimorphodon

Veteran Member
Joined
May 9, 2011
Messages
5,336
Location
Upper Triassic
Huh. I was also under the impression that the TRS-80 Model I used NTSC frequencies for its monitor output regardless of the region it was sold in. Apparently not. I didn't have time to watch your whole video yet but I did see your unit had the one-piece ALPS keyboard of later units so I thought it might be worth checking the service manual that's floating around for the Japanese-manufactured units to see if it mentions PAL frequency video as an option, but a quick scan didn't turn up anything.
 

simmiv

Member
Joined
Nov 2, 2017
Messages
37
Location
Australia, NSW
In the video I talk about the bodge wire on the back of the board. Now I know what it does.. They cut the trace between pin 2 on Z66 to pin 9 on Z32. The bodge wire goes from pin 2 on Z66 to pin 8 on Z32. Basically, the changed Z32 from divide by 11 to divide by 13. As the unit came with the warranty sticker untouched, I can only assume this is a factory mod to make the vertical 50Hz for Australia.
From this:
Z66 Z32
Pin 1 to 11 D output (8)
Pin 2 to 9 B output (2)
Pin 13 to 12 A output (1)

8 + 2 + 1 = divide by 11


To this:
Z66 Z32
Pin 1 to 11 D output (8)
Pin 2 to 8 C output (4)
Pin 13 to 12 A output (1)

8 + 4 + 1 = divide by 13
 

simmiv

Member
Joined
Nov 2, 2017
Messages
37
Location
Australia, NSW
I've now restored the TRS-80 Model 1 to a fully functional system. I managed to find some matching automotive paint for the back cover and hook the PAL version of the TRS-80 to the original Tandy Video Monitor, which is the US version. I used both the Oscilloscope and a Logic Analyzer to see the corrupted SRAM in the video memory space. All explained here..
 

richarjo

Member
Joined
Jun 6, 2022
Messages
20
Location
Tasmania Australia
I have a NTSC Model 1 and an Australian 50hz TRS-80 monitor. I thought doing the mod you've mentioned might be good so I added the wire to Z66-Z32. When cutting the trace between Z66 and Z32 I wasn't sure exactly where along the line - so I cut it right next to pin 9 on Z32 which is after a small solder junction. I did this because at the junction point it seems to go elsewhere and I didn't want to cut it at an earlier point. (Maybe that was wrong and pin 9 is used by another location from the junction also?) The trace is about 4mm long - not the main length from Z66. Now I have a screen split in two; the upper half lighter with a command line and the lower half darker - also with another command line. It still seems to have the 20cmx12cm display area I thought was because of NTSC on a PAL monitor. I'm wondering if the trace from Z66 is cut at a different point?
 

richarjo

Member
Joined
Jun 6, 2022
Messages
20
Location
Tasmania Australia
I've taken the initiative and completed the PAL mod on my TRS-80 model 1. I'd cut the trace in a co-joining area. Now Working well. Thank You for the information and inspiration.
 
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