simmiv
Member
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
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