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Finally, I found the problem! I checked the circuits again, against the schematic. Line by line, trace by trace. After hours work, I found that the trace between U13 Pin4 and U14 Pin4 was broken! I fixed it. Now the C64 is back to life!!! Thanks for your kindly help! BR, Wells.

I checked all RAM related circuits, resolder all RAM IC sockets, but still got garbage characters on the screen. Then I desoldered U7-6510, U17-PLA, U18-SID, U6-2114(color RAM) and tested them on another NTSC C64, they all works fine! I also desoldered U13/U25-74LS257, U14-74LS258...

Hi Ruud, Thanks, but in C64_Diagnostic_Instruction_and_Troubleshooting_Manual_(326070-01), it says Zero Page Bad means U7 6510 failed. I also think if 6510 failed, I can't get the blue screen, maybe it's wrong. Thanks, I will check it carefully this weekend.

Thanks! I will test it this weekend.

Hi, My C64 boot up with wrong character and with garbage message on the screen. I use Diag Test Cart to diagnose it, it says "NGZERO 0002E02". Because Zero Page memory is internal to the 6510, does it mean 6510 MPU is failed? I tried replaced all 4164 RAM chips, but it didn't change...

Thanks @dittman , That's great!!! I got it! A12 is another CS, hah! I updated the file, you may redownload it. ROM B (IC5)

Thanks! In datasheet, it says "The Chip Select input deselects the output and puts the chip in a powerdown mode." Doest it means when I need to read out the data, I should set Pin 20 at HIGH? Another question, I successfully read out data from IC6, why IC5 has this problem? they are in same...

Thanks, which type is correct? the label is same as ROM A.

If use western style keyboard, IC Z37 would not be installed. Japanese keyboard version would be installed a chip to store kana character.

IC61 is HM6116 2KB SRAM as video RAM. this machine is so strange, 16KB ROM, but 8KB is empty, 64KB RAM and 2KB video RAM, maybe has 4KB character ROM...

I took clear photos of this board, under CC BY-NC-SA 4.0 license, you may share and download it from here: http://geek-logic.com/files/TRS-80/TRS-80_Model_I.26-7061.Mainboard.jpg http://geek-logic.com/files/TRS-80/TRS-80_Model_I.26-7061.Mainboard_back.jpg You can see there is a custom ROM...

Thanks! Haaa... I know this guy who post on Instagram! haha.. HwH_Ricky, I will contact him. I'll try to boot it up and dump the ROM file. Thanks.

I bought a special TRS-80 Model I mainboard in flea market. It's so special, look like integrated with expansion interface. I can't find any information of this board. I think it was made in Japan (since TEC logo on it) like some Japanese version of M1. It was marked as "HE11E016530" and...

That's OK. Thanks!

Ah... I didn't notice this... I checked the board with a paper manual, it shows the signal Z50 pin 11 is 15.835Khz, as you mentioned. When I wrote this post, I open a pdf file to take a screenshot. In this pdf file, it says 11.835khz... Actually is should be 15.835Khz... I checked these two...

Thanks! I replaced Z57 with a 74HC04，the sync problem solved. The original 74C04 is failed on Pin8. After test, I found the Z80 CPU is also failed. I replaced it. Now the TRS-80 Model I works fine. Thanks for your kindly help!

Hi, I got a MODEL I (26-1001) recently, but it could not startup. When I push the power button, there is only a scanline on the monitor, just like lost h-sync. First I checked the power output, +5v, +12v and -5v are all normal. I tried to replace Z80 chip and all RAM chips, but it still...

Yes! I just want to save time for buying a new AC transformer and ignore the problem, but it cost me more time to replace and test it. Learned a lot on this case! :) There is a saying in Chinese, "欲速则不达", haste makes waste. Thanks!

Hi Daver2， Yes, I measured it. it still a bit lower than 5v, about 4.6v, I think maybe it dosen't matter. -_-b I'm WRONG! I ignored it at first, but I could not find any other problem, so at last I bought a new transformer and replaced it. Problem solved ... Thanks! BR, Wells.

Finally, I fixed it. The root cause is AC transformer, I replaced a new AC15V( 7.5v 0v 7.5v output) transformer. It works fine. I don't know why the original AC transformer is AC 10V. Maybe I can't use DC 5v input directly to drive the board -_-b. Thanks all you guys. BR, Wells.