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TRS-80 Model III FreHD issues

S

s_crumb

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Apr 20, 2020
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I just installed a FreHD with the autoboot ROM into my Model III and I'm not getting anywhere. I used someone else's known good SD card image and I can't seem to get it to boot.

To troubleshoot, so far I have replaced the ribbon cables from the FDC and the RS232 boards. I cleaned the drive connector on the bottom of the motherboard. I reseated the autoboot ROM in U106. I used a new cable from the FreHD to the motherboard. The stripe is towards the outside of the case. The FreHD.rom file is in the root of the SD Card.

On startup the FreHD LED flashes green one time but I get nothing on the monitor. Holding Break and booting the machine gives me nothing as well.

Does anyone have any thoughts as to what I may be doing wrong?
 
I had a similar issue on my Model 4p but at least I got an 'ID Error'. Turns out the ribbon cable was not properly seated. Also check the voltage. It only needs 5v as I recall. make sure its not connected to the 12v. Also make sure the boot files are in the root directory of the SD card and that the card itself is formatted to Fat32. I assume you have tried other SD cards?
 
Looks like I have bigger problems. At some point during the disassembly and reassembly, the computer has stopped working. There's no life from the monitor and the keyboard is not responding. I need to recheck my connections and get out the multi-meter.
 
COVID has significantly slowed down my troubleshooting! I didn't even have the energy to poke around with the meter. But I'm back!

I had a loose edge connector on the motherboard so I reflowed it....with no change. I reassembled the machine and started it up. No response. A quick light from the disk drive and nothing. Adjusting the monitor knobs doesn't even give me raster lines. So I checked the voltage on the monitor harness. I only got 2 volts from the power supply.

Previously I replaced the filter caps on both power supplies. Clearly there is now an issue somewhere in the power supply. I'm not exactly sure where to go next. I guess I will pull the power supply and check for obviously bad components. Any ideas?
 
without a boot disk in the drive, hold "break", then turn power on, you should see "CASS?" on the crt. power without doing that can show no raster if knob is not working but brightness may show the "cass?"

you said" I only get 2 volts from power supply" where did you check at the connector that plugs into the video board? or at power supply?
 
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Patrick,

No luck. I get no response from the keyboard. Break does nothing. Reset does nothing.

I got 2 volts from the wiring harness. I'll check at the power supply next.
 
Ok. Progress has been made. I replaced my OEM power supply with one of Jay's brand new replacements.

Same result. No life.

I discovered if the top connector is connected.....the one that goes to the main board, the power supply wont start. I disconnect that and it's happy. So there must be a short there somewhere.

I checked the continuity of all of the wires, to include the ground. Everything looks good. (Keep in mind I am testing this with the silver shield removed from the main board.)

Could it be that I am missing a ground somewhere or that I am just ignorant of how the main board works? Is the silver shield tied into the ground somehow? I've disassembled and reassembled this before and it worked. I'm not sure how this is the time I failed to do it correctly.

Another thought, the only thing I have done to the main board is I installed an auto boot ROM....in the correct orientation. I also replaced the flat cables to the RS232 and the disk controller boards.
 
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Thanks Patrick. I double checked. It's correct and in the correct orientation. The power supply turns on when it is disconnected from the motherboard.
 
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