Byte⏩Shift @byteshift69@mastodon.cloud

FYI for anyone with a #TRS80 #Model4 (or a #Model3 with a M4 video board, like me) If you have #flickering or #dimming issues, and have the TCE-made board (not RCA) then check the highlighted resistors (R315, R316, R318) #RetroComputing

I had a nice evening with some friends and a telescope (Brr, it was cold!) and then I finally fixed the damn thing! Very bright now, and the image is rock solid!

And also, I did bypass that INT BRT pot from a previous post, and it improved the display. It didn't fix the variable brightness issue, though.

Home from work, and preparing to get back to troubleshooting my #TRS80 Model III monitor brightness issues. In a previous episode, I mentioned that I didn't have a schematic that matched my board. On a hunch, I looked up the service manual for a Model 4 instead, and voila! My Model III has a Model 4 video board in it! Now I have a schematic :) #RetroComputing

Almost time for 38 Special!

I found one resistor in the signal path with a low value, and replaced it. Otherwise, everything looked fine. After reconnecting, though, it looks like there is a trimpot, which doesn't appear on any of my schematics, which may be flaky. It's labeled INT BRT and seems to be secondary brightness control. I'm going to try bypassing it and see what happens.

Some real funkiness on this card edge... Definitely going to clean that up and reflow the solder

...or not. Both pots check out. Bypassing the pots with resistors doesn't stop the flickering. Time to discharge the CRT and pull the monitor board!

And backordered.

Yikes, S-100 bus connectors are pricey!

Oh. That explains it. The signal makes a detour through the 'brightness' potentiometer on the way. I guess I found my problem!
Now to see if I have anything here close enough to replace it with.

I disconnected the connector from the monitor board and checked the other end. The video signal loses an entire volt while traversing the cable!

Well, there's honestly not much here. Pin 13 of U52 feeds it, with only a small capacitor across it. Guess I'll check that cap, and also make sure the cable to the monitor board has no issues.

Disconnecting the monitor brings the signal up to 4.55V peak-to-peak, which is where it should be. So either the monitor board is pulling it down, or the CPU board isn't sourcing enough current. I'm going to start on the CPU board, since it's physically easier, and has fewer shocky bits. 🌩️

Now trying to troubleshoot the issue (which has worsened over the past few days) of the monitor randomly dimming. I suspected oxidized connections or bad cabling, but wiggling has no effect. I did find that the video signal at the CPU board was only 3V peak-to-peak, when it is supposed to be closer to TTL levels (nominally 5V)

When I put this thing back together (with actual screws, even) I don't want to have to take it back apart for a while. I went through and used DeOxIt on every socketed IC in the thing.

I have made a mistake. I have followed more people here than I can keep up with. I don't seem to have time to read and interact with everyone's wonderful posts 😞

Cleaning up some things today so I can button up the #TRS80 Model III and get it out of the way. I deoxed all the pots and connectors on the monitor board, replaced the chassis ground wire and the screw where it connects (adding toothed washers for a better connection), and cut the volume control cable (from the previous amplifier/speaker mod) to add connectors for easily separating the halves of the computer.

This is the S-100 backplane that I'm having manufactured:
https://retrobrewcomputers.org/doku.php?id=boards:s100:backplanev4:start

If anyone has a source for cheap S-100 card edge connectors, I'm all ears!