Easy TRS-80 M100 (and NEC PC-8201a) factory RS-232 mod 40 years late!

Video sponsor: PCBWay: $5 for 2-layer PCBs with 24hr turnaround at www.pcbway.com/
In this video we take a look at an official TRS-80 Model 100 factory modification that was done to the RS-232 port. This was released as TSB 100-001 almost 40 years ago. The early M100s had too large a currently limiting resistor in the RS-232 output lines which caused compatibility problems with some other TRS-80 products as well as other RS-232 devices.
We discover what the problem is, we apply the modification as per the bulletin, and test the results to see how it performs. We also find this same modification also applies to the NEC-8201a which uses the exact same RS-232 circuit design and has the same problem.
Timeline:
00:00 - Welcome
02:46 - Problem demonstration and source
10:48 - Fixing the problem
16:32 - Testing our fix
18:17 - It works for the NEC PC-8201a too!
18:50 - Wrap up. Thanks!
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Credits
Chiptune Thing No. 3 "Victory" by Steven O’Brien / stevenobrien
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[@MrHowlinAlan]

These videos are so soothing. It’s like Bob Ross, but with electronics.

[@ravengelen]

Very timely!! I have my model 100 PCB on the workbench to recap, while I am waiting on a better desoldering unit to arrive. Will swap the resistors too, since the serial number tells me it has this issue. A recap is needed since the screen started to flicker due to random voltage fluctuations and eventually went out.

[@HennerZeller]

Thanks! I've updated the TRS-80 Model 100 schematic transcript I published on github to contain this information from the bulletin.

[@nickfifteen]

Haha I was wondering why this topic seemed to familiar! I had this exact same problem. It's nice to see what you meant by the voltage drop affecting the handshake signal. I guess we should be lucky we had one of those "rare" units. :D

[@Fezzler61]

Thanks. I just wait for the Hey Birt! BA run.

[@wimwiddershins]

I think there's a little "ham" in your reenactment.🤪
I also seem to have more 330Ω resistors than I'll ever use, about a 6' belt of the things...

[@NickNorton]

16:00 It's as you say, less energy dense. But hopefully the switch to Supercap instead of NiCad/NiMH won't be so leak prone.
Still a chemical device, but I hope yours never leaks. Let us know in 2063

[@dugmeister65]

Thanks for this. I'll check mine!

[@nuttick]

Hey Birt, Thanks for this. I finally got around to doing the swap in my NEC. All I could find was 390 Ohm resistors, but the laptop works well now with the Backpack Drive. Before the swap it wouldn't reliably connect to either the Backpack or the TPDD2.

[@HeyBirt]

Glad you got it sorted. The resistor value is not critical, they may have chosen 330ohms as it was in the correct range and they already were using that value, and it would already be loaded on the insertion machine.

[@tenminutetokyo2643]

DOOD...................................