What's on my bench
I bought a(nother) Tandy 100, as "not working/for parts." A quick exam revealed that it was executing code, but not showing anything on the display.
After recapping the board it still was not showing anything on the display. The Tandy 100 Service Manual (https://ftp.whtech.com/club100/doc/m100ServiceManual.pdf) has a good flowchart to debug these things; I knew it was just the LCD, though.
It requires +5V and -5V from the power supply circuit, but the PS was only producing +5V. After some more tracing I found there was a trace broken by the leaky capacitor that I replaced, so I added a bodge wire. Then, the PS produced 0.6V and 0.6V (instead of 5V and -5V). Ugh.
I desoldered the transformer, to maybe swap it with my other, working 100, but chickened out - I didn't want to possibly ruin the working 100... I soldered it back in, removed the bodge wire, and it was STILL producing 0.6 and 0.6V... I had soldered it in upside-down. So I removed and replaced it AGAIN, and it was producing 5V and nothing, so it didn't get worse.
I let it sit for a few weeks and got back to it today. I found that BOTH sides of the capacitor were disconnected from the circuit so I added the original bodge and another bodge wire, and success! +5v and -5v!
But...the LCD still didn't work. It also needs a contrast signal, which was pegged at -5V. Comparing to my working 100, it was supposed to be between 0 and +4V. Back to the board...
I found another broken trace (from a different leaky cap) between the contrast circuit and the connector to the LCD. A third bodge wire was added, and .... success!
Great work!
Chris Garrett Forgot to add: two keyswitches were dead (the A key and the caps lock, which is a special "latching" version of the Alps SKFL). Then I found 3 more which had a broken bit, so all 5 keyswitches were replaced. (I found a source: NEC MS-DOS laptops)
David Plass one of the less than ideal aspects of the C64U keyboard is the shift lock ... doesn't lock so that is good you found a replacement :)
Chris Garrett Ah, the lock KS was from an old Tandy 200 keyboard I got on eBay. All other keyswitches were from the NEC. (The NEC uses software lock, which is a nonstarter for the Tandy 100.)