I'm working on a TRS-80 Model 100.

Not long ago, I have done some capacitor repairs after experiencing random reboots. I've skimmed through some online manuals, and it seems that somewhere in the circuitry around the on/off switch is somehow correlated with the reset button. I replaced all the bad capacitors in this section of the motherboard in this picture I found online.

enter image description here

  • I replaced P with an electrolytic cap. 0.47μF, 50V.

  • I replaced O with an electrolytic cap. 1μF, 50V.

  • I replaced J with an electrolytic cap. 4.7μF, 50V although the original is a 25V.

  • I replaced M with cap. 33μF, 16V although the original was a 10V.

I've replaced all 10uF 16v capacitors that are used for the display. Recently, I had to replace B and D as it was a bad solder.

I've damaged the trace near b and d but was working just fine and it hasn't severed the trace completely.

I checked the other in the circled that wasn't mentioned above and they did not leak. Although I had to desolder them and put them back again. I made sure that working old capacitors were properly seated and connected through the solder. I even checked the non-polarized C, and it was not leaking so I had to re-solder it carefully.

I'm confused right now as what to do. All bad capacitors have been replaced.... I'm not sure why its doing this....

Works for a while then randomly reboots after entering the telecom. So which is why I checked the C capacitor (has to do with the modem?)

Somewhere in the service manual, it talks about a battery voltage detector. I suspect that since the capacitors in the circled area have gone bad is the culprit of the "reset." But I was mistaken. After replacing the bad capacitors I still was experiencing reset. So I decided I must have done an improper solder. I made certain that the solder sunk into the holes where the capacitors are supposed to be at and nothing but a black screen of death. I'm scratching my head right now. Did I short something by mistake?

Here is the service manual, look for page 50

enter image description here

Here is the capacitor list and map.

  • \$\begingroup\$ I'm really impressed you are taking on this project. It's good of you. The first thing I'd do is to re-draw the schematics, though, so that I can understand them better. That way, I can work forward one step at a time and make sure everything is working the way it should. The existing schematics you show are not well-drawn for understanding, so that is why it would be my next step given what you've already done to get things going. See my discussion here for a perspective on how to proceed on redrawing the schematics. \$\endgroup\$
    – jonk
    Mar 10, 2019 at 3:52
  • \$\begingroup\$ Does it do this when powered from fresh alkaline AA batteries? \$\endgroup\$ Mar 10, 2019 at 3:54
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    \$\begingroup\$ @TravisWells, Just to make it clear, replacing a lower voltage capacitor with a higher voltage one doesn't really affect the performance (see this). \$\endgroup\$ Mar 10, 2019 at 4:01
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    \$\begingroup\$ I can understand the schematics OK , but you need to make some DMM measurements and activate switches. Around the areas in Yellow then the PCS 40kHz clock with RESET hi going to the blue area of the DC-DC converter with +/-5V out i.stack.imgur.com/BQ6cA.png \$\endgroup\$ Mar 10, 2019 at 4:32
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    \$\begingroup\$ Could you be over-complicating this? A randomly re-booting PC was repaired: it turned out to be an intermittent reset push-switch (or the wires going to the motherboard). \$\endgroup\$
    – glen_geek
    Mar 10, 2019 at 15:47

1 Answer 1


I found out that the scorch from the soldering iron has interfered with the capacitors' legs being in conductive touch with the solder pads. I cleaned off the scorched stains with a sewing needle and a flat screw driver. I had trouble with the solder sinking into the holes to deep for the soldering iron. ( de-solder )


Especially to safely make contact without scorching the board I had to dab some solder ontop of it. I've accidentally yanked out a few pads because of accidental scorching. So if you pulled pads, the copper plating would be on the top side of the board. Just scrape safely till there's copper around the holes and solder the capacitors on the top side. Be careful consistent heating will weaken the copper plating and can peel off when scraping with a screw-driver. Wire-glue should be able to fix any scraped off copper plating.

I also found out that the old nicad battery that was pre-installed on the board doesn't hold charge at factory capacity. As a result it drains fresh batteries in a week if you leave it off. For anyone who encounters future problems replacing the battery should be the first thing to do so that you're not de-soldering, re-cleaning, re-scraping and re-soldering everything. And that new pack of batteries is dead.

Altogether, the problem was scorch interfering with the conductivity and the old nicad battery draining a new pack of batteries in a week. And of course pulled pads. Just de-solder all the capacitors carefully and clean out the holes thoroughly with rubbing alcohol. If you haven't replaced the nicad battery just make sure you remove the batteries when you're done.

Sunnyskyguy EE75

Thanks Sunnyskyguy EE75 for the edited schematic. It indeed was something wrong with the reset circuitry. It helped me find out that the scorch was partially responsible for constant rebooting.

Cap list/map


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