uno_bliss: Thanks for pointing out the name of that component. I googled and going to buy it to replace. The other SCR you mentioned has a little bit burnt mark on the PCB, but the SCR itselft looks ok. Probably saved by the large aluminum heat sink.
Jubooo: I modied the psu last year after it has been used with a step-down transformer for 1 year. After mods, it has been working fine. It was overheated last week after it was placed inside a narrow gap. The charger was very hot when roomba has this Error 5 and alert voice.
Maybe the mod caused increased current therefore it generates more heat than before. (Not sure about this, need expert's opinion.) But previously it was placed on the open ground so no issue. The charger only feels warm during charging.
Here's a few pointers for u:
1. Charger needs to be ventilated, cos it gets hot ~50 deg C during charging. Placing in enclosed cabinet will not result in charred SCR.
As stated in my previous post, charred SCR is due to bad contact between Roomba & Dock. Bad contact is 90% chance due to dust, rarely due to irregular floor surface since Dock contact is spring loaded giving some allowance.
2. Dock should be placed such that Roomba can make the approach from at least 1m away, during the 1m, Roomba can fine tune & dock nicely.
If dock is awkwardly placed such that Roomba must approach from the side, it won't have space to fine tune, and that results in mis-alignment & bad contact.
U'll recognise bad contact when u see Dock light flash on & off.
3. This overheat incident has nothing to do with the way u mod, nor the fact u used Transformer previously.
4. The multi-legged chip mounted to heat sink is not SCR, it's power Mosfet 5N60C. This chip is costly ~$6-$8, but since u can still charge, it's probably not spoilt yet. But the PCB charring implies it was put under tremendous stress before.
5. Physical appearance of electronic components seldom indicate their usability.
So when u say "the SCR itself looks ok", looks don't count. U literally need to de-solder and use Digital Meter to measure. And even that isn't 100% accurate cos Digital Meter uses only nominal load (ie. teeny-weeny stress). It's like u knock on a partition & it feels solid, so u lean against it and crash right thru.
6. Before going to Sim Lim Tower to buy replacement components, u should Google for Datasheet & print at least the synopsis. Cos don't expect Sim Lim people to know everything, some shops only know how to type in part no. & search their inventory.