DC Motor Controller Problem (SCR/Thyristor Controller)

[adamq]

I started a thread two days ago, which turned out to be caused by user error. My bad there.

So I was happily using my 7x12 mini lathe for about 12 hours yesterday when suddenly it made a nice pop sound and stopped.

Fuse blew. Replaced fuse. Blew again.

Open up the control box and one of the rectifier diodes has blown (D8020l). Fine.

Replaced it today with D6020L, the 600v comparator to the 800v D8020L.

Replaced fuse. All lights are fine.

Now the motor still won't turn, despite there being voltage at the terminals. Something isn't working right.

Some things I've checked:
- Pot used for speed control works fine (opposing scale resistance)
- there is voltage between terminals where motor connects on forward/reverse/neutral switch.
- motor works fine on its own (connected to 14.4v drill battery)
- no other obvious signs of component failure

It's like there's voltage but no available current. I don't know if that even makes sense. I'm not especially familiar with how these motor controllers work.

In the pic, the top two heatsinks are SCRs (BT151) and the remaining three (which form a triangle) are rectifiers (two D8020L original on bottom, one D8060L as replacement on top).

Any help is much appreciated.

 

[Minder]

Any of the bridge devices and the BEMF rectifier are prime culprits usually,
If a bridge component shorts, it usually takes out another one, either SCR or rectifier.
You need to test them all.
M.