buv18a transistor sub

[redwire]

Hmmm. You have to carefully figure out why the resistor burned up. If that transistor shorted C-E then the speed control would be stuck, not sure what the control board would do. (22Ω if base-drive would not burn up, this is what worries me).
The "field coil" as I call it seems to be the armature which is fed via carbon brushes or slip-rings on both generator and motor maybe, from the diagram. The shunt coil I am not sure it has these or what the stator is called.
It looks like a low voltage gen/motor so I would not hit it with too many kV to test insulation.

It's a weird forklift. The two 12V batteries run in series with stuff at the middle tap, the PID control and braking look scary too.

[petehall347]

motor is shunt-wound 4-pole motor
i have just got a tester back i gave away years ago that runs off 9v battery . 500v so inline with the service manual , bloody thing gave me a nasty shock earlier so i know its working . one thing bothers me is removing the brushes with motor in situ as manual says you need a pit or raise machine enough to get under it . i cant test insulation properly with brushes fitted . mast has to come off to remove motor .

[petehall347]

this manual doesn't really make sense .think i have cracked it though ..its test everything against everything but ignore A1 .looks like i can leave the brushes for now and only go there if insulation test fails or obvious burning and damage there .

[redwire]

I guess I'd give the motor/gen the sniff test as one 1/2W carbon resistor burning up makes only a mild stink?
I thought the schematic shows the stators (generator and motor) are connected together and floating (with a current-transformer monitoring that).
The field windings, it would be a hard short to GND I guess to overload the H-bridge.

[petehall347]

oh the fun of it all .turns out the transistors i got for less than a good price turn out to be used pulls and i dont have a curve tracer . yes for sure i will be putting my nose there . ah yes it did try moving backwards and forwards when i tried it . i guess it couldn't get any better with lack of field current so that to me narrows it down to the drive motor field circuit which figures with the blown transistor . maybe a wire has rubbed through i dunno .. i should find with the megger . am leaning towards field winding burned though with it smelling of burning .. will see sometime this week . just need to fix the board ready first . oh other problem i was sent 1/4 watt 22 ohm resistors ,.just waiting on 1/2 watt .

[petehall347]

bux20 fitted and new resistor .
insulation tests done about 50 meg according to the meter so looks good .
put boards back in and got the owner as a test pilot whilst i looked for smoke from the repaired board . and nothing seen .
it goes forwards and backwards now so whatever blew the transistor isnt doing it now . time will tell .

[petehall347]

redwire thanks for the help .