Dc Motor Controller Repair  - Page 1
Forum section: electronics / repair

author: Quefol
author_rank: Newbie
timestamp: July 02, 2025, 03:58:18 am
content: Hello,I am trying to repair a KBIC 240D DC motor controller. I have determined that the issue is with the bridge rectifier. The rectifier consists of 2 SCR's and 2 diodes. The problem am having is I am unable to get any part numbers off the diodes. All I can get is part # TYN1225RG off the SCR's. Would there be any way to determine what diode specs i would need based on the SCR's? It needs to be a 2 pin TO220 package. I found GD10MPS12A at Digikey wich has the same amp and Voltage rating as the SCR but with a forward voltage on 1.8v. Would this work? If not maybe someone could suggest a better replacement option.Thanks.

author: Ian.M
author_rank: Super Contributor
timestamp: July 02, 2025, 04:47:21 am
content: Seehttp://andysmachines.weebly.com/variable-speed-controls.htmlfor SCRs and diodes used in older KBIC240 and KBIC clone drives. There's also a full schematic for a KBLC-240D which is very similar to the KBIC series drives, which should help you figure out what's what on you board.I've had to replace those diodes due to swarf getting into the lathe control box and shorting their legs to the heatsink. My KBIC clone had MUR1660 dodes (600V, 16A), and was supposed to have an 8A fuse (which a 'smooth-brain' operator had previously 'upgraded' to 10A), but note I am in a 230V country!Note that the diodes are a fast silicon PN junction type not Schottky and/or SiliconCarbide!I believe the diodes should have a continuous current rating (as a pair) of approx 3x the max. rated motor current and 2x the AC line fuse rating.If the SCRs have failed, you should check their gate drive resistors, which may have gone open circuit when the SCR blew.

author: Quefol
author_rank: Newbie
timestamp: July 05, 2025, 03:52:38 am
content: Ian.m, Thank you for the info. How familiar are you with these. I managed to power the board up. (It has a green LED that lights up) but I seem to be only getting a 27mV output on the DC A+ and A- outputs. Should I not be getting 90v or 180v. It doesn't seem to change as well when I ajust the POT. Do you think this may still be a rectifier issue. Or maybe someone else reading this might be able to give some insight. I really don't want to spend to much time and money on this as I can get a replacement board for $140 CAD.

author: Ian.M
author_rank: Super Contributor
timestamp: July 05, 2025, 12:30:57 pm
content: If you are happy to throw money at the problem and replace the board so you don't have a machine down while you figure out the problem and repair the old board, do that, but its still worth fixing it so you've got a spare!CAUTION: The whole drive is MAINS LIVE including its control pot and inhibit switch wiring, so do not attempt to measure waveforms unless you have a CAT II or better rated isolated differential probe, a CAT II or better handheld scopemeter with CAT II or better rated probes, or an isolation transformer andknow how to use them safely!You wont get significant output voltage without a load on the drive as the SCRs wont stay on for longer than their very short firing pulses if they don't have a minimum load current. Connect a 60W or 100Wincandescentbulb between A+ (the bridge's controlled positive output from the SCR cathodes) and A- (which goes via the HP resistor to the negative side of the bridge), and repeat your tests! If its OK, the drive will act as a lamp dimmer. If using a 120V bulb load on a 180V output drive, don't turn the speed up all the way . . .Actual faults that can cause no output and are likely to occur if there's been a bad enough blowup to need power diode and/or SCR replacement include the HP resistor going open circuit, so check that's measuring something close to its nominal value. Also as I mentioned earlier you should check the SCR gate resistors, which can blow if the SCR does. Follow the tracks from the SCR gate pins and you'll find them.