Fixing a Japan-made overhead mixer, Yamato LR-51a ca.1980s

[Allphor]

I'm trying to repair this really beautifully-made Japanese overhead mixer. After opening everything up and cleaning/lubricating the hardware components I was amazed how well built this thing is. It was used daily in a lab but only for ~ year, and it's good as new... with one big exception: Some moisture got in the box holding the pcb w subsequent corrosion having ruined the power switch and the speed controller's "5p4m p9x" thyristors. I'm not familiar with thyristors, not so sure what's available today that i could replace them with. This mixer has very high torque, low RPM. 115VAC/50-60Hz/3.5A, it's DC motor is rated at 100W i believe.

Thank you for giving this a look - i really appreciate your time, friends

Al

 

[Harald Kapp]

ruined the power switch and the speed controller's "5p4m p9x" thyristors

I can understand corrosion of the power switch.

What makes you think the thyristors are affected? There are more sensitive components on the PCB likely to be affected by moisture.
Anyway, the 5P4M seems to be a 400 V / 5 A Thyristor. It is still available online. Or use any other 400 V / 5 A thyristor for replacement.

I was amazed how well built this thing is.

Obviously not that well built if moisture could enter the case. I can see no signs of gaskets or sealants to prevent this from happening.

 

[Allphor]

Hahaha -not that well-built, indeed! You're absolutely right on that front, HK - seems I opted for telepathy instead of explaining what I had in mind. In fact, the mixer's built with a rudimentary two-part design, the base w/ motor (A) and the controller box w/ PCB (B). The base is machined really well, robust gaskets and brass gears that look like a work of art! But you're 100% right, the control box certainly should've had some type of gasket and the level of craftsmanship was...good, nothing special.

I don't know if the thyristor(s) (there's three of them in series, visible in top of 1st picture) were part of the original problem. When I received it, the power switch was clearly kaput and i replaced it as well as the busted fuse. Then, the unit powered on for a brief period, then died with a popping sound. After inspection i saw what i thought were signs of damage/corrosion on the side of the 1st thyristor so i actually did replace it with a 5p4m purchased online. But when i tried to power up again, it made a popping sound again - and this time the replacement thyristor basically had exploded! I installed the 2nd one i bought as backup and the same thing happened, more or less.

Thanks so much for the help.... Clearly I'm in over my head- I'm guessing the root problem is not anything I've even considered!

 

[Harald Kapp]

the replacement thyristor basically had exploded! I

Sounds like an issue with the motor. Check the motor's mechanics. Does it rotate freely? Is the rotor or the gear somehow stuck?