B&H smoked a rectifier when plugged in

[Eric March]

Hello Everybody

I won an eBay auction for a Bell and Howell 1590 the other day. It arrived today with a blown bulb, which is an easy fix, but another issue. I plugged it in and ran the motor for about a minute when the magic smoke started billowing out.

I tore it down to the amplifier board and found smoked diodes. Anybody have some insight as to how this might have happened, or how to repair it?

[Steve Klare]

Hi Eric,

It's possible that something this bridge rectifier was driving drew too much current and overloaded it, but then again years and years of operation often makes semiconductors die a natural death. The devices in these machines are really ancient: they would have been dead decades ago if they were run continuously .

To me it looks like the second diode up from the bottom is the one that failed first. Since they usually fail short circuit, there should be one other diode that should be at least overheated.

With a little luck, you can clear the wreckage, clean up the board and solder in four new diodes (minding polarity, of course!), this one may come back to life again.

It's also possible that the motor was the root cause, so while you are at it you should check to see if there is someplace to lube it. If it's not running freely, it will draw more current.

It's even possible that the motor is an innocent bystander and the diodes just failed at the same time the motor was operating (Freud was wrong: there ARE coincidences!). If this machine has an AC motor, I can basically guarantee this.