My son sleeps exclusively in a Fisher-Price Cradle ‘n Swing. The swing stopped swinging during the night last night, so I took it apart to see if I could fix it before tonight. The swing is still in pretty good shape, though it is an older model we bought second hand from Once Upon a Child.
When I got the top open, I could smell the odor of burnt electronics. Fortunately, the bad transistor did me the favor of leaving a scorch mark to let me know what needed replacing. The scorch marks are a little hard to see in the first picture, but pretty obvious with the transistor clipped off.
The markings on the part indicate that it is most likely an ON Semiconductor MJE171G PNP transistor. Since we needed the swing tonight, I headed out to the local Radio Shack to see what they have on hand.
There I found a $2 Tip 42 PNP transistor with comparable specs. The only spec that is not up to the original MJE171G is the maximum emitter-base voltage — 7.0 V for the MJE171G, but only 5.0 V for the Tip 42. When I probed the drive circuit without a transistor installed, the maximum emitter-base voltage was less than 4.3V for any setting of the motor speed dial, so the Tip 42 will hopefully be fine. The picture below shows the Tip 42 installed in place of the MJE171G.
The motor still spins when power is applied, but needed a little push to get going. Rather than to save a second trip to the store for a motor, I also got a replacement motor while I was out.
A helpful Instructables page called Repair your FisherPrice cradle swing describes how to re-purpose the motor from an Air Wick air freshener as a replacement for the Fisher-Price swing motor. The local Meijer store had one for $5.
It is interesting that the control PCB pictured in the Instructables article has a TO-220 transistor with a big heat sink prominently mounted on a PCB. It looks like our, apparently older, swing must have had enough transistors blow out that the redesigned with a beefier transistor and better cooling. Had I read the article all the way through and seen the size of the heat sink on the newer model, I would have bought a heat sink, too.
After replacing the transistor and motor, the swing is back in working order. I am sure I could have bought the parts online from Digikey or Mouser for much less, but shipping would have eaten up the difference. Even more importantly, the total price was low enough that getting all the parts fast enough to repair the swing before bedtime was more worthwhile than saving a few dollars.