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Lorin Thwaits A geek says what?

I've got a receiver / amplifier that I bought like 15 years ago from a friend.  Transistor-based, and only 120 watts per channel, but still it's got amazing sound.  All discrete components.  Cleaner than just about anything I've heard.  Will drive down to a 4 ohm load very nicely.  Or at least it _was_ nice...  until about six months ago when an exposed speaker wire briefly made contact wth the FM antenna.  Ouch.  At the time I had some little 8 ohm bookshelf speakers cranked to around 90 decibels, kicking a fair amount of bass.  The output must have been swinging at about +/- 40 volts or so.  There were sparks, okay just one small spark.  After a brief puff of smoke, it was dead.  The fuse had burned.  I didn't have time to troubleshoot it back then, so I just put it up on the shelf in my storeroom.

Fast-forward to today, I'm preparing to head out to Ireland on Thursday to teach a Vista class.  I need a nice voltage converter to go from Ireland's 220V to anywhere between 100 and 130V, and got to thinking that the big transformer in that old receiver might just do the trick.

Any 120V transformer that has a 60V winding will deliver 120V when it's connected to 240V.  So my first test with the big transformer in the receiver was to see if it had a 60V winding (or even a tap near that voltage).  No dice.  The main winding delivers 90V, with no other taps.  Oh well.   But while I had it out, I decided to see just how cooked this thing really was.

I took a look around the tuner section, thinking that it would certainly be fried.  I didn't find anything burned, but also didn't want to tempt fate, so before testing it I disconnected all the lines going to the tuner section.  Then put a fresh fuse in, and turned it on.  It burned the fuse immediately.  So the problem was probably elsewhere.  I figured one or both of the output transistors was fried in one of the channels.  (It's got a big darlington pair for each channel.)  So I did what any brave experimenter would: I put a 15 amp fuse in place of the 5 amp fuse and turned it on, watching for smoke!  (Isn't that what you would do?)  Of course smoke and cracking soon followed.  As soon as it popped I unplugged it (took just like 2 seconds or so), and looked for the cracked parts.  Nothing, but the output section for the right channel was pretty hot.  There was the culprit.

After taking ten minutes to tear down everything I got out the big gun-style Weller soldering iron and desoldered the output transistors for the right channel (the pair on the right in the picture), put it all back together, plugged it back in, and everything jumped to life again!  Even the tuner is fine.  I'm a happy man.  Missing its right side altogether, but at least it's something.  I'll have to order a high quality pair of output transistors to replace it with.  Probably also have to get out the o-scope and make sure the preamp section is still alive.  Maybe this time I'll put in parts with a higher slew rate.

Glad to have tunes again.  But for the trip to Europe I'm still looking for a step-down transformer that will deliver at least 300 watts...

Posted on Sunday, September 30, 2007 7:53 PM Gadgets | Back to top

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