Here's my sad story about how some bad transistors blew up one channel of a Marantz 240. I had just completed rebuilding the
unit... a compete restoration with new capacitors, Pre-drive transistor, etc. I had spent a lot of time and money on this one. I
replaced all the output transistor because the ones that were in it were not original and were a mix from differing manufactures.
I had done all the voltage checks prior to installing the new transistors. All was well....
I slipped the new transistors in and started the unit up... seemed ok. I hooked up a 1khz test tone and turned the volume up a
bit. The wave form began to distort and zap... a small puff of smoke and the right channel went silent.
Well... I could not figure this one out. On investigation I found substantial damage to the channel... It was toasted
On a hunch, I cut the Transistors open. I am no expert on counterfeits, but the white glue that held the die directly to the back
plate seemed funny to me. I cut open a like part number form another source to compare. The difference was dramatic. I took
some photo's of the pair and sent them to a couple of highly regarded Audio engineers. They both came to the same
conclusion (independently)... The dies were too small.... Both were counterfeit!!! Rod Elliott has done a lot or research on the
counterfeit issue. His website covers the issue in depth (http://sound.westhost.com and
Well, even the one I thought was real seemed to be suspect. The next morning I spent a couple of hours going through my
inventory of transistors. I keep my transistors separated by source as well as type. So I took one example from each source
that I had any doubt about (I consider Mouser and Newark safe) and cut it open. Wow, was I surprised. Of six, I only found two
You might think... "well, he's buying transistor on ebay". Not so... I learned my lesson long ago about ebay sources. Do your self's
a favor and do not buy output transistors on ebay... risky business. Especially sources in China, or Hong Kong.
The transistors pictured above were all purchased from on-line electronics dealers. Sources I thought were safe. I sent the
dealer images of the transistors he sold me that blew up my Marantz, just to warn him he may have counterfeits in his inventory.
I never received a reply.... which I find strange. Either he knows and does not care, or he just does not what to believe it. I am
not going to name sources here as I am no expert on this matter. But be warned... the problem of counterfeit transistors is far
more wide spread then I had ever imagined. I am getting counterfeits from sources that are fairly large distributors of electronics.
Needless to say I trashed about a quarter of my output transistor inventory.
The ones below are Real... the ones above are suspect. Some things to note about fakes...
1) Die is too small
2) The connecting leads are thin and flimsy
3) Lettering is often different (orientation, or marking order, etc...)
4) Sometimes (but not always) the lettering will wipe off easily with a solvent (know as the "acetone test")