Fender Rhodes germanium to silicon transistor swap

[MichaelNuzum]

Hey everybody- thanks for all the input.

As far as what killed the power transistor, it would be outside my experience to be able to tell, but I have a strong suspicion one of the screws that mounted the transistor ended up getting shorted to chassis. As I mentioned, the transistor was mounted to the rear chassis wall, with a proper mica spacer and heatsink compound. But the screws that went through the case of the transistor and into the chassis wall were mounted with some rubber isolation washers that seemed pretty dry rotted and squished down. This is completely a guess: but I think when the amp was removed from the keyboard, it was enough movement to jostle that screw and washer, shorting the transistor case to ground and killing the transistor. Again, this is just a guess.

I haven't had the time to swap components, but I should be able to dive into it in the next few days. I will report back on what I come up with.

Again, thanks so much for all the help.

[Jazz P Bass]

Google "TO3 transistor bushing" if you need to purchase new ones.

[doctor]

620 is the standard (E24) value.

[mattikari]

For future needs, I want to add the following to this discussion. In my case, while swapping the (dead) transistor to a MJ15023 seemed to work on my bench, it did not work when I installed the PSU back in the piano. After a lot of research it turned out that in fact to make sure the silicon transistor works, the 330 ohm resistor on the base needs to be changed to a higher value, since Vbe of a germanium transistor is lower than on a silicon one. Probably a 470 one would have been enough, but I went to 620 ohm and now it works.

The problem with the stock resistor was that the startup current with both power amp modules and the preamp was too high, causing the regulator not to start. It surely was right on the limit, as when I turned the vibrato on on the preamp, causing one of the lamps to be turned off on the starp, it worked fine. I could even switch the vibrato (really a tremolo) off and everything worked. And it worked without changes with the Vintage Vibe preamp as well, as it does not use incandescent bulbs. FYI!