In our last episode of Soldering Iron Justice, our intrepid technician was befuddled by a capacitor that was really a thermistor. Could the part be located? Would it in fact enable the repair? Could sonic mojo be restored to the Stax Earspeaker Energizer, SRD-7?
It turns out, the answers were and are: yes.
Mouser Electronics to the rescue! They sell Murata thermistors, in this case 4.7 ohms @ 140 volts. They are a fraction of the size of the torched thermistor, but I can live with that especially since the prior examples were bent to fit and one lay across a wire wound resistor which contributed to its demise. I purchased 2 for the sake of symmetry, a buck each.
My original effort on the SRD-7 was to replace the pair of 1 mfd 160 volt electrolytic caps with fairly compact polpropylene 250 volt caps. The new thermistors were a bit tricky to install because they were nearly too small for the leads to reach, but with a bit of gentle coaxing they reached and the solder grabbed.
In the photo above, the thermistors are the brown disks above the left hand transformer. The right one is torched. A black wire wound resistor is next to each thermistor. The 1 mfd film caps are the white lozenges in the middle of the board. The small, loose caps at the top were the originals, now removed. Time passed... and the thermistors were subsequenty replaced.
I brought the SRD-7 up on the variac without any smoke so I hooked up the pair of Stax SR-X Mark 3's. The energizer was driven by the trusty Advent 300 and when it was lit I heard sweet, sweet music.
Electrostatic headphones are ear candy deluxe. If everyone on earth had a pair connected to an Advent 300 there would be no war. Tripping Daisy would be everyone's favorite band. The sound is effortless and extended. Listening to Austin's classical station, KMFA, I could clearly hear the crappy digital sideband hiss, but the music sounded so good I really didn't care. I am not sure I can give higher praise without an eargasm.
Given the age of the SRD-7 (introduced in 1971) it would seem to me that the first order of business for anyone who owns one should be to replace the two electrolytic caps. I also wonder about the location of the thermistors since the originals are large and easily bent over to touch the large resistors next to them. Not good. Only 4 screws separate one from peace of mind, so open it up and make sure the thermistors are not bent over, touching the resistors. Only 2 more screws need to be removed to actually work on the unit.
If you have a chance to acquire a pair of Stax headphones, I highly recommend that you do. You will thank me. And you're welcome.