SAE X-25A Schottky diodes

[stevieg123]

Hi all, recently picked up one of these amps with one ch. distorted. Tracked down the offending parts to the two Schottky diodes CR14 and CR15. See attached link to print of output section: http://www.wardsweb.org/audio/docs/X25_schematic2.tif the parts are marked TRW 11-0202, and with what I now believe is date code, 7917. I originally thought the 7917 may have been the TRW in house part # so I tried finding a cross for that #, that led me to a 1N2977 13 volt 10 watt zener in a stud mount package. Since these schottky diodes are in stud packages, I thought I was on the right track. I was not. After closer inspection, what I need is the schottky diodes and not zeners. Any help from you guys in this would be greatly appreciated as I really want to hear this amp thur my M-L's to see if what everyone says is true about these old amps, plus, I really do need to sell it after I listen to it for a while to pay for some recent health problems. Thanks in advance, Steve

[indianajo]

Dead PCAT switcher power suppliies are full of schottky double diodes in the 2 or 3 amp versions, 45 v PIV max. 'this is a TO220 or TO247 package, screwed to the heat sink on an insulator. You can also buy these from distributors. Good luck on the stud mount, I can't even get stud mount recitifers for car battery chargers anymore, only TO220 double rectifiers with one lead cut off to make it a single diode. Had to put a real heat sink in the battery charger that was struck by lightning. Get your mica or thermalloy heat sink insulator on the same order, or salvage it from the dead PCAT power supply. I discovered by salvaging, that mica insulators are not the thing now, they make some green mesh plastic that doesn't need heat sink compound. Mouser doesn't seem to sell mica insulators , and newark doesn't seem to sell the thermalloy stuff for TO247 (TO3P).

[stevieg123]

Hey Jo, thanks for the tips, I really think I need to get some more facts about the original schottkys before I start stuffing salvage parts into her. Mainly 'cause it's a real P.I.T.A. to put the diodes in this thing as the entire output board needs to be loose from all the output transistor sockets (12 x 3) leads to solder/unsolder to uninstall/reinstall the board with the diodes on it. Any ideas of someone, anyone who used to work for TRW, SAE, or anything like that who may have original specs on those schottky diodes??? Anyone,??? Thanks, Steve

[EchoWars]

You say the originals are stud devices? Seems that all you can find now are TO-220 or TO-247 packages. In any case, look at the schematic...I can't say that I totally understand what SAE was trying to do with this output stage, but the diodes are obviously there because of R67 and R68 and to prevent DC from getting to the output line.



That being the case, I'd not get too hung up on lots of specifications...besides the style of the case, there's only two specs that are important - standoff voltage and current-carrying capability. There are any number of devices available at Digikey that are rated for large voltages and current, such as the TO-220 style On-Semi MBR40250G (rated for 250V & 40A). The problem is that mounting might be a issue, and the device will need adequate heatsinking.

Unless someone here can give me a good reason not to, I'd be tempted to remove R67 and R68, and replace CR14 and CR15 with a jumper wire...as a test at least, to see that the amp is capable of undistorted output. I can't say I'm thrilled with this resistor & diode thing, as it just seems like one more way to have something go wrong and send boatloads of DC to the speakers so you can watch them go up in flames. (Edit: Oh...this is a class A amp? Then the output stage makes a little more sense.)

Also, FWIW, this thread really belongs in the Solid State forum. I'd ask a moderator to move it there.

[stevieg123]

Hello Echo, yeah, that output section doesn't resemble anything I've seen in my 30+ years of servicing big amps of all kinds. If you look at it, all the emitter resistors of both banks of outputs, are tied together and then the full total output signal is passed thru each one of the diodes to the load, while the two resistors, R67 & R68 are there, seemingly, to keep the outputs biased on all the time (class A)?

What I did when I got this amp was to tear it down, checking all likely parts from most likely to be bad, down to least likely to be bad, hense, thses big, high current, stud diodes were in fact, my absolute last parts to check. When I removed the wires attached to the terminal end of each one, I used my "curve tracer" in my bench to test them. Sure enough, one was a perfect looking diode junction, and the other was almost a pure resistance reading. So, then I decided to just pull one from the good channel and sub it into the bad channel just to make sure the bad channel would work with two good diodes in it. Well, long story short, I think the heat involved in removing the diodes from the good channel caused both of them to go bad during removal. They both had checked good before heating them up with the big 300 watt Weller that was required to unsolder the stubs. Ok, now out of 4 original schottky diodes, I have only one, the good diode in the bad channel, left now to test and try to figure out it's parameters to try to find currently available replacements. Any ideas for a test jig (power supply, amp/volt meters, etc) hooked up so as to give me the forward turn on voltage, and any other critical specs?

Thanks, Steve

[EchoWars]

If the amp is running in Class A, we don't really care about Vf. Like I said before, mounting style, standoff voltage, and current-carrying capability are what you need to be concerned with.