SX-780 Output Modules

[Watthour]

STK-0050 replacements. I had a pair I had picked up from AudioLab or Georgia a few months ago, since the other sources I had been using were showing no inventory and long lead times. Electronix, B&D, and MAT were cleaned out.

Today I got a 780 with an open E-C on one half of the right channel module. After circuit checkout, resistor replacement, and installation of one new module, the unit was run on the dim bulb for initial checkout. The left module was then replaced just for the sake of symmetry. The unit will now undergo some testing and use for a while on speakers and resistors. If the modules tolerate a reasonable workout I'll report back.

One thing I noted was that the replacement modules were much lighter (mass) than the originals. Of course, I tagged and saved the good module for future use.

 

[Watthour]

After a thorough run-in for 20-something hours, resetting DC offset as close to 0.00 mV as possible (1.2/2.2 mV respectively), the voltage drop across the 0.22Ω emitter resistors is a consistent 2.35mV in all four positions. If my calculator is working correctly, that should be in the area of 10.7mA bias current. It's not wonderful, but it's better than some of the reported measurements I've been able to dig up for some of these modules.

Hopefully it is enough to kill a little of the XO distortion. My ears can't tell, and I don't have the really good source generator to measure accurately with a scope, but I can throw up a waveform once I get it hooked up to what I have just to check it out. I can at least check it at 1 KHz and 4KHz.

 

[Keymaster]

Congrats! I replaced one of my STK's twice in my 780 when I got it and fried both of them. Played great up to about 1/3 volume for as long as I wanted. The minute I tried going to half volume, the moment I turned it back down, pop! I'm wondering if mixing a new power pack with an original on the other side could possibly be the culprit???

 

[bktheking]

I pulled a "lighter" one out of one today, it was blown.