Darlington Powerpack

[Cosmic Charlie]

Hi to all, nice forum. The other day a friend gave me an SX-580 for helping him hang a door. He said it worked last time he used it (3 or 4 yrs. ago) Well I got it home and blew out a 16th" of dust, and plugged it in, lights up,stereo light lights tape monitor source lights light. The tuning needle moves with switching stations. I get no sound, when I push the A or B speaker button I get a faint crackle in the left speaker and the wattage indicator jumps a little. I checked all 4 visable fuses (is there more that I may have missed), sprayed all pots and switches with deOxit and progold, manipulated all switches and pots for about half an hour. Still no sound. I know I should probably get a manual, but I don't really want to invest $25 into a unit I'm not sure I can fix or would be worth $25 when I'm done. Does anyone have any ideas to what I could check, is there a method or a link to check the Darlington powerpacks, do they go bad, can new ones be aquired. anyone that may have some insight I would be very grateful. I don't see any burn marks on any of the circut boards. It's a sweet looking receiver and my friend said the sound was clear and loud, it cleaned up nice. Anyone know of a cheap manual vendor Thanks for your consideration. CC

 

[B3Nut]

www.electronicsic.com might have the power pack IC's for that unit, they could be at fault. With the speaker switch engaged (and no speakers connected) measure the speaker outputs for DC voltage. Replacing the power packs wouldn't hurt, they aren't very expensive. I had a 680 that had no sound in one channel...bought a new output IC and it came back to life. The 580 and 680 have their power packs socketed, so you just remove the screws and pull the old one out. Clean the heatsink and apply thermal grease to the new chip, and pop it in and screw it down. Pizza cake.

TP

 

[Cosmic Charlie]

Hey, B3nut thanks for your reply. I checked DC voltage at the speaker terminals and got 2.1 rt. and 000 lft. when i switch powerpack Rt to Lft it reads vice versa. I checked Ohm on the pins at 2 and 5 I get 100 Ohms difference between powerpacks one read 280 the other 180 does that mean its bad should I replace both? That link looks positive on replacement power packs has my number only $19 apiece. Anything else you can think of I should check. Thanks CC

 

[B3Nut]

Not sure on what the resistance measurement is measuring within the IC as I don't have the pinout handy. $19 seems a bit steep...I wonder if www.electronix.com would have them, I bought an STK-0050 from them for $6. Worth a shot replacing them at least...

TP

 

[Cosmic Charlie]

Okay, I replaced the Darlingtons, no sound. I looked over the boards a little closer and noticed a burn on the bottem of, I guess its the amplifier board. The componant that corrosponds on top, looks like a transister in its own heatsink, screwed to a peice of channel standing upright, there is only one, is this the voltage regulator, can I test it, could it be my problem or part of my problem. I removed the burnt part, it has a symbol like Mitsubishi 3 diamonds in a circle with numbers D 712 under that 8ZH-C I cant find the part or the value for the part, can anyone help me. I'm very new with electronics but would like to learn. Thanks or no thanks CC

 

[B3Nut]

Ah, a classic case of an overheated regulator transistor. Wasn't sure if this afflicted the 580's...it's a common failure on 780's. Try re-soldering the leads to that transistor and see if that brings it back up.

TP

 

[EchoWars]

That transistor is a 2SD712. With Japanese transistors, the '2S' is assumed.

It may not be a bad transistor, but simply desoldered itself since this one gets so hot (as B3Nut suggested). Test it with the diode test function on a multimeter, and if its OK, put it back. If you don't know how to test a transistor with a diode tester, Google is your friend.

If it tests bad, put a On-Semi MJE15032 in there and it'll probably never give you any more trouble.

 

[Cosmic Charlie]

Thank you both so much, will do, and report back. Wish I'd seen that sooner. CC

 

[Cosmic Charlie]

Well guys, I checked the transistor and got 1185 hFE. I checked dc offset got 12mV per side. Not knowing the specs for the transistor I thought I'd read somewhere in these forums that as long as its not 0 or real low that it was probably good. While the trans was out I lost my stereo ind. light and the tape mon source light, back on with reinstallation. Fuses all check good, power pacs new, any suggestions. I'm not ready to give up yet. I probably should have skipped the power pacs and got the svc man. Thought it may be an easy fix. (not with my luck) Any way anything anyone can think of to check I'm open to ideas. Is there any other known problems with this line of receivers. Thanks CC

 

[EchoWars]

You need to be checking supply voltages, both regulated and unregulated. As far as the cost of a manual, think of it as an investment in your education, not money down the drain for a 'not-worthy' unit.

 

[Cosmic Charlie]

The manual is on its way. I guess i really do need it