JP1 Remotes

[kupakai]

I have an old Yamaha RX-S70 mini system, and my wife, in attempt to keep the dust off, covered the receiver's top with cloth, and of course something blew out. I was hoping it was just a fuse, but it would burn out immediately if I replaced it. I do see two large capacitors with white hardened stuff on the base, but it looks like it could have been always there (this burned out a long time ago, I just haven't gotten around to it). I know this is a bit vague, but can anyone suggest where I can start looking to see what the problem might be?

Thanks

[The Robman]

Zaphod should be along later with the experts opinion, but if you have capacitors with gunk around the base, it means they're blown and should be replaced. Now the question is, what else needs replacing as I doubt the capacitors died alone, but for that you'll need Steve (Zaphod).

In the meantime, look around the PCB to see if you can spot anything else that looks blown.
_________________
Rob
www.hifi-remote.com
Please don't PM me with remote questions, post them in the forums so all the experts can help!

[zaphod7501]

I'm not normally here in the middle of the day but I'm taking a short vacation, so here goes. The white stuff was probably just glue used to keep the caps from physical oscillations so it's probably not important.

The odds are that the outputs blew (don't put more fuses in, you'll do more damage). There are two types of output circuits possible: discrete output transistors and power ICs (STK-xxx typically). Accidentally crossing speaker leads (however briefly) is as likely a culprit as overheating.

Discrete transistor shorting is potentially the cheapest fix but the most likely to have additional parts besides the power transistors failed. To test you would discharge the main filter capacitors http://www.hifi-remote.com/forums/viewtopic.php?t=7563 and find the large transistors mounted on a big heat sink (hunk of aluminum) and check for shorts between leads with a multimeter that can resolve down to at least 10 ohms. Note the type marked on the part (most have the jdec type number with the 2S leading digits missing - a B1620 would be a 2SB1620 when trying to find one). Actually getting to and replacing power output transistors (or almost any other part) can be very difficult on mini systems. At this point you can generally unsolder the shorted pair, replace the fuse and see if the unit will work normally (with one set of speakers inactive). Generally disconnect the speakers and test initially with headphones. If it works, you could buy a pair of transistors, solder them in, and be good to go; or there could be other problems that would be way beyond DIY.

Power ICs are easier to spot (large rectangular 1/4 inch thick blocks attached to a heat sink with 12 - 30 wire leads out the bottom). Even with a schematic it might not be possible to determine if one is shorted. You generally have to unsolder all the leads on one (there could be 2 or 3 in a set) and test by replacing the fuse (no other way around that) to see if it operates. The good thing is that there is usually no other circuitry to fail, it's all inside the IC. Some of them are relatively cheap, some are extraordinarily expensive, like the RSN-xxx series that Panasonic uses.

It's not too bad in a shop situation but it could be a little overwhelming for a beginner or hobbyist. Plain receivers are usually much easier than mini-systems and some HTIBs can be downright bizarre.
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Just call me Zaphod (or Steve) --- I never should have started using numbers in a screen name but I just can't stop now.

[kupakai]

Thanks for the tip Rob and Steve!

I checked out the transistors and found two sets of transistors labelled A1694 and C4467. One pair seems ok, but both transistors of the other pair shows shorting between all of the leads. I'll try disconnecting the shorted pair and test it as you said. Hopefully, this will be the problem, but if not, at least I get to finally shop for a new system.

[zaphod7501]

The replacement transistors will be called 2SA1694 and 2SC4467. MCM Electronics has them for about $6 each. Other sources will be equivalently priced. Don't use generic subs except as a last resort.

If you can't get at them easily to unsolder, for test purposes you can cut the leads with side cutters (they're already obviously bad).
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Just call me Zaphod (or Steve) --- I never should have started using numbers in a screen name but I just can't stop now.

[kupakai]

I cut the leads to the bad transistors and replaced the fuse to power it up, and lo and behold, I get sound out the the headphones I connected! It looks like the front VFD display burned out, but everything else seems to work. Now I'll just have to order the parts and see if there's anything I can do about the display (probably not), while I replace the transistors.

Thank you very much! My wife asked a service center about repairs, and they wouldn't look at it without a non-refundable deposit (actually, not an unreasonable request), and we figured it wouldn't be worth the cost of the repair, but you honed in on the problem right away. Now, I might be able to get it to working condition fairly inexpensively. Thank you again, Steve! And thank you Rob for a great site!

[The Robman]

That's what we're all about! Hope you get the display fixed also.
_________________
Rob
www.hifi-remote.com
Please don't PM me with remote questions, post them in the forums so all the experts can help!

[zaphod7501]

Sometimes when outputs fail they take out multiple fuses or fusible resistors. Look around.
When you are changing outputs, you might want to check the soldering on the small power regulators. These are usually moderately sized transistors mounted on small heat sinks and heat can cause them to become unsoldered.
_________________
Just call me Zaphod (or Steve) --- I never should have started using numbers in a screen name but I just can't stop now.