Yamaha RX496 short ckt.

Jan 28, 2008 4 Replies
B

My Yamaha RX496 Tuner-Amplifier has a problem in its Power Supply Ckts. It appeared that the internal thermal fuse in the primary transformer is blown. I have by-passed that fuse, but when turning on the amplifier it draws well over 5 Amp at 240 VAC. Only when I disconnect the two RED-wires running from the Impedance switch to the Main PCB will return Prim Current to a low 100 mA or so level.



I dunno what's really connected to these 2 red wires, I belief it will be a rectifier bridge and some Big Cap smoothing Filter finally feeding the final output stage.



Doing a straight forward Trouble Shooting on Bridge rectfier and Elco as well as Output transistors did not show any suspects.



Is there any help out here telling me the most likely cause of error? Mark Z, if you are still around and can tell me an equivalent or the original ckt diagram ?



Thanks Brasto


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M

This almost NEVER happens unless someone has defeated the primary fuse and in this case, probably the turn-on relay as well.

If the outputs are really good,check the filter caps and look to see if a transistor insulator may have shorted through to the heastsink.

I'll try sending the manual direct.

Mark Z.

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B

Mark, as allways a great TY for your support. I now have a clue where the two red ones run to. I also see a few jumpers which I can break to isolate the short. I'll keep you posted on results......

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B

Mark, Problem solved, Yamaha restored to manufacturers specs. I used an external adjustable powersupply to feed the main pcb of the Yamaha. It appeared that as soon as I crancked the voltage above ~~13VDC on the Bridge rectifier inputs that the current sink avalanched from a few mA to a full short.

Problem appeared to be the bridge rectifier S4VB20 which passes the normal VOM checks but fails when voltages above 13 VDC are applied.

Again thanks for your support. Bram

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M

Never should have taken out the power transformer thermal link. Somebody overfused the thing or otherwise defeated the manufacturer's protective measures. Glad you got to the bottom of it, but now you're on the hook from a liability standpoint if it burns down somebody's house, even years later. The correct option is to also replace the power transformer with a new one. Of course this throws off the cost equation - the transformer is going to be the most expensive part in the thing.

Mark Z.

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