Yamaha RX-A430 no go.

I was asked if I could fix this amplifier, it came with no background so I haven't turned it on yet? What damage can an amplifier sustain if it is set to 8 ohms and is connected to 6-ohm speakers? I can't find anything wrong so far with this amplifier except for the incredible amount of dust and grime and the impedance setting?
 
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mbz

Member
2013-07-05 1:12 am
Use cautious power up procedures, ie, dim bulb tester, variac,,, and monitor psu
voltages and bias allowing for some transients during power up. Relay click is good,
possibly the dust has found its way into the switches??? Any pre-out/main-in? would
be a good test...
 
I took it apart and cleaned it up, all the transistors and diodes read Ok so with trembling hands and a DBT I fired it up no smoke or problem but I only heard one relay click in and that was on the main power supply board going to the primary on the transformer? The positive and negative power rails seem to be alright at 32.50v It's supposed to be 33.3v going by the Service Manual so that's close enough, the other voltages are looking Ok so far. I'm just getting used to this service manual, it covers about 8 other models so it's a bit of a headache finding my way around. I will check the ICs, voltage regulators, etc, and if I run into trouble you'll be sure to hear from me.
 
The DC Offset is 9 ohms on the front main speakers and less than that on the rest. I had it playing music in stereo on the two main speakers for about 4 hours today, then I cranked some Led Zep up only for the amp to go into protection after 15 minutes, the heatsink wasn't hot and the transformer was just warm. I've been picking bits of speaker wire out of the amp I have about 15 one centimeter pieces now, I can't find any component that has failed so I suspect these bits of copper wire is what the problem is? I disconnected the speakers and plugged in headphones when I turned the amp on I got a message on the display saying I should check the SP (Speaker) wires?
 
Red, the offset is measured in volts (hopefully, millivolts). Was 9 ohms a typo? One of the things that causes an amp to 'go into protection' is DC offset on the outputs. Hopefully it wasn't 9 volts!

No mate, it was a typo. It was probably less than 9 mv DC Offset because the amp hadn't had time to settle down. So far I have pulled the amp apart and checked everything and can't find a thing wrong except for some poor soldering so I reflowed those joints and assembled it back together. I turned it on with a DBT and got a slight glow on a 72-watt bulb so it looks like a short somewhere? It looks like a normal bias glow on the DBT but when it was working the other day there was no glow at all? Do you think it would be Ok to switch it on to do some voltage checks to track this short down? I'm used to working on Sansui amplifiers, the Yamaha service manual for this is full of acronyms with no index to find out what they mean? So its been slow going trying to figure things out? I'll do another DC Offset check and get back about it soon if you think that's Ok?
 
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