Help with Yamaha amp

I’m hoping somebody can give me direction. I have an Yamaha AX – 900 amplifier. While listening to records the unit suddenly made a loud pop and both speakers became silent. This is what I’ve done so far. After opening the unit on the top I examined all the capacitor. None were blown or bulging. I looked over the board and transistors and do not see any cracks or burnt soldering spots. Then I removedthe base of the unit did the same and did not see any burnt areas. When I turn the unit on there is a smell that develops very faintly near these two very large caps So I shut it off. I’ve read that the transistor outputs tend to lead to this problem. I have not checked voltages across these transistors. But I assume that’s my next project.There are some large Capacitors so I’m a little cautious about touching anything. I don’t know if the preamp is working correctly.Anyway I’m at my limits. I would appreciate any help. I don’t know if it’s worth fixing. It’s an old amplifier from the 1980s. But sounded pretty good …still. Any and all help is appreciated.
Brian

Use google to find the “service manual” (I see a few out there), this will list everything you’ll need to test and give more information than you’ll need. Start with the electronics commandment “thou shalt check voltages”, and see which areas aren’t getting their appropriate voltages. Then use the block diagrams to back trace what is malfunctioning.

Sky,
I appreciate your help. I’m gonna start researching which service manual people find most helpful. Can you point me in a starting direction? Preamp vs amp. Caps, transistors etc. I have a multimeter but the task ahead seems dounting. I even thought it might be at the RCA plugs or speaker connection.

I know Ohms law but not much more about electronics. How careful do I need to be about the big caps? Even the small ones?

Thanks again for your input.

good day,

I take you are looking for advice on how to diagnose and fix. Welll to you first question. you want to take about 5k ohm about 1w or 2 watts and touch the terminals of the big caps with the resistor and not your hands. You could get very hurt by big caps that have signification charge. but if this unit has not been used in a month or two charge leakage on the capacitors has most likely cause the voltage on the capacitors to be at zero. Just be safe.

You then want to look at electrolytic capacitors (metal canisters with on side with painted dash. Unit has two solder connections and is polarized) to see if the capacitors are distended. Next look for the fuses… often before or after the full bridge rectifier( if there is one). This should be right next to any power transformer (big blocky iron thing. has for solder connections to pcb board). clean the board and connectors. This should be the first step in evaluating the condition of the board and help you diagnose the issue.

Other steps that may help in determining what is broken on the board.

find a schematic. I looked i could not find one.

Understand what type of amplification is being used. (two stage with pre op amp and power op-amp… Or switch mode amplifier(i quit if i go to this or just see if there is a replacement board from manufacturer).

a good book on diagnosis could do wonders try half price books, barnes nobles, or amazon.

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Thanks for your help ceramic. I found some schematics I didn’t check voltages on the two large caps. They were 130 and 123 with the machine on and then I ran the voltage with the machine off. One pair charged to the full 72 V and the other pair only charged to 50 V. Before both started diminishing I assume the capacitor that in charge fully is not working properly but whether this is a problem or not I still don’t know. I think I’m going to bring it to a Yamaha certified repair center. Before any greater damage develops. I think everyone for their help.

Found this -

Disconnect your speakers and check them. Hopefully that loud pop didn’t result in a blown speaker coil or filter.

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I checked the two large cabs that are Label 22,000 micro Faraday at 71 V. One cap peaked at 130 V and the other at 123 V when the unit was turned on. With the unit shut off one maximized at 72 V and the other at 50 V before they both discharged.

So I assume there’s a short somewhere. And checking this board may be way over my head. Also I checked the transistors. Half of them read .4 V, one read 14.4 V and three were 0 V. Not sure what to make of that

Is it even worth taking to a technician and spending $100 to find out what it is.?

Being an 80’s unit, it’s likely your electrolytic caps are leaking which may have caused a bridge or short inside, which is why you’re reading full line voltage on one. Typical life of those caps is 30-40yrs if you’re lucky, so it’s not unusual. As long as it didn’t blow anything other than a protection diode or two (and the pop didn’t get your speakers as noted above), replacing all the electrolytic caps would be a good start regardless of whether the problem originated there or not.
Take note before pulling anything of location, size, voltage, capacity, and orientation (+/-) if you have any issues reading schematics. Draw yourself a block diagram or print out a top-down photo and take notes on it directly.

Take photos from different directions as well so you have a record of where things go and what direction the caps need to be.

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