Yamaha AX-500 Problem

DrDreas

Member
Hey guys,

So I am trying to get this AX-500 working, I've had one previously and really liked it and kinda regret selling it. This other one came along cheap and looks clean as, but the relay is not tripping. I've had a really quick look, but anything beyond simple and obvious I usually leave to my tech. I've searched via these forums and google to no real avail, but the following has been observed thus far:

I noticed I think it was C191 and 192 were bulging and open at the top so that was the first thing I addressed. I found a couple of caps the same value in my Onkyo parts mule (the two biggish grey ones on the pics) and that didn't do anything, but the strange thing is they are getting quite hot. This leads me to assume the problem is not the caps but whatever was responsible for wrecking the others I replaced.

5b7acb032dc0e7f37723d55bea1c7206.4198.jpg


The area surrounding Q113,117 and 111 seems to be getting hot also, if the darkness around that area is any indication

656a34c78111f4440810deb945e0c806.4198.jpg


At this point I have taken it to my repair guy. He says there is around 6vDC offset and I think he referred to a "ripple current" that is present in a lot of places? He noted C201 (the greeen one?) and mentioned that he couldn't clearly identify it's function, that the output transistors were not the problem and that whatever was going on was not specific to the left or right channel.

Here are some excerpts from the service manual

Screen Shot 2015-05-23 at 12.46.00 am.jpg

Screen Shot 2015-05-23 at 12.47.29 am.jpg

Screen Shot 2015-05-23 at 12.47.49 am.jpg

Now I found this bit of text that poses a question relating to it:

"Does anyone understand the unusual VAS current loading arrangement used here? The voltage amplifier stage is loaded by a current mirror. Unusually the input side of the current mirror is programmed by R208 and C197, via Q117. The base of Q117 is biased mid way between the base of the two driver transistors, Q121 and Q123. The other side of R208 and C197 is bootstrapped to the output rail via electrolytic capacitor C201. It looks like some kind of feed-forward error correction around the output stage. Or maybe there is another logical explaination why this arrangement is used. Any comments?"

And also this from another AK thread relating to this amp:

Screen Shot 2015-05-11 at 11.04.24 pm.png

Not being much of a tech myself I have no idea, but am hoping this triggers a piece of info someone with some experience on this or similar Yamaha model may have. The reason I like this tech is that he is very honest and open about work he performs, plus his experience usually means he is very quick at troubleshooting...and less time spent with trial and error removal and testing of components means I usually avoid the "arm and a leg" repair charges these guys can be notorious for! but he's feeling like a lot of time could be wasted here on something that turns out to be simple so I'm hoping to help narrow it down for him and save some cash! Is there anything obvious that I could pass on to narrow down the list of culprit components here?

Many thanks in advance :rockon:
 
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6 Volts of DC Offset is a lot. Your amp seems to be in protection mode. The relay won't click.

Can you pull those "burned" transistors (Q111, Q113, Q117, etc) off and test them?

VR106 and VR107 trimmers look that bias have different values at test points.

Can you take AC and DC voltage measurements with a multimeter?

Is your unit an EU model?

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The resistor I pointed out in the photo looks burnt. If it is.Pull and check the two output transistors closest to the two large capacitors.:scratch2:

R226 should be around 4.7Ω. Base limiting resistor (fusible) for output transistor Q126.
 

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Could just be a shadow in the photo. But if it is burnt theres probably a few more as well. Even some diodes and other transistors can't be ruled out.:scratch2:
 
You have non-original output transistors, probably fake and blown. See the single green one? Originally there were two of those. ;)
 
Another strange thing is that grey capacitor "pushing away" the surrounding components... Is that a stock cap? Which value is that? :scratch2:

A few more pictures (taken in day light) would help further.

2aq2qa.jpg
 
6 Volts of DC Offset is a lot. Your amp seems to be in protection mode. The relay won't click.

Can you pull those "burned" transistors (Q111, Q113, Q117, etc) off and test them?


Is your unit an EU model?

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It's an Australian model. I was hoping to avoid doing it this way, but it seems we might have to start pulling bits one by one :/

The resistor I pointed out in the photo looks burnt. If it is.Pull and check the two output transistors closest to the two large capacitors.:scratch2:

R226 should be around 4.7Ω. Base limiting resistor (fusible) for output transistor Q126.

I'll check on that resister, thanks. The output transistors are apparently not the problem

Another strange thing is that grey capacitor "pushing away" the surrounding components... Is that a stock cap? Which value is that? :scratch2:

Those are the 2 (Visibly stressed) That I replaced, that are now heating up. Same value as original, but physically a little bigger
 
I was hoping to avoid doing it this way, but it seems we might have to start pulling bits one by one :/

Hi,

If the resistor avionic pointed is burnt, I would check the output transistor(s) first. :thmbsp:
 
Output transistors have been ruled out. The man with the soldering iron has said "It has to be a power supply issue, one that is common to both channels". I mentioned to check the resistor that looks burnt but that doesn't seem to be the case here. It's a weird one...thanks for the replies, but it looks like "exploratory surgery" is now the only option :D:yes::no:
 
Output transistors have been ruled out. The man with the soldering iron has said "It has to be a power supply issue, one that is common to both channels". I mentioned to check the resistor that looks burnt but that doesn't seem to be the case here. It's a weird one...thanks for the replies, but it looks like "exploratory surgery" is now the only option :D:yes::no:

Shouldn't be to tough. Its not a complicated power supply. Make sure you have your +/- 16 volts . Key is equal + / - voltage at this point.D125 and D126 both are 16 volt zener diodes and is the good test point for the +/- 16vdc.
And DC imbalance (offset) back in the pre-amp circuit will produce a DC offset at the power amplifiers output.
 
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Ding ding ding! Problem has been found. For those playing at home:

"It looks like some muting transistors (Q137, Q138) have gone faulty, prob due to over voltage when that Zener (D125) had a dry joint on it. Unbelievably both L & R muting transistors failed in exactly the same way"

Thanks again for the replies :)
 
Hi!

I know it's an old thread but maybe someone is still interested in this. I too have an AX-500 which I attempted to recap, but eventually failed to start the amp after it (power led on, but no click from relay). As suggested by Avionic, I measured the DC voltage at D125-126, at D125 I've got 16.45V, at D126 it is jumping between 4.19 and 4.8. As my knowledge is close to 0, can I ask somebody to help? :) I removed 1 output transistor Q126 as it is shorted, the replacement part should arrive this weekend. What should I check? (solder joints are ok, no shorts)
 
I'm still waiting for the replacement transistor, anyone have an idea what should be done to find the cause of the problem?
 
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