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Old 16th May 2011, 02:27 PM   #1
harrydg is offline harrydg  Belgium
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Default yamaha ax 470 problem

Hey all,

First of all, i'm new here. Second: i don't know a real lot about amps/electronics... just the basics but i'm learning quickly
now to business:
i've gotten a yamaha ax470 amp for repairs (from a theatrical group)... What went wrong? some genious thought it would be a good idea to avoid having to by a mixing panel, and just said: put 1 pair of speakers on the right channel, other pair on the left channel and crossfade like that. (i don't know if this is relevant).
But then... since he fancied a beer but was so concerned he put a towel over the amp... which overheated. Result: "click"... no more sound. After some time, it came back on... and off again.

What do i think happens: thermal protection fault. Should be relatively easy to fix, just slap the guy and desolder the "sensor" and replace it with a new one. Without much thinking i looked inside, saw the 2 large cooling blocks with 2 transistors attached to each, and then, with a lot of cooling paste: some very small thingy which made me think: ah... thermal sensor?
so i desoldered it, went looking on what it is, but couldn't find anything on the web, but the closest i see is: 2sc2603 (it says 603 27E on the transistor?)
anyway, i thought... hell, try it without thermal protection first... and while pushing the power button, i got a nice white light from the fuse... and off course everything is gone now...

To make things a bit more clear and for those who need more info on what i did (wrong). and to make you look no further:
http://harry.enzoverder.be/YAMAHA%20AX-470%20SM--.pdf
the service manual...
The ones that i desoldered are: Q117 and Q118

Now off course the questions: is it possible that this blew up my amp? if not, i'll look later today on what else i might have missed when reassembling.

Second: how can i fix the initial problem?

(the inputs worked fine before a started fiddling with it, output as well, except off course the cutoff after some time)

Thanks in advance! (ow... and please, if someone knows any good reads on this, it'd be very much appreciated)
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Old 16th May 2011, 04:13 PM   #2
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yeap !!! your amp is blown for good ... importand is that you did it in a very funcy way ... actually this was an original and one of the best laughs i had in my life .....

The transistors you removed are not thermostats or part of the thermal protection ... the specific transitors are parts of a specific circuit that is called Vbe multiplier and its a circuit that automatically adjust the bias of your amplifier while monitoring the temparature developed on the heatsinks depending on the use of the amplifier

Removing the transitor might cause many troubles such is blown outs or drivers ...Though i think that your outputs have been blown before this happend ...

To repair this has to be done by someone that understands basics or a tech guy ... i dont think that this will be cheap

Answer to your next question is NO .... no one here will help you ore quide you through to repair this since for you to do that means understanding the basics which obviously you dont ....

To be more practical you need to verify voltage in all stages of the amplifier according to the schematic , then you need to check each and every semiconductor arround the stages of the amplifier and then check every diode and every resistor .... then replace every shorted semiconductor or open resistor ...replace the transitors remooved and you are done ...

sorry regards sakis
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Old 16th May 2011, 04:29 PM   #3
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By desoldering Q117 and 118, you have removed the bias control from the amplifier circuit so therefore over-biased the transistors and more than likely blown the output transistors up too. The reason it is mounted on the heatsink is that BJT's (bipolar junction transistors-the ones used in that amplifier) have a positive temperature coefficient so as the temperature increases, they allow more current to flow. This is undesired and can cause `thermal runaway` in amplifiers where something similar to a chain reaction happens where the current through the transistors keeps on increasing until their limits are reached.

The purpose of this small transistor mounted on the heatsink is to act as a sort-of thermal sensor. Instead of feeding back to the control systems in the amplifier though (which control the inputs and outputs and display etc.), it is an integral part of the amplifier circuit and helps control the bias current in the output transistors. It does this by being mounted in contact with the heatsink so as the temperature goes up, the current through the output transistors is decreased so they remain within their limits.

Removing those transistors without knowing what they are for has almost certainly made your initial problem worse. It may have been an easy fix previously but now it may be more difficult and may require testing some parts to see if they are dead.

My first suggestion would certainly be to re-fit the removed transistors (it is unlikely that there is anything wrong with them) and desolder the main output transistors (physically larger than the ones you removed and 2 bolted to each heatsink). You will then need a multimeter to test them according to the procedure here. You will also need the datasheet for each transistor to figure out which lead is which (base collector and emitter).

Datasheet for 2SA1491
Datasheet for 2SC3855

It would also help to know what kind of knowledge you have along with what test equipment you have too.

Edit: Sorry sakis, didn't see your reply before I started writing mine!
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Old 17th May 2011, 07:57 AM   #4
harrydg is offline harrydg  Belgium
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bleh... i knew i should have looked at the schematics first... but hey... the amp was broken, now it's still broken. Nothing lost here...

i'll still resolder the transistors, replace the fuse and see if you're right about "everything is gone"...

btw. this is exactly the reason for the question at the bottom of my first post...
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