Problem with Yamaha EMX200

[blandBill]

Hi. I have an EMX200 which will overheat and blow output xisitors, w/o fan coming on. I have traced problem up to the 2ea Temperature resistors- STR60R70; Part number IF 006210; also have 60L on small rectangular case. 1 reads over 600k cold and other reads around 400k cold---about 45 degrees F. I am supposing these are not correct, but cannot find any info on them or replacements. Looks like the EMX 150, 200 and 300 used the same parts. I'm thinking these are thermistors. Does anyone have experience with these PA amps and can help me? My kids use this for their 'kids band' and getting it up would be wonderful THANKS!! Bill

[Enzo]

I don't have the schematic handy at the moment, but what are they wired to? If in series with mains, then they probably ought to be opening at hot. If they are in the fan circuit, try hitting them with a heat gun and see if they close when hot.

Get the schematic for the unit and look up the fan circuit.

Also, check to see that the bias adjustment is not simply too hot.

[Steve Conner]

Ghetto fix, just wire the fan to run full speed all the time!
But do check that the bias is right as Enzo says. Also make sure the load is good, no shorted speaker cables etc.

[blandBill]

Progress being made.

Hi group!
My thanks to those who have given replies to my EMX problem.
Here's what I have done since first posting. I have been successful-finally- at tracing the wiring from the fan control circuit to the thermistors. That's what they are! I have placed a dual 300k pot-one section across each suspected thermistor- and starting with high resistance and going toward low resistance, have found that at approx 3.4kohms the fan will turn on. I then went to various parts houses online and became frustrated with trying to figure out which thermistor to buy. I then went to ebay and found a listing for thermistors with an approx. 3.3k resistance at 50 degrees C. and with the correct mounting. And have ordered 2 pair-an extra pair in case of some problem. I am now waiting for them to come in.
I have a copy of the repair manual, but in it I have seen no values given for idle current on the output xisistors. Any one have a clue?

Thanks again for your help and ideas!!
Bill

[MarkusBass]

This is not a thermistor. It is called thermal resistor but in reality this is just a thermal switch. It is normally open and after reaching 60 degrees Celsius it becomes closed. The fan should normally turn all the time at low speed and after the output transistors reach 60 degrees the switch is closed and the fan starts turning at full speed. So maybe you should fist check whether you have low speed. If not, there should be two resistors (68 Ohms) in series with the fan. Aren't they open? You can check the switches by heating them > 60 degrees (Celsius). They may be OK but resistors in series may be open. There is also a fuse in series with the fan - I hope that you checked if it's not open .
Edit: if ~3k resistor helps, it is rather the switch that failed. Can you confirm that you have low speed of the fan?
Such thermal switches are available from Mouser. Don't use thermistors - the current is to high and they will fail. Post a photo of the switch and someone will help you to find similar component in Mouser catalogue.

Mark

[blandBill]

MarkusBass, Thanks for your suggestions. The 2 resistors and the fan fuse are OK. Checked them earlier. As I read the circuit, the fan does not have a low speed. The fan is controlled by a 3 transistor circuit whose output transistor closes relay RY801(if I have read the diagram's blotchy numbers). The power going through STR60R70, Temperature Resistor(one for each bank of output transistors) has to be very low, as they feed directly into the base of TR801, which is directly connected to TR802 which is directly connected to TR803, whose collector is driving the coil on RY801. Therefore the fan is either OFF or ON at full speed. At least this is what I understand of this circuit. My main trouble with diagnosing this circuit is the poor copy which I have, and the poor wiring diagram of Yamaha. Not to mention the lack of usable part numbers. I will check with Mouser, as I can understand what you're suggesting these parts actually are. Thermal switches would be a better solution than thermistors. If I have problems with finding the parts, I will pull one, shoot it and Photoshop it if necessary. Thanks again!

[Jazz P Bass]

Yamaha EMX200 Service Manual

Here is a clear copy of the EMX200 schematic.
The fan circuit is on the second one.
Page 35.
K-5

[g1]

RY503 (thermal switch?) is also in the circuit, bypassing the 10P10P68x2. RY801 bypasses the 2P32K.
You got the fan to turn on by closing relay 801, perhaps when it is closed RY503 will put it in high speed mode.
Are those parts no longer available from Yamaha?

[MarkusBass]

OK, now that I see the schematic I have to correct myself - sorry for confusion .
We are talking about two different parts related to the fan speed control:
you talk about TM601 (and this is definitely a termistor), and I'm talking about RY503 thermal switch (which, when closed, changes the fan speed to full).
You are right how it works: when the amp is switched on, the fan does not turn at all. When the heatsink gets warmer, the circuit with three transistors closes the relay (this is caused by the thermistor) and the fan starts turning. But there are two resitors (68 Ohms) in series so the speed of the fan is low. When the amp gets hot, the switch RY503 is closed (shorting 68 Ohms resistors) and the fan turns with full speed.
So if the thermistor failed, the fan does not turn at all and this causes that the amp fails. It means that you are right regarding diagnosis of the failure.
You are also right regarding the value of the thermistor - it should be between 2.2k and 10k. Most probably the value 3.3k is correct. It can be calculated (if you understand how the circuit works) and if you really need it, I can calculate it for you.
For the relay to be activated you need Tr802, Tr803 conducting. For this you need Tr801 non-conducting. It means that the base voltage of Tr801 has to be below 0.65V (let's say 0.6V). And there is voltage divider: +12V -> 33k -> thermistor. Knowing the Ohm's low you can calculate that this happens with 2k resistance. It means that the thermistor value should be slightly higher, most probably 3.3k. With 2,2k the fan will be turning with low speed most of the time. So you first guess was very accurate.
Sorry for confusion with my first post. I thought that you were talking about the fan speed switch.

Mark