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Yamaha m4 Bias at -2.8 on one Chanel

tested the 2sa1050b and got a ~500 reading on one connection and nothing on the others, and on the 2sc2460 it was 500 both ways so i think they are all ok. I will have to look at the left channel board again for something shorted. the 2sc2460's have a plastic like sheet under them, and the 2sa1050 does not is that because of the pnp, npn circuitry? any ideas where i should look next.
 
saevox said:
tested the 2sa1050b and got a ~500 reading on one connection and nothing on the others, and on the 2sc2460 it was 500 both ways so i think they are all ok. I will have to look at the left channel board again for something shorted. the 2sc2460's have a plastic like sheet under them, and the 2sa1050 does not is that because of the pnp, npn circuitry? any ideas where i should look next.
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Its not plastic. Its mica insulators. There is your shorted problem!.. If there not both insulated. The power rail is shorted.
http://www.mouser.com/ProductDetail/Keystone-Electronics/4662/?qs=sGAEpiMZZMsQtlBhqKq43VRctYvTpGwy

Raw B+/- is applied directly to the output transistors metal case (collector)-- the aluminum heatsink is fastened to chassis ground. There must be and insulator between them!


Mica--
http://en.wikipedia.org/wiki/Mica
 
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The white pasty stuff is thermal compound - usually zinc oxide mixed with a silicone based oil. This is not a sufficient insulator. Its just there as a microscopic void filler and needs to be used with the sheet mica insulators.
 
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Are you testing them this way with your multimeter ?

1. B-C forward
2. B-C reverse
3, B-E forward
4. B-E reverse
5. C-E forward
6. C-E reverse
B-C and B-E depending on PNP or NPN should have a voltage reading in either the forward or reverse. But not both. C-E will be open in both directions.
 
I was doing a test with the diode test function, from what I read one is a Pnp the other an npn. Not sure which was base or emitter put tried with red test point at base or emitter and then tested the other 2 with the black, and then tried the other base/emitter with red and did the same. But looking at my right channel it looks like it has mica under all 4 t03's is that my issue, does this mean this unit was probably serviced before and they were left out and the grease was isolating until now? The unit was working for several months.
 
does this mean this unit was probably serviced before and they were left out and the grease was isolating until now? The unit was working for several months.
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I guess, or you over looked the mica due to and excessive amount of old thermal compound. I'd pull the A1050's and double check. Clean all the old compound off and verify either there is or is not a mica sheet there or not.
 
You can also measure the resistance from the transistors metal case to the metal chassis ground. Should read in the megohms or infinity (over range).
 
So after much confusion on my part, I pulled the transistors again and the 2sa1050b does NOT have the mica, just a bunch of grease, a lot more than the other ones (2460.) I pulled the working right Channel and it also has no mica. So it seems to be correct...one is npn the other pnp. Unless by some odd chance I am missing 4 mica sheets......
 
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