Replacement power transistors for Yamaha G100-212

[J M Fahey]

1) TO3 case pinout



2) please post here the power amp schematic you are using.

Thanks.

[Enzo]

Remember, the case is C, the case meaning the metal body of the transistor. IMportant to measure from B and from E to the case. E to C is the most common pair to short.

In the lower right of your schematic, are there not a series of printed notes, saying things like all resistors in ohms, and all capacitors in MFD, etc?

This drawing should be close, but at least shows the notes.
http://bmamps.com/Schematics/Yamaha/...e%20Manual.pdf

[Jimmy74]

here's the schematic I'm using...it's similar to the one Enzo posted but still different...I'm guessing that the diode is exactly what I'm looking for.

Yamaha G100 series i service manual -05_zpsvc4seuf4.jpg Photo by jimmy6122 | Photobucket

[Jimmy74]

Ok one 2SC783 E to C gives over 5 Mohms, B to C gives over 3 Mohms
the other 2SC783 E to C gives over 3 Mohms and B to C over 210Kohms
the 2SC483 E to C = over 36.8Kohms and B to C = 13kOhms

[J M Fahey]

When reading transistors "ohms" values don´t mean much (except that 0 is 0 in any scale),that´s why most multimeters incorporate a "Diode Test" function.
Rather than speak of "ohms" they pass a small current (a couple mA) through the diode junction (transistors also have them inside) and display shows the mV across it.

So we expect around 0.7V across BE or BC in one way, infinite or out of range the other way, and infinite CE both ways.

On some Darlingtons (TIP142/147) or power fets (IRF240) there is an "upside down" CE or DS diode built in so in one way they must show it; if they short it´s usually both ways.

So please re do those tests but on the diode scale.

[Jimmy74]

ok here goes:

1) 2SA483 BC = 1.2V (one way only +ve on B -ve on C)
EC = 0.5V (one way only +ve on C -ve on E)
BE = 0.5V (one way only +ve on E -ve on B)

2SC783 BC = 0.5V (one way only +ve on B -ve on C)
EC = 0.5V (+ve on E -ve on C) 1.8V rising ( +ve on C -ve on E)
BE = 0.5V (one way only +ve on B -ve on E)

2SC783 BC = 0.5V ( +ve on B -ve on C) 1.8V rising (+ve on C -ve on B)
EC = 1.1V rising (one way only +ve on C -ve on E)
BE = 0.5V (one way only +ve on B -ve on E)

[The Dude]

Those readings are odd. On one hand, it seems that the transistors are shorted E-C. On the other hand, you are only reading the "near short" one direction/polarity, which is puzzling. Are you measuring them out of circuit?

[Jimmy74]

no they're still soldered to the board

[The Dude]

If they were shorted, you'd probably have the same reading both ways. My guess is that you are reading something else in the circuit that is across C-E. If you want to be sure, you could unsolder the emitter lead and retest C-E.

[Jimmy74]

sorry for such a lengthy absence...getting back to this amp...I proceeded to replacing all the TO3 transistors, electrolytics and burnt resistors on the board. But when mounting up the output transistors I noticed that the mounting screws aren't isolated from the heatsink, that is they don't have the usual plastic spacer washers for the screws...just the mica washer for the transistor itself.

Is this normal in these amps?

thanks
J.

[Enzo]

Look at the socket. Don't the screw holes have a lip molded into the socket so the screws remain centerd in the oversized chassis holes? The screws make contact with the transistor body (the collector)and make the connection to the socket.

[Jimmy74]

Yes you're right...sorry was getting confused with the screw holes used to mount the protective grill.

[Jimmy74]

I've just realised that I need a dummy load, scope and wave generator to bias this amp...is there any way to do this without that equipment?

[Jazz P Bass]

Nope!

At least, not properly.

The half way voltage & the bias can be set with a decent volt meter.

If you did not touch VR3, the symmetry may be all right.

G100 Test.pdf

[Jimmy74]

definitely didn't touch any of the VR's...I'll proceed to doing those 2 tests