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A couple months ago, the former General Mgr of CenterStaging, LLC brought me his aged Yamaha CR640 Receiver that needed work. I managed to restore the power supply and recapped a lot of the unit, beginning with the power supply and areas around the Power Amp stages, along with the Phono preamp and PS filters for the Tuner/Receiver area. A week after he had it, be brought it back, it having blown the mains fuse. I found one of the two Power Amp Modules had shorted power amp xstrs. IG02980 Hybrid amp modules, long out of production. These work in concert with an IG02950 PreDriver hybrid module, also long out of production.
Yamaha_IG02980 Power Amp Module Schematic.pdf
Yamaha IG02950 PreDriver Amp Module.pdf
After some searching, I took a chance on a pair of modules I found in Germany. One didn't measure good at all, the other, which had the excess length of the narrow portion of the PCB terminals cut, but long enough, I think, to pass thru the PCB, the power xstrs measured ok, but the Bias Network (a discrete Darlington NPN circuit with a resistor/diode bias string between the top/bottom of the network straddling the bases of the pre-driver xstrs of the output xstr circuit was measuring open circuit.
I had forgotten my Fluke 8060 in Diode Test mode only displays the results on the 2V range, while it's 1mA current source is delivered from an 8VDC source. To measure the full potential of this network, I needed a second DC Voltmeter to display that greater than 2V, found the later revision bias network present in the newer IG02980 module. After measuring the bias network between Pins 7 and 4, I found the bias circuit having an extra semiconductor junction in the network, and WAS measuring good. But, in the CR640 unit there is NO bias adjustment provisions...it's fixed within the Power Amp module. There is a pair of resistors between pins 0 and 7, and between pins 4 and 1, which measure 240 ohms. In the Yamaha circuit, the PreDriver module is coupled to Pins 0 and 1, which includes those 240 ohm resistors, while pins 7 and 4 there is an electrolytic cap across this fixed bias network.....typical of circuits like this in a wide variety of discrete amps.
Being wary of that one module that looked like it might work, I found another supplier who had new IG02980 Power Amp modules and bought those. Brand new, never installed, but they too had this later generation bias network. I installed one of these and proceeded ahead to see if I could adjust the bias, as the other working module had 29mA current flowing thru the emitter resistors (0.47 ohm). The new module's 0.47 ohm emitter resistors had NO Current flowing thru its' emitter resistors. I was UNABLE to get ANY current flow thru them in a Prebias condition. I had tried adjusting a 5k 10-turn helipot across the lower 240 ohm resistor between pins 4 and 1, to see if I could get the voltage across pins 7 and 4 to increase, but without any luck.
The nominal voltage on the Right Ch's IG02980 pins 7 to 4 was 3.71V, with 27.4mV between pins 8 & 3, which is 29mA thru the 0.47 ohm emitter resistors. That of this new IG02980, voltage across Pins 7 and 4 measured 3.26V, and have had no success with increasing that to start turning on the output stage.
The unit DOES Work, driving speaker as well as 8 ohm resistive load. I don't measure any distortion difference. Just has no prebias, and I have NO idea HOW to increase that potential to get some current flow similar to that of the older revision of this IG02980 module.
Anyone have any suggestions on how to trim up these newer power amp modules for similar idle bias current in the output stage?
Yamaha_IG02980 Power Amp Module Schematic.pdf
Yamaha IG02950 PreDriver Amp Module.pdf
After some searching, I took a chance on a pair of modules I found in Germany. One didn't measure good at all, the other, which had the excess length of the narrow portion of the PCB terminals cut, but long enough, I think, to pass thru the PCB, the power xstrs measured ok, but the Bias Network (a discrete Darlington NPN circuit with a resistor/diode bias string between the top/bottom of the network straddling the bases of the pre-driver xstrs of the output xstr circuit was measuring open circuit.
I had forgotten my Fluke 8060 in Diode Test mode only displays the results on the 2V range, while it's 1mA current source is delivered from an 8VDC source. To measure the full potential of this network, I needed a second DC Voltmeter to display that greater than 2V, found the later revision bias network present in the newer IG02980 module. After measuring the bias network between Pins 7 and 4, I found the bias circuit having an extra semiconductor junction in the network, and WAS measuring good. But, in the CR640 unit there is NO bias adjustment provisions...it's fixed within the Power Amp module. There is a pair of resistors between pins 0 and 7, and between pins 4 and 1, which measure 240 ohms. In the Yamaha circuit, the PreDriver module is coupled to Pins 0 and 1, which includes those 240 ohm resistors, while pins 7 and 4 there is an electrolytic cap across this fixed bias network.....typical of circuits like this in a wide variety of discrete amps.
Being wary of that one module that looked like it might work, I found another supplier who had new IG02980 Power Amp modules and bought those. Brand new, never installed, but they too had this later generation bias network. I installed one of these and proceeded ahead to see if I could adjust the bias, as the other working module had 29mA current flowing thru the emitter resistors (0.47 ohm). The new module's 0.47 ohm emitter resistors had NO Current flowing thru its' emitter resistors. I was UNABLE to get ANY current flow thru them in a Prebias condition. I had tried adjusting a 5k 10-turn helipot across the lower 240 ohm resistor between pins 4 and 1, to see if I could get the voltage across pins 7 and 4 to increase, but without any luck.
The nominal voltage on the Right Ch's IG02980 pins 7 to 4 was 3.71V, with 27.4mV between pins 8 & 3, which is 29mA thru the 0.47 ohm emitter resistors. That of this new IG02980, voltage across Pins 7 and 4 measured 3.26V, and have had no success with increasing that to start turning on the output stage.
The unit DOES Work, driving speaker as well as 8 ohm resistive load. I don't measure any distortion difference. Just has no prebias, and I have NO idea HOW to increase that potential to get some current flow similar to that of the older revision of this IG02980 module.
Anyone have any suggestions on how to trim up these newer power amp modules for similar idle bias current in the output stage?
Attached Files
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