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I'm repairing a Yamaha M-50 that had a blown channel. I replaced the output transistors TR135/TR139 (2SC2706) with NJW3281G and TR137/141 (2SA1146) with NJW1302G. The driver transistors TR131 (2SC2238) and TR133 (2SA968) were replaced with TTA006B,Q and TTC011B,Q respectively. The emitter resistors (0.33Ω) measured good and were not changed.
I then followed the service manual instructions to adjust the -B power supply, photocoupler, DC offset and idling current for both channels. I am getting signal passing thru both channels to the speaker terminals. However, I'm noticing that the channel that I replaced the outputs for has about 20% less signal at the speaker terminals (or even at test point TPO prior to the speaker relays).
The hFE on the output transistors for both channels (working, plus repaired one) are right around 100 for all four sets so they are pretty close to each other. The hFE on the drivers of the good channel are 123, whereas the channel I am repairing has the drivers closer to 170 hFE.
I confirmed both channels are getting the same level signal to the Pre drive board from the front panel volume controlled input signal, but also confirmed that the outputs of the pre-drive board to the Main board show this approximately 20% difference in levels going to the main board.
Given the channels have NFB tied into the pre-drive circuitry, I'm trying to understand if the lower signal output is related to the NFB, or if it's not just creating a strong enough signal to be amplified.
The file called service has clearer schematics (and all of them), whereas the other file has the adjustment procedure/settings on page 8 of 25 but is missing the non-power supply schematics.
Any guidance would be appreciated. Thanks!
edit: The relevant part of the schematic is on page 16 of 28 of the hfe_yamaha_m-50_service.pdf file.
I then followed the service manual instructions to adjust the -B power supply, photocoupler, DC offset and idling current for both channels. I am getting signal passing thru both channels to the speaker terminals. However, I'm noticing that the channel that I replaced the outputs for has about 20% less signal at the speaker terminals (or even at test point TPO prior to the speaker relays).
The hFE on the output transistors for both channels (working, plus repaired one) are right around 100 for all four sets so they are pretty close to each other. The hFE on the drivers of the good channel are 123, whereas the channel I am repairing has the drivers closer to 170 hFE.
I confirmed both channels are getting the same level signal to the Pre drive board from the front panel volume controlled input signal, but also confirmed that the outputs of the pre-drive board to the Main board show this approximately 20% difference in levels going to the main board.
Given the channels have NFB tied into the pre-drive circuitry, I'm trying to understand if the lower signal output is related to the NFB, or if it's not just creating a strong enough signal to be amplified.
The file called service has clearer schematics (and all of them), whereas the other file has the adjustment procedure/settings on page 8 of 25 but is missing the non-power supply schematics.
Any guidance would be appreciated. Thanks!
edit: The relevant part of the schematic is on page 16 of 28 of the hfe_yamaha_m-50_service.pdf file.
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