Hello diy audio community. I currently have a Technics SU-V9 on my bench that I'm attempting to repair. I know my way around tube amps well enough, but my knowledge of anything transistor-based is sorely lacking, so I've come here seeking advice.
I believe the main problem with this amp is related to the protective circuitry which engages the main DC voltage rails. The amp powers up and doesn't blow the fuse, but neither of the internal relays click on.
I'll attach the service manual which has the schematic for reference… There are two transistors (Q601 and Q606) which I believe activate the thyristors for the DC + and - rails (D601 - D604), but I don't think they are turning on - Q601 collector and Q606 emitter both read 0V when they should be +16 and -14, respectively. Tracing things backward, Q603 and Q605 don't appear to be conducting, either. Neither does Q505, which seems to be what drives Q603 and Q605. Q505 has 0.1V on its emitter, when it should have 4.7V. Finally, some of the pins on IC501, the muting/relay drive circuit, don't show the correct voltage, but at least there's 3 volts on pin 9 (DC power to the chip itself).
And that's about as far as I've been able to get with it. Like I said, my knowledge of solid-state transistor logic is pretty weak...
Some other observations, which I'm not sure if they should be cause for alarm: the DC + and - rails after the thyristors read about +20 and -30 volts, and drift by +/- a few volts every couple seconds. There is also a very slight ticking sound coming from the power transformer, which is in sync with the fluctuations on those DC + and - lines.
The AC voltages from the power transformer secondary seem okay and are stable (I think around ~40V for one pair and ~50V for the other). And the DC + and - supplies prior to the thyristors also seem okay and stable, about +47V and -46V.
Phew, sorry for the long post. If you're still paying attention, any ideas what might be causing this behavior or what to chec
k next? TIA!
I believe the main problem with this amp is related to the protective circuitry which engages the main DC voltage rails. The amp powers up and doesn't blow the fuse, but neither of the internal relays click on.
I'll attach the service manual which has the schematic for reference… There are two transistors (Q601 and Q606) which I believe activate the thyristors for the DC + and - rails (D601 - D604), but I don't think they are turning on - Q601 collector and Q606 emitter both read 0V when they should be +16 and -14, respectively. Tracing things backward, Q603 and Q605 don't appear to be conducting, either. Neither does Q505, which seems to be what drives Q603 and Q605. Q505 has 0.1V on its emitter, when it should have 4.7V. Finally, some of the pins on IC501, the muting/relay drive circuit, don't show the correct voltage, but at least there's 3 volts on pin 9 (DC power to the chip itself).
And that's about as far as I've been able to get with it. Like I said, my knowledge of solid-state transistor logic is pretty weak...
Some other observations, which I'm not sure if they should be cause for alarm: the DC + and - rails after the thyristors read about +20 and -30 volts, and drift by +/- a few volts every couple seconds. There is also a very slight ticking sound coming from the power transformer, which is in sync with the fluctuations on those DC + and - lines.
The AC voltages from the power transformer secondary seem okay and are stable (I think around ~40V for one pair and ~50V for the other). And the DC + and - supplies prior to the thyristors also seem okay and stable, about +47V and -46V.
Phew, sorry for the long post. If you're still paying attention, any ideas what might be causing this behavior or what to chec
k next? TIA!