Re: Repairing Alesis M1 Active mk2 Monitor Speakers
Been a while since I last looked at the schematics on these...
But with Q1 being the main switching MOSFET... if you have fluctuating voltage between pin 5 (primary ground) and pin 7 (UC3844 VCC voltage), then that means the UC3844 is likely OK and trying to start up. It also suggests Q3 and Q4 might be OK too. However, Q1 may or may not be good. One way to know is to measure the resistance of R15 (0.22 Ohms). You should get a very low resistance on your multimeter - same as if you were to short the probes together (i.e. lowest resistance your multimeter can measure.) If R15 is good, then Q1 is probably OK too and there is something else preventing the PSU from starting up. In that case, check diode D5 and resistor R5 (10 Ohm). These supply the UC3844 with power when the PSU is in steady-state operational mode.
If D5 and R5 are OK, check the output rectifiers on the secondary side, D10 through D13, along with the filter electrolytic capacitors, C17 through C24. If these are good, see if the PSU runs when disconnected from the amplifier board (measure voltage output across C18, C20, C22 and C24.) For t his test, it might be a good idea to connect a 100-300 Watt halogen bulb in series with the Live (240V AC in your case) line on the input. This is to prevent damage to the PSU, should some component fail suddenly.
If no voltage on the output at C18, C20, C22, and C24, time to check more components on the primary side. Start with Q3, Q4, and Q5. But before that, make sure the primary caps, C6 and C7 are discharged. BE CAREFUL around these - a faulty primary could have these still charged close to 340V total, which can definitely hurt you if you touch the wrong parts or damage your multimeter. You can discharge these safely with a low-power device, such as a hot glue gun, soldering iron, or portable lamp. Just connect the plug from one of these across each cap, and that should discharge the caps pretty gracefully. Once C6 and C7 are discharged, then check Q3, Q4, and Q5.
Q4 is a PNP transistor. With black MM probe on its Base terminal, you should get around 600 mV reading on your MM when you touch either the Emitter or Collector terminals with your red probe. For Q3, you should get the same readings, but just repeat the test with the red and black probes reversed compared to Q4 (since Q3 is an NPN transistor.)
Q5 may be a bit tricky. Due to transformer T2A, you'll probably see a short-circuit / very low resistance / 0 mV reading between Base and Emitter terminals. But for Base and Collector terminals, measure it like Q4 and you should get around 600 mV again, if I'm not mistaken.
Been a while since I last looked at the schematics on these...
But with Q1 being the main switching MOSFET... if you have fluctuating voltage between pin 5 (primary ground) and pin 7 (UC3844 VCC voltage), then that means the UC3844 is likely OK and trying to start up. It also suggests Q3 and Q4 might be OK too. However, Q1 may or may not be good. One way to know is to measure the resistance of R15 (0.22 Ohms). You should get a very low resistance on your multimeter - same as if you were to short the probes together (i.e. lowest resistance your multimeter can measure.) If R15 is good, then Q1 is probably OK too and there is something else preventing the PSU from starting up. In that case, check diode D5 and resistor R5 (10 Ohm). These supply the UC3844 with power when the PSU is in steady-state operational mode.
If D5 and R5 are OK, check the output rectifiers on the secondary side, D10 through D13, along with the filter electrolytic capacitors, C17 through C24. If these are good, see if the PSU runs when disconnected from the amplifier board (measure voltage output across C18, C20, C22 and C24.) For t his test, it might be a good idea to connect a 100-300 Watt halogen bulb in series with the Live (240V AC in your case) line on the input. This is to prevent damage to the PSU, should some component fail suddenly.
If no voltage on the output at C18, C20, C22, and C24, time to check more components on the primary side. Start with Q3, Q4, and Q5. But before that, make sure the primary caps, C6 and C7 are discharged. BE CAREFUL around these - a faulty primary could have these still charged close to 340V total, which can definitely hurt you if you touch the wrong parts or damage your multimeter. You can discharge these safely with a low-power device, such as a hot glue gun, soldering iron, or portable lamp. Just connect the plug from one of these across each cap, and that should discharge the caps pretty gracefully. Once C6 and C7 are discharged, then check Q3, Q4, and Q5.
Q4 is a PNP transistor. With black MM probe on its Base terminal, you should get around 600 mV reading on your MM when you touch either the Emitter or Collector terminals with your red probe. For Q3, you should get the same readings, but just repeat the test with the red and black probes reversed compared to Q4 (since Q3 is an NPN transistor.)
Q5 may be a bit tricky. Due to transformer T2A, you'll probably see a short-circuit / very low resistance / 0 mV reading between Base and Emitter terminals. But for Base and Collector terminals, measure it like Q4 and you should get around 600 mV again, if I'm not mistaken.
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