Hi guys
I'm trying to repair this powered mixer. Sorry this is a bit long but I want to give as much info as possible to help get an answer,
As soon as it powers on the protect light comes on. There are relays that connect the speakers to each channel of the amplifier but they never click. If I connect a 60W light bulb in series with the mains supply it comes in bright then goes to maybe 30-50% brightness - it seems to dim a little further over time
I have power to the amplifier, I think it was as about plus/minus 50-60V for the main amp, and plus/minus 15V to the mixer. There are some other voltages marked on the silk screen near the ribbon cable that connects the main amplifier board to the mixer (5V, 15V etc) and they also check OK.
Just to mention the +/-60V supply reads about +/- 30V with the light bulb limiter in circuit so it looks like this amp is drawing about 30W in quiescent state, either with or without the preamp board connected.
All the output devices check OK in circuit, the emitter and base resistors check OK. I have the amplifier PCB on the bench, mixer ribbon disconnected to make it easier to work on.
I've traced the circuit back to the driver or pre-driver stage and the DC voltages read the same on both channels (this is the four transistors bolted to the PCB either side of the large electrolytic capacitors).
Measuring the NPN output transistors on both channels (base-emitter) I have about 0.65V and the PNP output transistors (base-emitter) ones I have about 0.57V. This is the same for both channels - also if I measure from ground to the junction of the emitter resistors on both channels I get about 0.06V so it looks like the output transistors are biased correctly and I don't have one or both channels presenting DC voltage to the output. By the way I don't have speakers connected at the moment.
So to me all that looks OK but when I was putting the amp PCB back in the cabinet I noticed something is getting warm on the rear edge of the PCB behind the heat sinks.
So I used my thermal camera and I can clearly see that one transistor (Q140) is getting warm/hot. This transistor is type BF393 and checked OK in circuit so I removed it and confirmed it is OK on my DCA55.
I then looked for a schematic and found one (attached) but it does not match my PCB as there are no transistors Q140, Q141, Q142 etc on the schematic.
So I'm a bit stuck now. There is a whole row of these transistors behind the heat sink, 16 of them in fact. Two sets are in blocks of six and have 2x BF393 plus two pairs of MPSA42 and MPSA92 (complimentary NPN/PNP) then there is a block of four more in the middle of the row.
As none of these transistors behind the heat sink seem to exist on the schematic, and I have to confess I have no idea what they are supposed to be doing, I don't know how to proceed now. Quite clearly something is wrong if one is getting hot and I do suspect this is causing the Protect error but that is just a guess
Can someone help me understand and repair this?
Cheers
Rich
I'm trying to repair this powered mixer. Sorry this is a bit long but I want to give as much info as possible to help get an answer,
As soon as it powers on the protect light comes on. There are relays that connect the speakers to each channel of the amplifier but they never click. If I connect a 60W light bulb in series with the mains supply it comes in bright then goes to maybe 30-50% brightness - it seems to dim a little further over time
I have power to the amplifier, I think it was as about plus/minus 50-60V for the main amp, and plus/minus 15V to the mixer. There are some other voltages marked on the silk screen near the ribbon cable that connects the main amplifier board to the mixer (5V, 15V etc) and they also check OK.
Just to mention the +/-60V supply reads about +/- 30V with the light bulb limiter in circuit so it looks like this amp is drawing about 30W in quiescent state, either with or without the preamp board connected.
All the output devices check OK in circuit, the emitter and base resistors check OK. I have the amplifier PCB on the bench, mixer ribbon disconnected to make it easier to work on.
I've traced the circuit back to the driver or pre-driver stage and the DC voltages read the same on both channels (this is the four transistors bolted to the PCB either side of the large electrolytic capacitors).
Measuring the NPN output transistors on both channels (base-emitter) I have about 0.65V and the PNP output transistors (base-emitter) ones I have about 0.57V. This is the same for both channels - also if I measure from ground to the junction of the emitter resistors on both channels I get about 0.06V so it looks like the output transistors are biased correctly and I don't have one or both channels presenting DC voltage to the output. By the way I don't have speakers connected at the moment.
So to me all that looks OK but when I was putting the amp PCB back in the cabinet I noticed something is getting warm on the rear edge of the PCB behind the heat sinks.
So I used my thermal camera and I can clearly see that one transistor (Q140) is getting warm/hot. This transistor is type BF393 and checked OK in circuit so I removed it and confirmed it is OK on my DCA55.
I then looked for a schematic and found one (attached) but it does not match my PCB as there are no transistors Q140, Q141, Q142 etc on the schematic.
So I'm a bit stuck now. There is a whole row of these transistors behind the heat sink, 16 of them in fact. Two sets are in blocks of six and have 2x BF393 plus two pairs of MPSA42 and MPSA92 (complimentary NPN/PNP) then there is a block of four more in the middle of the row.
As none of these transistors behind the heat sink seem to exist on the schematic, and I have to confess I have no idea what they are supposed to be doing, I don't know how to proceed now. Quite clearly something is wrong if one is getting hot and I do suspect this is causing the Protect error but that is just a guess
Can someone help me understand and repair this?
Cheers
Rich
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