Hello folks,
I apologize ahead of time for the diagtibe!
Been working on a Mackie SR1232 power amp section of this powered monitor. I think it has 2x12" woofers & a horn.
The first issue is I received it from another tech who could not repair it and actually went backwards shorting out some FET output transistors. FETS Q11 & 17 have been replaced an surrounding circuitry been checked. the symptom once the amp was restored to it's original condition w/ he received it is the same. Past a certain volume level the sound gets all flabby & distored.
My take on the power supply arrangement to the rails of the output stage is that up to a certain power demand, +-25V is supplied through D27 & D3. Past that level the circuit in the power supply from Bridge rect D1 composing of U1 and Q11 & Q17 switches in the higher voltage via FB thru U1.
I'm again surmising (guessing really) that this circuitry endeavors to keep the + and - rails equal as it appears to only 'regulate' the negative supply thru L3 & L2 via the feedback to U1 Pins 1 & 2.
However, when you pass a certain level L3 (or L2 can't recall) in the power supply squeals as if there is an inordinate load on the negative higher voltage supply to the output stage that appears to kick in at that point.
Again in hopefully a slightly educated guess to isolate the issue, I believe that Q21 and Q23 to the right of the output stage act as crowbar protection if there is any inordinate DC offset from the output stage, so I disconnected the drains of those 2 FETS and can see that the circuit that supplies the higher +- voltage to the output stage does indeed try supply the higher voltage to the + half (I only went up to 100V with volume level in effort to not blow shit up) but the negative half has some kind of issue where it most likely is drawing too much current that causes L3 or L2 to squeal and not supply the higher voltage.
Issue I have is that I cannot find any obvious bad components in the Neg side of the output stage that would be causing this in ordinate current draw so I'm still not certain if the issue is in the output stage or this weird circuit with L2 & L3 in it.
I've also tried to figure a way to disable some of the protection circuitry to the right of the output stage & cannot figure a safe way to isolate that circuitry.
I have replaced the opto-IC-1 and Q17 & Q11 as mentioned previously.
Weirdly IC-1 outputs a square wave to the FETS Q11 and Q17. I really don't see any PWM generator anywhere there. I suppose it may be a pwm type output that is ultimately filtered thru L3 & L2.
Anyway I know this is kinda like the car mechanic hears a long explanation from the consumer when the mechanic a the end of it says: 'well maam, if you hold the car up to the phone, maybe I can tell you what's wrong with it'!
Anyone had any experience with this beast?
Thanx, Glen
I apologize ahead of time for the diagtibe!
Been working on a Mackie SR1232 power amp section of this powered monitor. I think it has 2x12" woofers & a horn.
The first issue is I received it from another tech who could not repair it and actually went backwards shorting out some FET output transistors. FETS Q11 & 17 have been replaced an surrounding circuitry been checked. the symptom once the amp was restored to it's original condition w/ he received it is the same. Past a certain volume level the sound gets all flabby & distored.
My take on the power supply arrangement to the rails of the output stage is that up to a certain power demand, +-25V is supplied through D27 & D3. Past that level the circuit in the power supply from Bridge rect D1 composing of U1 and Q11 & Q17 switches in the higher voltage via FB thru U1.
I'm again surmising (guessing really) that this circuitry endeavors to keep the + and - rails equal as it appears to only 'regulate' the negative supply thru L3 & L2 via the feedback to U1 Pins 1 & 2.
However, when you pass a certain level L3 (or L2 can't recall) in the power supply squeals as if there is an inordinate load on the negative higher voltage supply to the output stage that appears to kick in at that point.
Again in hopefully a slightly educated guess to isolate the issue, I believe that Q21 and Q23 to the right of the output stage act as crowbar protection if there is any inordinate DC offset from the output stage, so I disconnected the drains of those 2 FETS and can see that the circuit that supplies the higher +- voltage to the output stage does indeed try supply the higher voltage to the + half (I only went up to 100V with volume level in effort to not blow shit up) but the negative half has some kind of issue where it most likely is drawing too much current that causes L3 or L2 to squeal and not supply the higher voltage.
Issue I have is that I cannot find any obvious bad components in the Neg side of the output stage that would be causing this in ordinate current draw so I'm still not certain if the issue is in the output stage or this weird circuit with L2 & L3 in it.
I've also tried to figure a way to disable some of the protection circuitry to the right of the output stage & cannot figure a safe way to isolate that circuitry.
I have replaced the opto-IC-1 and Q17 & Q11 as mentioned previously.
Weirdly IC-1 outputs a square wave to the FETS Q11 and Q17. I really don't see any PWM generator anywhere there. I suppose it may be a pwm type output that is ultimately filtered thru L3 & L2.
Anyway I know this is kinda like the car mechanic hears a long explanation from the consumer when the mechanic a the end of it says: 'well maam, if you hold the car up to the phone, maybe I can tell you what's wrong with it'!
Anyone had any experience with this beast?
Thanx, Glen
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