Hi,
I've got a Dynaudio BM6A Mark II monitor i'm trying to repair.
i initially changed the 4 output transistors and the 4 driver transistors following the procedure indicated in the service manual (attached below).
I put 1A fuses for the biasing phase, plugged the monitor to a current limiter bulb.
Bias potentiometers set to minimum and to 21mV 2 minutes after switch on from cold (quite a pain in the *** to do ...).
i left it plugged for 20 mins, everything seamed fine.
I plugged some music to it, everything's still fine, music playing.
5 mins later the LF fuses blew and my current limiter bulb lighted up.
LF Output transistors dead.
I Changed them, remade bias setting carefully, plugged some music, everything seams fine and some minutes later bulb lighted up again, LF fuses blew, transistors dead once again.
So before i spend the day changing transistors only to blow them,
i think i could use some help ahah.
As you already guessed i can understand and solder things but i'm not an electronic expert, so please be indulgent.
I've read about many problems with Dynaudio monitors, but as far as i know an amp is an amp, 1+1=2, and it once worked so there should be a not so difficult way to make it work again ...
Any advice on what to check and what to try before i throw this thing by the window ?
Thanks in advance,
Fred
I've got a Dynaudio BM6A Mark II monitor i'm trying to repair.
i initially changed the 4 output transistors and the 4 driver transistors following the procedure indicated in the service manual (attached below).
I put 1A fuses for the biasing phase, plugged the monitor to a current limiter bulb.
Bias potentiometers set to minimum and to 21mV 2 minutes after switch on from cold (quite a pain in the *** to do ...).
i left it plugged for 20 mins, everything seamed fine.
I plugged some music to it, everything's still fine, music playing.
5 mins later the LF fuses blew and my current limiter bulb lighted up.
LF Output transistors dead.
I Changed them, remade bias setting carefully, plugged some music, everything seams fine and some minutes later bulb lighted up again, LF fuses blew, transistors dead once again.
So before i spend the day changing transistors only to blow them,
i think i could use some help ahah.
As you already guessed i can understand and solder things but i'm not an electronic expert, so please be indulgent.
I've read about many problems with Dynaudio monitors, but as far as i know an amp is an amp, 1+1=2, and it once worked so there should be a not so difficult way to make it work again ...
Any advice on what to check and what to try before i throw this thing by the window ?
Thanks in advance,
Fred
Turning it into a passive loudspeaker could be a solution once you are sure electronics is beyond your repair capabilities.
The amplifier is uncomplicated. Opamp + 2BJT + 2MOSFET. Bipolar power supply + IC stabilizer (+ -) 15v.
Solve problems one at a time. Power supply first. Then Opamp. Last but not least are drivers and output transistors.
Solve problems one at a time. Power supply first. Then Opamp. Last but not least are drivers and output transistors.
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Hey, Fred, I just managed to fix my Dynaudio BM6A yesterday afte rmessing with it for several months,
for me - the original problem was with one of the diodes going down and getting slightly distorted sound because of this, but while my friend was doing the repair, he also did some rotations of the bias variacs and we thought that this should not be that much important for it at least to work, and we had long array of troubles with output transistors getting blown, etc, etc. Only then I found out how important the bias knbos are for getting it to work for more than 5 seconds =)
I think it can be more that 5 or 10 minutes to warm up to stable position, try lowering the variacs to minimum and may be just do a 1/10 turn and wait with this setting, for me it jumped from 15mv to 25mv easily during the course of 2-3 minutes. May be because of flunctuating power voltage also.
And at first, I even connected it to sound source when bias voltage was something close to 1-2 mV and it already sounded "normal".
So my suggestion is to try setting it to a bit lower bias voltage even after 20-30 minutes of power and also - are you trying it with transistors on the radiator or just hanging in the air?
for me - the original problem was with one of the diodes going down and getting slightly distorted sound because of this, but while my friend was doing the repair, he also did some rotations of the bias variacs and we thought that this should not be that much important for it at least to work, and we had long array of troubles with output transistors getting blown, etc, etc. Only then I found out how important the bias knbos are for getting it to work for more than 5 seconds =)
I think it can be more that 5 or 10 minutes to warm up to stable position, try lowering the variacs to minimum and may be just do a 1/10 turn and wait with this setting, for me it jumped from 15mv to 25mv easily during the course of 2-3 minutes. May be because of flunctuating power voltage also.
And at first, I even connected it to sound source when bias voltage was something close to 1-2 mV and it already sounded "normal".
So my suggestion is to try setting it to a bit lower bias voltage even after 20-30 minutes of power and also - are you trying it with transistors on the radiator or just hanging in the air?
Hey Raymaxer,
thanks for your message !
I still put my BM6A up on the bench from time to time when i'm willing to give it another shot, but result is always the same ahah.
I always put the transistors back against the radiator before firing it up but I never tried with a lower bias setting,
the service manual asks for 21mV after 2 minutes warming up, which is what i tried to achieve.
How much did you left your speaker with for it to work ? something like 15mV after 30 minutes ?
Do you feel it's fixed or do you still handle it with care ahah ?
I wanted to get a second BM6A back from the studio to compare measurements, but a monitor is not quite handy on a bike.
I'll give a try to lower biais setting, i'm not a pair of mosfets shy...
Thanks a lot for your message.
Fred
Looking into the schematic of the amplifier I would say that it should be checked without output transistors and checked for stability. Measure voltages on gate points and compare, positive and negative are not necessarily the same value (positive or negative).
Next point will be checking NTC, is it's resistance falling with temperature rise.
Suggestion is to replace 5k trimer for idle current adjustment with multi turn type, it provides more precise adjustment. Music vibrations inside the box sometimes can damage connection on component, so idle current can rise with bad connection on trimer.
This amplifier shouldn't be a problem.
Next point will be checking NTC, is it's resistance falling with temperature rise.
Suggestion is to replace 5k trimer for idle current adjustment with multi turn type, it provides more precise adjustment. Music vibrations inside the box sometimes can damage connection on component, so idle current can rise with bad connection on trimer.
This amplifier shouldn't be a problem.
Thanks for your reply Pitbul.
1 turn trimmers indeed are a pita to adjust precisely...
I'll check it all when i'll get back to it.
"This amplifier shouldn't be a problem", it's pretty clear to me on the schematics, but as anything can go stupidly wrong and because i'm quite a noob, this amplifier totally is a problem ahah
Thanks again for your time.
I dont think this one can be checked properly “without output transistors”. The outputs are common source amplifiers, not the usual unity gain followers.
But if the bias stack is good and the drivers havent failed there really isn’t anything else that can cause catastrophic failure. Other than the output transistors themselves being bad or being driven with square waves at a couple hundred kHz. Isolate the entire bias stack and check it - make sure the pot isn’t intermittent and the NTC works (heat it with a soldering iron and watch the bias voltage drop). If that is good new transistors should work. 2N3904/6 are under rated IMO, but will usually work. 40 VCEO is pushing it but usually they will take a LOT more. But something rated higher would give you certainty and you could rule that out. Getting fake mosfets, however is a more likely cause of failure.
@Friedd, do it as I said, measure voltages on output (should be around 0V with reference to GND) and measure voltages over R8 and R29 and also over R72 and R50, it will represent your Vgs of the output transistors (MOSFETs).
wg_ski described you how to check idle current circuit (bias) in more precise way.
Do not put any signal on the input (short it), measure voltages for one day (put it live on the bench).
wg_ski described you how to check idle current circuit (bias) in more precise way.
Do not put any signal on the input (short it), measure voltages for one day (put it live on the bench).
Hey Fried! It's still working 4 days since putting it back. =) Try holding it for 20-30 minutes on the lowest or very close to lowest setting, IMHO and then setting BIAS difference to 15-20 mV, not more. But I am a noob too myself =)
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Being a noob myself, and having this amp constantly blowing, i can only see you as the successful noob among us 😉
I'll let you all know the result of all this.
once again, thank you all for your messages.
i'm struggling with this amp too. i have distortion when i rise volumes ..i dont know if its because its going out of power supply rails or something else...