Hello folks
I have tinkered with this for hours now and it's driving me nuts
Last year, I saved a Dynacord PowerMate 1000 from the dumpster. It is a power mixer (a mixing desk with a built in power amplifier)
One of the output channels was completely toast after a severe meltdown, having burnt several resistors and with no exception, all output devices (MJ15022+MJ15023).
All output devices have been now replaced, and I replaced a lot of resistors as well. The problem is, though the amplifier itself works fine now, the protect circuit stays active for some reason. I can measure output signal before the output relay, and if I cut one end of R428, the protect circuit deactivates and both channels play fine. This is of course not a good solution, since I have effectively bypassed all protection, leaving both speakers and amplifiers at risk of damage. There is no DC on the output (less than 10mV) and quiescent current is also fine.
I suspect that I for some reason just can't see the forest for the trees, and it may be something simple. On the positive side, I have another functioning channel that I can do comparative measurements on.
On the good channel, I measure almost full negative VCC on the collector of Q335, on the defective channel, Q335 has opened up and pulled the collectore close towards positive VCC activating the protection circuit through R428 and D318.
I have attached a link to part of the schematic, with my handwritten DC voltage measurements (those I found relevant).
http://c-kronborg.com/billeder/20150214_224833.jpg
Hopefully, some of you guys can help me decipher this mystery (which probably isn't a mystery anyways 😉
I have tinkered with this for hours now and it's driving me nuts
Last year, I saved a Dynacord PowerMate 1000 from the dumpster. It is a power mixer (a mixing desk with a built in power amplifier)
One of the output channels was completely toast after a severe meltdown, having burnt several resistors and with no exception, all output devices (MJ15022+MJ15023).
All output devices have been now replaced, and I replaced a lot of resistors as well. The problem is, though the amplifier itself works fine now, the protect circuit stays active for some reason. I can measure output signal before the output relay, and if I cut one end of R428, the protect circuit deactivates and both channels play fine. This is of course not a good solution, since I have effectively bypassed all protection, leaving both speakers and amplifiers at risk of damage. There is no DC on the output (less than 10mV) and quiescent current is also fine.
I suspect that I for some reason just can't see the forest for the trees, and it may be something simple. On the positive side, I have another functioning channel that I can do comparative measurements on.
On the good channel, I measure almost full negative VCC on the collector of Q335, on the defective channel, Q335 has opened up and pulled the collectore close towards positive VCC activating the protection circuit through R428 and D318.
I have attached a link to part of the schematic, with my handwritten DC voltage measurements (those I found relevant).
http://c-kronborg.com/billeder/20150214_224833.jpg
Hopefully, some of you guys can help me decipher this mystery (which probably isn't a mystery anyways 😉
Hi,
Check transistors Q335 and Q334 for short. Also check resistor R427 resistance and also if you have a -VCC voltage at one side of the R427.
Check transistors Q335 and Q334 for short. Also check resistor R427 resistance and also if you have a -VCC voltage at one side of the R427.
Hi,
Is the mixer part attached to the poweramp section?
I had several of these very fine power mixers in repair.
I have to look at my repair reports to remember the problem causing the protection circuit to do it's job.
I try to come back later today, but I think it will be monday...
Is the mixer part attached to the poweramp section?
I had several of these very fine power mixers in repair.
I have to look at my repair reports to remember the problem causing the protection circuit to do it's job.
I try to come back later today, but I think it will be monday...
Yes, everything is connected as it should, and when I cut R427, it all works fine and plays out of both channels, I just don't want to put any speakers or the amplifier itself at risk of damage, because the protect circuit is disabled.
R409,R411 or R412 to high resistance.Also possible voltage of D314 to high.Q337 perhaps suffered from the melddown to.
Mona
Mona
I haven't checked my papers yet, but...
Do you have the whole set of schematics for this mixer?
Next step is to understand the protection circuit.
Find out what can trigger the protection circuit.
Check for problems at any of these possible triggers.
The rest is for tomorrow
Do you have the whole set of schematics for this mixer?
Next step is to understand the protection circuit.
Find out what can trigger the protection circuit.
Check for problems at any of these possible triggers.
The rest is for tomorrow
I have the whole schematic yes. I have narrowed it down to this part of the circuit, since when I cut the resistor R428, the protection circuit deactivates, so it is definitely activated from this part of the circuit.
I will check that tomorrow
Thank you all for your help. Hope to have this baby up and running again very soon
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