Hi All
Just picked up a DC300A and looking to get into shape for use as a subwoofer amp. Thanks to any who can assist.
So far one channel seems OK, but the other channel has a -0.6V DC offset. I have started the troubleshooting procedure in the service manual. The output DC offset adjust, which I had hoped would be the solution, has no effect. Neither does the input offset adjust. I was able to use these controls to null out the offset on the other channel, which was only a few mV to start with.
Next step I checked the +/- 10V supply to IC1. The negative is -9.52V (close enough I guess, since I believe it is a 5% part), but the positive is only +6.5V. This leads me to suspect the Zener D4 needs replacing.
My confusion at this point is that I wonder how, if this is the cause of the DC offset, it is only affecting one channel? Would this IC power supply issue not affect both channels?
Thanks to any who can provide some input.
Best Regards
Mark
Just picked up a DC300A and looking to get into shape for use as a subwoofer amp. Thanks to any who can assist.
So far one channel seems OK, but the other channel has a -0.6V DC offset. I have started the troubleshooting procedure in the service manual. The output DC offset adjust, which I had hoped would be the solution, has no effect. Neither does the input offset adjust. I was able to use these controls to null out the offset on the other channel, which was only a few mV to start with.
Next step I checked the +/- 10V supply to IC1. The negative is -9.52V (close enough I guess, since I believe it is a 5% part), but the positive is only +6.5V. This leads me to suspect the Zener D4 needs replacing.
My confusion at this point is that I wonder how, if this is the cause of the DC offset, it is only affecting one channel? Would this IC power supply issue not affect both channels?
Thanks to any who can provide some input.
Best Regards
Mark
Any shorted capacitor to the +10 line could allow current to sneak in and mess up the value. There are two input offset pots and two output offset pots, how all that interacts part of the strange & weird design of this amp.
I'd say check the D4 for the plus side out of the circuit, say with a 12v battery charger plus a cap plus a series resistor. Zeners are high failure parts. However, low voltage caps can be shorted by output device failures. You may have bought the result of an incomplete output repair. After D4 is proved good, look for caps on that +10 line with the same DC value on both sides of the cap.
IC1 could also be bad but I wouldn't replace it first, it is exotic and expensive.
Search the forum for this amp, there is another long thread where a guy fixed one. Wasn't this exact problem though.
One option if you lack patience to fix this weird circuit, is to salvage the case, transformer, fan, heat sinks, output devices, for a PA amp. Replace the driver board with something more conventional like a LME49810 IC or something. Another candidate for a output transistor driver is the TDA7293 IC with suitable idle current biasing diode stack mounted on the output transistor heat sinks. You don't need the DC output capabilities of this amp for a sub-woofer. The DC rating of this amp is for driving DC actuators for diesel motor speed control or hydraulic jack leveling of equipment or something like that. We used to use something like this in geophysics to drive the ground with a 1/2 to 25 hz sine wave- hydraulically, with 3000 hp pumping the actuators. They would use the DC capability of this to lift actuators in one direction, say in lifting the Cape Fear lighthouse to move it (PBS Nova program).
If this amp actually drifts into DC offset it could burn your expensive woofer coils. Great package, wrong driver circuit for PA work.
I'd say check the D4 for the plus side out of the circuit, say with a 12v battery charger plus a cap plus a series resistor. Zeners are high failure parts. However, low voltage caps can be shorted by output device failures. You may have bought the result of an incomplete output repair. After D4 is proved good, look for caps on that +10 line with the same DC value on both sides of the cap.
IC1 could also be bad but I wouldn't replace it first, it is exotic and expensive.
Search the forum for this amp, there is another long thread where a guy fixed one. Wasn't this exact problem though.
One option if you lack patience to fix this weird circuit, is to salvage the case, transformer, fan, heat sinks, output devices, for a PA amp. Replace the driver board with something more conventional like a LME49810 IC or something. Another candidate for a output transistor driver is the TDA7293 IC with suitable idle current biasing diode stack mounted on the output transistor heat sinks. You don't need the DC output capabilities of this amp for a sub-woofer. The DC rating of this amp is for driving DC actuators for diesel motor speed control or hydraulic jack leveling of equipment or something like that. We used to use something like this in geophysics to drive the ground with a 1/2 to 25 hz sine wave- hydraulically, with 3000 hp pumping the actuators. They would use the DC capability of this to lift actuators in one direction, say in lifting the Cape Fear lighthouse to move it (PBS Nova program).
If this amp actually drifts into DC offset it could burn your expensive woofer coils. Great package, wrong driver circuit for PA work.
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Thanks indianjo for your suggestions. I will check out the caps as well as the zener. I have found a schematic, which helps with the debugging - so far I only had the parts placement. For some reason the service manual posted online has the schematic removed. I will post my results after further debugging.
Thanks, Mark
Thanks, Mark
There were a lot of links to the schematic in this thread:
http://www.diyaudio.com/forums/solid-state/248973-crown-dc300a-repair-restore.html
There were also bits of the schematic posted including this op amp area.
The links in the 2006 thread that bing comes up with first were dead. Try those in this thread only 2 years old.
http://www.diyaudio.com/forums/solid-state/248973-crown-dc300a-repair-restore.html
There were also bits of the schematic posted including this op amp area.
The links in the 2006 thread that bing comes up with first were dead. Try those in this thread only 2 years old.
Thanks for the help! I was successful in getting this amp working well again!
Turned out the DC offset was related to 10 V power supply for the IC - bad Zener. Replaced both 10V Zeners (D4,D5), and was able to adjust the DC bias to 0V on both channels. Before this the IC was seeing +6V/-10V.
Next I went to check the bias, and found that it was out of spec in one channel. Turned out what I think is the voltage doubler circuit had a bad cap C5. Replaced all electrolytic caps, as well as the bias transistors Q101 and 201. Now with a tweak to the bias resistor on one channel, I was able to get both channels within the bias voltage range of 310-345 mV.
Now get clean output power well over 150 watts into 8 ohms.
Looking forward to using this amp to power some subwoofers I am building.
Thanks again for the help.
Regards
Mark
Turned out the DC offset was related to 10 V power supply for the IC - bad Zener. Replaced both 10V Zeners (D4,D5), and was able to adjust the DC bias to 0V on both channels. Before this the IC was seeing +6V/-10V.
Next I went to check the bias, and found that it was out of spec in one channel. Turned out what I think is the voltage doubler circuit had a bad cap C5. Replaced all electrolytic caps, as well as the bias transistors Q101 and 201. Now with a tweak to the bias resistor on one channel, I was able to get both channels within the bias voltage range of 310-345 mV.
Now get clean output power well over 150 watts into 8 ohms.
Looking forward to using this amp to power some subwoofers I am building.
Thanks again for the help.
Regards
Mark
Congratulations on your success.
See this thread http://www.diyaudio.com/forums/solid-state/304337-crown-dc300aii-resurrection.html
about concerns about the long term stability of the DC driver DC300a which uses pots - which oxidize over time and go open.
options include a protection circuit to disconnect the speakers if DC occurs on the output too long, or
replacing the whole driver board with a white oak brand driver board designed for a phase linear product which has archaic parts no longer availble. Like your central op amp.
See this thread http://www.diyaudio.com/forums/solid-state/304337-crown-dc300aii-resurrection.html
about concerns about the long term stability of the DC driver DC300a which uses pots - which oxidize over time and go open.
options include a protection circuit to disconnect the speakers if DC occurs on the output too long, or
replacing the whole driver board with a white oak brand driver board designed for a phase linear product which has archaic parts no longer availble. Like your central op amp.
I bought a can of deoxit but never used it. I don't know. I think it may contain cramolyn the secret ingredient that dissolves oxides but also promotes oxidation so that you have to use it again sooner than later. Sells more cans of deoxit that way.
I just know pots wipers are copper, brass or tin. Those oxidize, and small current small voltage signals can get blocked by that after time. Like the tiny feedback and zero adjust currents to the op amp at the heart of the DC300.
See all the workarounds for the bias current adjustment pot in all transistor class AB amps (except those that replace the pot with a resistor). Some day, maybe 12 years from now, a pot wiper will stay open circuit at turnon and DC will occur. IMHO. DC300's are designed to produce DC, and speakers don't like it. So think that through, and make a choice.
I just know pots wipers are copper, brass or tin. Those oxidize, and small current small voltage signals can get blocked by that after time. Like the tiny feedback and zero adjust currents to the op amp at the heart of the DC300.
See all the workarounds for the bias current adjustment pot in all transistor class AB amps (except those that replace the pot with a resistor). Some day, maybe 12 years from now, a pot wiper will stay open circuit at turnon and DC will occur. IMHO. DC300's are designed to produce DC, and speakers don't like it. So think that through, and make a choice.
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