Hello everyone! I am a first time poster, but have been appreciating this forum's knowledge and information for sometime.
I have a problem with my Crown DC300A amp. The IOC lights have started to come on when I have the channel set to twelve o'clock or higher. When initially turned on, the audio is audibly very distorted, but then becomes less distorted with time. It never fully goes away and IOC lights continue to flicker with the audio.
Before, the IOC lights would come on only if I was feeding audio to the amp while I powered it off. I understand this behavior is normal? It also did not have audible distortion before on power up at all. It immediately sounded correct and the IOC lights did not come on. I have had the amp for about 5 years.
Does anyone have any suggestions on how to start repairing it? I have a little experience with minor repair, but if it is really involved, I might send it out. Thanks!
I have a problem with my Crown DC300A amp. The IOC lights have started to come on when I have the channel set to twelve o'clock or higher. When initially turned on, the audio is audibly very distorted, but then becomes less distorted with time. It never fully goes away and IOC lights continue to flicker with the audio.
Before, the IOC lights would come on only if I was feeding audio to the amp while I powered it off. I understand this behavior is normal? It also did not have audible distortion before on power up at all. It immediately sounded correct and the IOC lights did not come on. I have had the amp for about 5 years.
Does anyone have any suggestions on how to start repairing it? I have a little experience with minor repair, but if it is really involved, I might send it out. Thanks!
Without taking a cover off, you can make 2 checks with a VOM. You can check if there is DC coming out of the speaker jacks. More than 200 mv, there is a problem.
If not that, you can put the amp on a load like an 8 ohm resistor, or more likely an 8 ohm speaker that you don't mind damaging. Either should have a 150 W rating. You play music of a relatively constant volume then turn the amp up until you hear distortion. Measure the AC voltage coming out of the amp just before distortion. A 150 W/ch amp should put out nearly 49 v into 8 ohms. A lot less, you may have a power supply problem.
Note that cheap DVM (~$30) with a 200 v AC scale produce random numbers on music. They are designed to measure 50 and 60 hz power line frequencies. This measurement is best done with an analog vom, available for about $30 on ebay. Or an oscilloscope, which requires an investment in easily broken scope probes, which I last saw for $50 each. There are also $150 RMS DVM, which I have avoided buying because they lie about ultrasonic oscillation. Anything above 7000 hz, a fluke RMS DVM will not see or indicate. Waste of money IMHO.
At this age the power supply problem is very likely dried up rail (main voltage) caps.
Distortions can also be caused by dried up coupler caps, or connectors that are coated with oxide and don't allow music to flow. A special kind of connector is the volume pot, and the tone control pots, which can lose contact due to age or high wear and cause distortion.
Oxidized connectors can , with the AC power off, be reseated to scrape oxide off. Volume pots can be sprayed out with contact cleaner such as that from CRC or LPS. Warning contact cleaner may be flammable, no smoking, open flame, pilot lights, electricity turned on or off within 10m of spray. Set up a fan first before spraying to dissipate the fumes.
There are several other DC300A threads on this forum, also more particularly the PA forum, which is the kind of amp this is. For details see those threads.
Personally, because a DC setting pot wiper oxidation might destroy a speaker worth much more than an old amp, I'd buy something else. These amps are designed to produce massive continuous DC outputs for controllling actuators and hydraulic valves. If a pot wiper comes loose from the setting, they will, and fry a speaker.
I've bought Peavey PV4 and CS800s amps used for $20 and $100 and repaired them. They are cheap in North America, protected against DC output, and documentation is available & complete.
If you are tempted to probe inside the amp, make sure you use one hand at a time. Voltages over 24 from one hand to the other can stop your heart. Use an alligator clip lead to connect the meter negative to speaker ground. Do not wear jewelry on hands, wrists or neck. 1v at high enough current can burn your flesh to charcoal. there are energy storage devices, capacitors, inside PA amps that have typically twice the rated output voltage in them, and the may not self discharge with the AC power unplugged. This amp may have 100 v capacitor charges in it, with enough energy to burn the end off a meter probe. Wear safety glasses, defective amps sometimes blow the top off parts.
Best of luck.
If not that, you can put the amp on a load like an 8 ohm resistor, or more likely an 8 ohm speaker that you don't mind damaging. Either should have a 150 W rating. You play music of a relatively constant volume then turn the amp up until you hear distortion. Measure the AC voltage coming out of the amp just before distortion. A 150 W/ch amp should put out nearly 49 v into 8 ohms. A lot less, you may have a power supply problem.
Note that cheap DVM (~$30) with a 200 v AC scale produce random numbers on music. They are designed to measure 50 and 60 hz power line frequencies. This measurement is best done with an analog vom, available for about $30 on ebay. Or an oscilloscope, which requires an investment in easily broken scope probes, which I last saw for $50 each. There are also $150 RMS DVM, which I have avoided buying because they lie about ultrasonic oscillation. Anything above 7000 hz, a fluke RMS DVM will not see or indicate. Waste of money IMHO.
At this age the power supply problem is very likely dried up rail (main voltage) caps.
Distortions can also be caused by dried up coupler caps, or connectors that are coated with oxide and don't allow music to flow. A special kind of connector is the volume pot, and the tone control pots, which can lose contact due to age or high wear and cause distortion.
Oxidized connectors can , with the AC power off, be reseated to scrape oxide off. Volume pots can be sprayed out with contact cleaner such as that from CRC or LPS. Warning contact cleaner may be flammable, no smoking, open flame, pilot lights, electricity turned on or off within 10m of spray. Set up a fan first before spraying to dissipate the fumes.
There are several other DC300A threads on this forum, also more particularly the PA forum, which is the kind of amp this is. For details see those threads.
Personally, because a DC setting pot wiper oxidation might destroy a speaker worth much more than an old amp, I'd buy something else. These amps are designed to produce massive continuous DC outputs for controllling actuators and hydraulic valves. If a pot wiper comes loose from the setting, they will, and fry a speaker.
I've bought Peavey PV4 and CS800s amps used for $20 and $100 and repaired them. They are cheap in North America, protected against DC output, and documentation is available & complete.
If you are tempted to probe inside the amp, make sure you use one hand at a time. Voltages over 24 from one hand to the other can stop your heart. Use an alligator clip lead to connect the meter negative to speaker ground. Do not wear jewelry on hands, wrists or neck. 1v at high enough current can burn your flesh to charcoal. there are energy storage devices, capacitors, inside PA amps that have typically twice the rated output voltage in them, and the may not self discharge with the AC power unplugged. This amp may have 100 v capacitor charges in it, with enough energy to burn the end off a meter probe. Wear safety glasses, defective amps sometimes blow the top off parts.
Best of luck.
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Hello Indianajo and thank you for your reply and advice!
I have an analog multi-meter and an oscilloscope, but I've been wanting to better learn the oscilloscope, so maybe this is a good chance for that. I will make sure to to take the proper safety precautions.
If I measure DC coming out of the speaker jacks, does this also likely point to dried up capacitors in the power supply? Is there anything I should just automatically replace besides the old capacitors?
I've had the amp in service for about 5 years in my living room and it has, up to this point, preformed like a champ. I have a DC speaker relay that is supposed to protect my speakers from being fried. All this to say, I have an attachment to this old amp at this point, so I want to fix it if possible.
I have an analog multi-meter and an oscilloscope, but I've been wanting to better learn the oscilloscope, so maybe this is a good chance for that. I will make sure to to take the proper safety precautions.
If I measure DC coming out of the speaker jacks, does this also likely point to dried up capacitors in the power supply? Is there anything I should just automatically replace besides the old capacitors?
I've had the amp in service for about 5 years in my living room and it has, up to this point, preformed like a champ. I have a DC speaker relay that is supposed to protect my speakers from being fried. All this to say, I have an attachment to this old amp at this point, so I want to fix it if possible.
Hello Indianajo and thank you for the reply!
This will be a good chance for me to better learn my oscilloscope.
Would DC voltage from the speaker jacks also point to dried up capacitors in the power supply? Is there anything else I should automatically replace? I will check out those other threads as well.
I will also make sure to take the proper safety precautions.
I've had this amp for 5 years and its preformed very well up until this point. I'd like to rescue it if possible, but if that starts becoming too problematic, I will check out those other amps for sure.
This will be a good chance for me to better learn my oscilloscope.
Would DC voltage from the speaker jacks also point to dried up capacitors in the power supply? Is there anything else I should automatically replace? I will check out those other threads as well.
I will also make sure to take the proper safety precautions.
I've had this amp for 5 years and its preformed very well up until this point. I'd like to rescue it if possible, but if that starts becoming too problematic, I will check out those other amps for sure.
DC coming out of the speaker jack is most likely on this amp, DC adjustment problems. Besides the pots that could cut loose at any point, there is a low voltage DC supply fed by a zener diode that has been posted about on previous threads.
However, the classic cause of DC on speaker jacks is output transistor meltdown, causing 100's of other parts to exceed their rated voltage and be opened or shorted. 127 parts I found wrong in my PV-1.3k that had shorted output transistors.
Low wattage out, ie 2 or 4 vac out the speaker jack on music, instead of 49 vac, points pretty emphatically to to dried up mains capacitors.
However, I would never allow this amp to touch my $600 a pair speakers ($600 each new). The possibility of DC out is high with this amp, since it is designed to produce DC out. This amp was the best thing for stage shows about 1975, but a lot of water has flowed under the dam. Bands in 1975 and 76 didn't have to worry about DC adjustment pots oxidizing. That is a 5 to 10 year problem. It has been 45 years since 1975.
If I inherited one (I'd never bid on one for ebay) I would take both driver boards out and salvage the case, transformer, rectifiers, heat sinks, output transistors if not blown, fans, controls etc. Almost any other driver board design would be better for driving speakers. The output transistors are Class AB emitter followers, and hence are compatible with nearly all modern driver designs. 1975 was full of such oddities as the SWTC Universal Tiger, which had strange and weird totem pole output design and was known for setting fire to resistors on stage.
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