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Did I destroy my power amp?
Did I destroy my power amp?
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Old 11th August 2019, 04:51 PM   #1
JaneyBleep is offline JaneyBleep  United States
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Join Date: Dec 2015
Location: Midwest USA
Default Did I destroy my power amp?

Recently I acquired a Peavey 1.3k power amp from my local church. Whenever I would turn it on,the cooling fan did not want to start spinning on it's own so I gave it a nudge and it would spin fine.Then I decided to replace the fan with a computer desktop 120mm fan,installed the desktop fan and turned on the power amp,fan started up just fine. Thirty minutes later,the fan stopped working and the power amp started to get really hot.I turned it off,opened the cover and found a blown resistor.
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

This it what the resistor would look like in it's normal state:
Click the image to open in full size.

I turned on the power amp back on and it just makes a buzzing sound on left and right channels.

I'm guessing the mismatching voltage of the desktop computer fan made the resistor blow,i have no idea.

My question is,is it just a matter of replacing the resistor and installing the proper voltage cooling fan or did i kill the power amp?
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Old 11th August 2019, 06:15 PM   #2
MAAC0 is offline MAAC0  Portugal
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You must find out first what actually this resistor is doing. If it is powering the fan then replace it and use a fan with the same voltage or You can even get a mains voltage fan.
As what the buzzing concerns, this amp needs some cleaning... Measure for DC on the output, if there is no DC probably it's fine.
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Old 11th August 2019, 07:23 PM   #3
amplidude is offline amplidude  Denmark
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Quote:
Originally Posted by JaneyBleep View Post
Recently I acquired a Peavey 1.3k power amp from my local church. Whenever I would turn it on,the cooling fan did not want to start spinning on it's own so I gave it a nudge and it would spin fine.Then I decided to replace the fan with a computer desktop 120mm fan,installed the desktop fan and turned on the power amp,fan started up just fine. Thirty minutes later,the fan stopped working and the power amp started to get really hot.I turned it off,opened the cover and found a blown resistor.
Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.

This it what the resistor would look like in it's normal state:
Click the image to open in full size.

I turned on the power amp back on and it just makes a buzzing sound on left and right channels.
Looking at this

I'm guessing the mismatching voltage of the desktop computer fan made the resistor blow,i have no idea.

My question is,is it just a matter of replacing the resistor and installing the proper voltage cooling fan or did i kill the power amp?
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File Type: jpg Screenshot_20190811-202214_Drive.jpg (346.9 KB, 89 views)
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Old 13th August 2019, 04:46 AM   #4
JaneyBleep is offline JaneyBleep  United States
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Quote:
Originally Posted by MAAC0 View Post
You must find out first what actually this resistor is doing. If it is powering the fan then replace it and use a fan with the same voltage or You can even get a mains voltage fan.
As what the buzzing concerns, this amp needs some cleaning... Measure for DC on the output, if there is no DC probably it's fine.
I took the power amp to the local audio shop and they told me the resistor blew because i put a mismatching cooling fan in the power amp.
As for the amp itself ,it is fine,both channels fully function.
The buzzing sound was coming from a faulty speaker wire as i found out later after further inspection.
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Old 13th August 2019, 05:50 AM   #5
wiseoldtech is offline wiseoldtech  United States
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Exchanging parts without the knowledge or research beforehand always results in things like this.
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Old 13th August 2019, 06:47 AM   #6
Boydk is offline Boydk  Denmark
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Quote:
Originally Posted by wiseoldtech View Post
Exchanging parts without the knowledge or research beforehand always results in things like this.
Really living up to your nick with all that helpful info.... are you???
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Old 13th August 2019, 07:28 AM   #7
wiseoldtech is offline wiseoldtech  United States
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Quote:
Originally Posted by Boydk View Post
Really living up to your nick with all that helpful info.... are you???

Indeed!.... you should always speak truthful.
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Old 13th August 2019, 01:16 PM   #8
indianajo is offline indianajo  United States
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It is an 120 VAC fan. I think it says that on the label, whereas computer fans are usually 24 VDC. Also usually says that on the label.
The resistor is to slow the fan down for quiet when the amp is not hot. A thermostat shorts the resistors. Buzzing could be form interference from the open AC line running around the case.
You need to check the speaker connections for DC to make sure you have not blown the amp before connecting any speaker to it. About 100 mv max DC.
I fixed one of these badly blown, but it took 20 output transistors, 3 drivers and 101 cheaper parts. OTOH I fixed a PV-4c (low power cousin) with 4 output transistors and some new mains filter caps. It was a good education in PA amp features. I used the schematic from eserviceinfo.com, a DVM and an analog VOM with a 2 vac scale to trace the music through the front end. No scope required.
I usually get my parts from newark in SC, although digikey is closer to westerners to MN and mouser is closer in TX. Only the 3180 transconductance amp is unavailable. Peavey even has that but you have to call them on the phone to get parts.
Put a filter over the fan grill; dust is what kills these amps usually if a speaker wire short doesn't get them. Don't use 1/4 phone plugs in temporary setups, they short if pulled out. Use dual banana plugs.
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Last edited by indianajo; 13th August 2019 at 01:26 PM.
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Old 13th August 2019, 04:31 PM   #9
JaneyBleep is offline JaneyBleep  United States
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This was the original cooling fan in the Peavey :
Click the image to open in full size.
Yes,you are correct about me not taking the time to find the proper replacement online ,especially when they are readily available .

In the meantime ,this is the cooling fan from a Dell desktop that i wired to a wall adapter :
Click the image to open in full size.
Click the image to open in full size.
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Old 13th August 2019, 05:26 PM   #10
JMFahey is offline JMFahey  Argentina
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Try to get the mains powered fan.

Besides matching voltage, they typically blow 2 or 3 times mor air (CFM=Cubic Feet per Minute) than same size DC ones.

Cooling a PC supply is light duty; but a couple pounds worth of aluminum heatsinks plus lots of heat dissipation is for the grown ups only
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