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Old 22nd February 2013, 12:50 AM   #51
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An analog meter would probably have been better, but at the noise and scratchieness changes, the AC voltage goes wild. I'm measuring in reference to ground (between the two main filter caps) and I get readings of 8V and jumping to 50 and 120V and back down. Pretty odd.

I'll make a quick video.
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Old 22nd February 2013, 01:06 AM   #52
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I just turned mine on and it isn't making the scratchy sound as it comes and goes.
And my controls have been cleaned yet so the are a bit dirty.
You will get some hum in the preamp if the gains are up and there is nothing plugged into the input as it is open in that condtion and it as tremendous gain.

As I leave it on some times it will start a loud scratchy sound all by itself.
I have left it on for days and forget about it and then it will start up and I then say to my self what the heck is that and discover that I left the amp on.

I am expecting this is what you are experiencing.
I really would like to know what is causing this as well.
Past experience has showed me that cold solder joints and bad caps can cause this as well as a noisy transistor.

The main issue is to determine if it is in the preamp stage or is it the power amp.
I have seen controls go bad as well where they would open on one side of them giving them the appearance that the are working,sorta.
The only way to check these it to take it out or isolate it from the rest of the circuit by lifting on end of any resistors connecting to it and then checking its function with an ohmmeter.
Sometimes it can be done in circuit depending on how it is configured.

jer
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Old 22nd February 2013, 01:08 AM   #53
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Which voltage rail are you measuring?
That looks a bad cap to me!!
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Old 22nd February 2013, 01:11 AM   #54
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Here is the link to the video

Renown 400 Hum - YouTube
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Old 22nd February 2013, 01:13 AM   #55
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If it is the 15v rails how are those voltages.
They are regulated by CR49 & CR50 zener diodes some times these go bad and become leaky and cause the voltage on that rail to jump around.

jer
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Old 22nd February 2013, 01:19 AM   #56
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Both jump around. The main voltage for the amp and also the preamp. I actually get 27V and -27V from the smaller preamp caps. I believe it is then regulated to 15V by R160 and R161 to provide a solid +/- 15V
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Old 22nd February 2013, 01:27 AM   #57
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Cool thanks for the Video.
I don't think that the caps are bad on the 55V supply because the amp is quiet when it is disconnected from the preamp.

I would check the 15v Rails and zener's.
You can try just tacking in a filter cap with some clip leads and see if the hum goes away.
But make sure that both the amps cap, and, the spare cap, is fully discharged before you add it.
Because the voltage from the spark from connecting the cap while the power is on could be high enough to take out an op amp and create more problems.

jer
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Old 22nd February 2013, 01:29 AM   #58
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Check those 15V rails to see if I get 15V and also for any AC?
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Old 22nd February 2013, 01:34 AM   #59
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yes.
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Old 22nd February 2013, 01:39 AM   #60
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Okay.

CR50 spits out -3VDC (should be -15VDC) and 0.032VAC

CR49 spits out 15.3VDC and and goes wild on AC. Sometimes it's 5VAC then it jumps to 10VAC, then down to 579mV of AC
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