Peavey XC-400, need some help with repairs.

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KiLL3Rw0Lf
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Peavey XC-400, need some help with repairs.

Post by KiLL3Rw0Lf » Mon Jul 17, 2017 6:07 pm

Thanks for looking. I am attempting to repair a Peavey XC-400. When it is plugged in and powered on it puts out this super annoying, loud sound. It sounds about like it's shorting out, but I don't see any obvious shorts. It's starting to annoy me a bit. I feel as though it is a super simple fix, but I can not see it. I figured I'd ask and I hope that someone will help me quick as I'd like to get it done ASAP. I will attach a link to Google Drive at the end of this where you will see all mentioned pics and the schematics for the board along with a diagram of where the unknown parts are..

I have so far checked/replaced the smaller "filter" caps, the audio transistors, the power transistors, the power supply, and I replaced the power cord. I had to replace the power cord due to the ground prong being broken off. I believe there may have also been a very slightly higher amperage fuse in it then listed and it was blown, unfortunately. Can someone help me here? What else could it be? I also noticed these blue, what I believe to be capacitors. They read ITW .01 M 125VAC DVL. I believe "ITW" is the brand, 125VAC is, of course, the voltage (125 volts, alternate current), and I believe .01 M to be the same as .01uF, and I don't know what DVL is. :roll: Could it be these? I got no reading on my multimeter no matter what settings I had it on and which polarity I used. Well, all except for a VERY fast number cross the screen on the resistance test, however I could never get that reading again.

Thanks to everyone who replies. No matter if it helps or not your comment is appreciated. Have a nice day.
https://drive.google.com/drive/folders/ ... sp=sharing

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Enzo
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Re: Peavey XC-400, need some help with repairs.

Post by Enzo » Mon Jul 17, 2017 11:03 pm

XC400? not XR400?

First off, is your loud noise a loud HUM? Look at the speaker cones. DO they move on e direction and stay there? If so that means the amp is making DC, and it will damage speakers.

On the other hand, if you have a bad filter cap, you will also get a loud hum, but no DC. Those are the large blue cans. And make sure they don't just need resoldering.

Don't worry about the blue caps on the power switch. If that fails you will be blowing circuit breakers.

So disconnect the speaker, and monitor for DC voltage on the speaker jack. DO not connect a speaker or load until we have no DC there.

Your main power rails are 52 volts on the drawing, so do you have around 50-55vDC one of each polarity? If one reads about half, then that one probably lacks a filter cap.

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KiLL3Rw0Lf
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Re: Peavey XC-400, need some help with repairs.

Post by KiLL3Rw0Lf » Tue Jul 18, 2017 7:16 am

Enzo wrote:XC400? not XR400?

First off, is your loud noise a loud HUM? Look at the speaker cones. DO they move on e direction and stay there? If so that means the amp is making DC, and it will damage speakers.

On the other hand, if you have a bad filter cap, you will also get a loud hum, but no DC. Those are the large blue cans. And make sure they don't just need resoldering.

Don't worry about the blue caps on the power switch. If that fails you will be blowing circuit breakers.

So disconnect the speaker, and monitor for DC voltage on the speaker jack. DO not connect a speaker or load until we have no DC there.

Your main power rails are 52 volts on the drawing, so do you have around 50-55vDC one of each polarity? If one reads about half, then that one probably lacks a filter cap.
Definitely XC-400, unless Peavey's faceplate is lying to me. Lol. Yes, it is a loud hum. Thank you for the help. So, poke multimeter on speaker output, got it. I've heard that it hurts the amp to be turned on without a load, but for such a short amount of time I guess it would be ok.

Also, stupid me made another mistake. XD. The other blue cap goes to the audio out, not audio in. My coffee wore off yesterday so when I posted this I was a little tired and had to take all photos and make diagrams then. :D

I appreciate you bearing with this noob. I have years of reading but little experience with larger curcuits.

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Re: Peavey XC-400, need some help with repairs.

Post by Enzo » Tue Jul 18, 2017 12:00 pm

It doesn't harm the amp in the slightest to operate without a load. That caution refers to tube amplifiers, not this solid state one.

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Re: Peavey XC-400, need some help with repairs.

Post by KiLL3Rw0Lf » Tue Jul 18, 2017 2:23 pm

Enzo wrote:It doesn't harm the amp in the slightest to operate without a load. That caution refers to tube amplifiers, not this solid state one.
Ok, that's also what I read. "Ok for ss, bad for tube". Due to all the mixed comments I just decided not to try it on such an amp. I am currently out and about. Will have to test your suggestions once I get back.

To my understanding, if both filter caps are out, on the output, it will show +/-50vDC, if one is out is will show +/-25vDC, if none then it will be clean or no DC, correct?

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Re: Peavey XC-400, need some help with repairs.

Post by Enzo » Tue Jul 18, 2017 5:54 pm

In the power supply, AC voltage from the transformer is rectified into pulsing DC. Ther is a large filter capacitor for each polarity. if the filters are good, both +52 and -52v supply should be present and free of ripple. If a supply has a bad cap, then the filtration is lost and there will not be 52v of smooth DC, instead you would get the pulsing raw DC. That will measure roughly half or 2/3 as much. If both caps have failed, then both supplies will read way low.

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KiLL3Rw0Lf
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Re: Peavey XC-400, need some help with repairs.

Post by KiLL3Rw0Lf » Fri Jul 21, 2017 6:55 pm

Enzo wrote:In the power supply, AC voltage from the transformer is rectified into pulsing DC. Ther is a large filter capacitor for each polarity. if the filters are good, both +52 and -52v supply should be present and free of ripple. If a supply has a bad cap, then the filtration is lost and there will not be 52v of smooth DC, instead you would get the pulsing raw DC. That will measure roughly half or 2/3 as much. If both caps have failed, then both supplies will read way low.
Ok, I finally got the time to sit down and look at this. To my knowledge, it looks like one of the filter caps what placed on backwards by whoever last took it apart. Looking on the bottom of the board you can tell where they were once taken off. There is rosin at the solder joints and they look soldered by hand, not machine-y, if you know what I mean. I haven't taken them off myself yet. I wanted your comment before doing so. I don't want to apply heat if not necessary.

You can see for yourself in the following link: https://drive.google.com/drive/folders/ ... sp=sharing

In the photo, named Filter_Capacitor-Front.PNG, you can see that one of the caps has the printed side facing the front with the other cap facing the back. Now, I believe from the printed side that the left side is positive and the right is negative. Am I correct in thinking that or is it backwards? In the Schematic I have out lined the caps in red for convenience. I have also put notes in the photos for convenience.

Anyways, We have a +52V and a -52V as you said and as the schematics say (I'm still figuring this schematic reading thing out and I think I might have it). In order to get -52V one cap has to be backwards. To get that backwards output cap, how they are placed on the board is they both have to be the same. In photo Filter_Capacitor-Front.PNG they need to be (- to +)(- to +) because the ground pin from the Transformer comes in between them.

Like this: +52V out (+ to-) Transformer ground (+ to -) -52V out. Or shorter: +52V(+<-)^(+>-)-52V, (^) representing ground since the ground icon is not on the keyboard. It really should be though. XD

Is this correct? Sorry if I don't make much sense. I'm bad at explaining things because I do it in a way I understand, not what others understand. lol

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Re: Peavey XC-400, need some help with repairs.

Post by Enzo » Fri Jul 21, 2017 7:11 pm

If the cap was backwards, it would usually blow a fuse. If you have +52 and -52 in the amp, and both seem fairly clean, then likely the cap is not backwards.

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Re: Peavey XC-400, need some help with repairs.

Post by KiLL3Rw0Lf » Fri Jul 21, 2017 7:25 pm

Enzo wrote:If the cap was backwards, it would usually blow a fuse. If you have +52 and -52 in the amp, and both seem fairly clean, then likely the cap is not backwards.
It is blowing fuses. I used a slow blow in it because that is all Radio Shack had (bought when they were still open) in 8ohm glass fuses. Since it's slow blow it takes a second to blow. Hopefully the prolonged exposure due to slow blow didn't cause major issues.

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Re: Peavey XC-400, need some help with repairs.

Post by Enzo » Sat Jul 22, 2017 2:29 am

Oh, sorry, I thought you said you had 52v supplies going.


Well then perhaps it IS backwards, remove it and see if fuses still blow.

The large 5000uf 55v caps have not been available for many years. We use 4700uf 63v caps in their place.

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Re: Peavey XC-400, need some help with repairs.

Post by KiLL3Rw0Lf » Sat Jul 22, 2017 6:59 am

Enzo wrote:Oh, sorry, I thought you said you had 52v supplies going.


Well then perhaps it IS backwards, remove it and see if fuses still blow.

The large 5000uf 55v caps have not been available for many years. We use 4700uf 63v caps in their place.
XD. Sorry for the confusion. I'll track down which one needs flipped and see if that works. I believe it will be the right one though. Thanks for the help.

I saw that not too long ago. I believe the actual one Peavey suggests it #30320146. I'd think using a smaller capacitance cap would cause a ripple. Maybe not noticably since it's close to the original 5,000uF.

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Re: Peavey XC-400, need some help with repairs.

Post by KiLL3Rw0Lf » Sat Jul 22, 2017 7:18 am

Enzo wrote:Oh, sorry, I thought you said you had 52v supplies going.


Well then perhaps it IS backwards, remove it and see if fuses still blow.

The large 5000uf 55v caps have not been available for many years. We use 4700uf 63v caps in their place.
I also just found these:

http://www.tedss.com/23N502M0635Q1A1

They aren't too badly priced. They wouldn't fit like the originals but neither does Peavey's suggestion. Lol. They are about the same price as Peavey's suggested, if not cheaper. Actually Tedss has a whole list of 5,000uf caps here:

http://capacitor.tedss.com/5000uF?sortB ... ortOrder=A

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Re: Peavey XC-400, need some help with repairs.

Post by Enzo » Sat Jul 22, 2017 10:50 am

4700uf is well within 10% of 5000uf, your original 5000uf part could have measured as low as 4500 and be within that spec. The difference won't be audible. In fact your original might well have been a 20% part. Big filter caps are not precision parts.

Here is one at Mouser that costs under $5 with wire leads.
http://www.mouser.com/ProductDetail/Nic ... BXSewMA%3d

And in a snap-in also under $5:
http://www.mouser.com/ProductDetail/Nic ... Grpfquk%3d

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KiLL3Rw0Lf
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Re: Peavey XC-400, need some help with repairs.

Post by KiLL3Rw0Lf » Sat Jul 22, 2017 12:13 pm

Enzo wrote:4700uf is well within 10% of 5000uf, your original 5000uf part could have measured as low as 4500 and be within that spec. The difference won't be audible. In fact your original might well have been a 20% part. Big filter caps are not precision parts.

Here is one at Mouser that costs under $5 with wire leads.
http://www.mouser.com/ProductDetail/Nic ... BXSewMA%3d

And in a snap-in also under $5:
http://www.mouser.com/ProductDetail/Nic ... Grpfquk%3d
That is a good point. XD

So apparently the blue film was put on backwards. Once I finally got it desoldered I noticed that there were little emblems on the feet and that one of the feet was a specific shape to allow it to fit in only one slot. I then noticed that on the film there was one of the same emblems beside the word NEGATIVE. So it is placed correctly. Now back to the drawing board.

To test the positive and negative 52V do I need to desoldered anything after the caps to keep that voltage from going beyond the cap. Just in case the cap is bad?

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Re: Peavey XC-400, need some help with repairs.

Post by Enzo » Sat Jul 22, 2017 4:41 pm

Cap can easily have been damaged running backwards. But I just bring up the voltage and find out. A shorted cap might blow fuses, but won't stres the transistors.

No load on the amp output, and look up "light bulb limiter" and make one and use it.

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