60 hz hum coming from amp

[jgcable]

I have a Randall Warhead that has taken a hell of a beating. First let me say that this amp was severely destroyed during shipping and I spent countless hours so far straightening the chassis and replacing broken parts. I got it to work and here is the problem. The clean channel is perfect. Perfectly silent and it appears to be problem free. The distortion channel is getting a wicked 60hz hum. As you increase the gain the hum gets louder. Plugging a guitar cord in or not does not affect the hum whatsoever. It sounds like a bad ground but I checked the central ground point and it is fine. Now I am thinking that it is the filter caps. Do any of you know what could be causing this hum. It is a low (typical) 60 hz hum. Thanks!! JG

[jgcable]

Re: 60 hz hum coming from amp

Bump.. anybody have any ideas?

[taxistationary]

Re: 60 hz hum coming from amp

Yeah, it's probably the amps caps. The best thing to do is check them yourself:

Take a multimeter and set it to VDC (voltage in direct current). Take the back probe and ground it. Then find the caps (ussually big bllack round things on the curcuit board) and get the amp to where you can acess their terminals on the underside on of the board. Then take the positive probe and try both of the terminals on all the caps. On one side of the terminal (+) you should get a reading somewhere around 400-500, and on the other side (-) you should get a reading of zero. If you get the 400-500 on the positive side, and on the negitive side you a reading above zero, then the cap is leaking DC into your curcuit, which meens it needs to be replaced.

If thats not the problem then that really could meen a lot of things could be happening. I gave you this test b/c caps in amps are usuually the problem. Be careful when you do this!! Good luck...

[Dreamland_Rebel]

Re: 60 hz hum coming from amp

just wait for pete's post. but i'm sure there could be several "links in the chain" that could be the culprit. i'd take it to a good technician.

[jgcable]

Re: 60 hz hum coming from amp

This amp was pretty trashed. Its solid state too. It has many capacitors of various sizes and values in it. There are several very large ones on the main board. Are these the filter caps? None of the caps have vented as far as I can see. How do I tell which ones are the filter caps?

[Twisteramps]

Re: 60 hz hum coming from amp

I'm not real sure about solid state amps. In a tube amp, the filter caps are going to be the largest ones in the amp.

Pete

[Cleveland Metal]

Re: 60 hz hum coming from amp

That's just what Warheads sound like man... [img]/images/graemlins/wink.gif[/img]

[Newc]

Re: 60 hz hum coming from amp

Stick a jumper lead on one side of the cap (-) and attach the other end to the chassis ground?
Better yet, attach a jumper to the chassis ground and get the amp humming, then start touching other ground points till you get sound but no hum.

Make sure your insurance is current [img]/images/graemlins/poke.gif[/img]

[jgcable]

Re: 60 hz hum coming from amp

[ QUOTE ]
That's just what Warheads sound like man... [img]/images/graemlins/wink.gif[/img]

[/ QUOTE ]

Cleveland.. I crap you not. This amp even in the condition it is in, sounds pretty frikkin good. I will tell you one thing for sure. Its LOUD! Really loud. It has more gain than any human being could ever need. With the gain channel on normal and set at 5 it has more gain than a rectifier dimed out. The gain channel has a pull boost feature that doubles the gain. You can leave it on 2 and do any type of death metal. The effects sound great and like I said.. this is one loud mutha. I need to fix it.

[jgcable]

Re: 60 hz hum coming from amp

How do I figure out which caps are the filter caps? There is a ton of small caps and 4 really large ones. I am gonna take the entire amp apart and rebuild it. The pots are PC mounted and 5 of them are sheered off at the shafts and seemingly impossible to get. The pots hold the circuit board in place when they are fastened to the chassis. I am thinking about using standard chassis mounted pots and making flying leads to the circuit board. I can then attach the circuit board to the chassis using standoffs. This amp has a ton of components in it. I am also thinking about making a new enclosure for it. The Warhead enclosure is probably 40lbs of its total weight of 56lbs. It is REALLY heavy and way bigger than it needs to be. There is at least 8" on each side of the cabinet that is completely empty and probably 6" of headroom. Its too bad that Randall no longer supports the Warhead or the Cyclone because it makes it really difficult to figure out what replacement parts to get. I think it is worth the investment in time and labor for me. I like messing with stuff like this.

[Cleveland Metal]

Re: 60 hz hum coming from amp

[ QUOTE ]
Cleveland.. I crap you not. This amp even in the condition it is in, sounds pretty frikkin good.

[/ QUOTE ]

Something MUST be wrong with that one then... Hehehe...

[jgcable]

Re: 60 hz hum coming from amp

I guess I see the good in alot of things that aren't really that good!

[Cleveland Metal]

Re: 60 hz hum coming from amp

I had extremely high hopes for that amp, till I played one for an hour or two.

But, as long as you enjoy it, that is absolutely all that counts.

[jgcable]

Re: 60 hz hum coming from amp

On paper it looks great. Internally there are many reasons why it should sound great and many reasons why it shouldn't. The circuit boards are robust but many of the PC mounted components and pots are as low quality as it gets. Many of the solder joints were marginal, the knobs are the cheapest plastic I have ever seen. It does have an interesting feature on the main board. There is a red and a green LED soldered to the board that light up when certain modes are activated. I have no idea why they are there because you can't see them when the chassis is in the enclosure. I am gonna use mil spec replacement parts and pitch those plastic elcheapo knobs and install Alco aluminum knobs. I am also thinking about removing the graphic EQ from the path. It is a really cheap one and I bet it only adds cheese and noise to the amp. I wish I knew more about SS amps but I guess I will learn on this one.
Now... what the heck is causing that 60 cycle hum???

[jgcable]

Re: 60 hz hum coming from amp

[ QUOTE ]
Yeah, it's probably the amps caps. The best thing to do is check them yourself:

Take a multimeter and set it to VDC (voltage in direct current). Take the back probe and ground it. Then find the caps (ussually big bllack round things on the curcuit board) and get the amp to where you can acess their terminals on the underside on of the board. Then take the positive probe and try both of the terminals on all the caps. On one side of the terminal (+) you should get a reading somewhere around 400-500, and on the other side (-) you should get a reading of zero. If you get the 400-500 on the positive side, and on the negitive side you a reading above zero, then the cap is leaking DC into your curcuit, which meens it needs to be replaced.

If thats not the problem then that really could meen a lot of things could be happening. I gave you this test b/c caps in amps are usuually the problem. Be careful when you do this!! Good luck...

[/ QUOTE ]

I tested all the big caps per your instructions by grounding the black probe on my meter and touching the red probe to either of the 2 terminals and here are the results:

4 large caps 4700uf @ 63V

Cap 1 -353 and 0
Cap 2 -335 and 0
cap 3 -33 and 0
cap 4 -33 and 0

2 medium sized caps 2200uf @ 35V

cap 1 472 and 0
cap 2 1041 and 0

1 medium sized cap 1000uf @ 63V

473 and 0

I don't get it!