BX4500H Hum in preamp section

[progdrums]

Has anyone had a problem with a loud hum in the preamp section of a BX4500H. I have read all the threads on this unit and could only find ppl who had problems with the power amp. Symptoms are a loud hum all the time however when an instrument is connected you can hear that the tone controls, ultra bass etc all seem to be working but hidden under the hum. The amp section works fine when driven with line level. I would hate to scrap this amp when it looks like it should be an easy fix.

[Enzo]

It doesn;t matter if someone else had this problem, nor does it matter what brand of amp it is. YOu have a humming preamp. SO we approach any amp the same way.

Isolate the problem. If you turn all the controls to zero, does the hum go away? If so, then which controls have to be up to hear it, and which ones have no effect?

The input jacks are on a small circuit board of their own are they not? Unplug that little board. Hum stay or leave?

The op amps run on +/-15v power supplies, so find them. I assume you have a pair of 15v Vregs on the power amp assembly? Are both producing 15v of CLEAN DC? Or is one down to like 9 or 10v with tons of ripple?

Powr OK? Then look at each op amp. Is there unexpected DC on any output pin? Pins 1 and 7 on the 8-leg dual op amps, and the four corners of the 14 pin ones.

[progdrums]

Approach to amp service not the same for all.

I get that you are trying to be helpful but you are stating the obvious when it comes to troubleshooting techniques.

Yes it does matter if someone else had this problem. Why reinvent the wheel. If the same problem keeps showing up and can be fixed or better yet modified so that it doesn't happen again that's great. So YES it does matter if someone else has this problem.

I don't approach each amp the same way. If you do and that works for you, that's great. In my life that is totally unrealistic. I look at the value of the amp. A guy just relisted this model on ebay with a starting bid of $150 and no one is bidding again. They sell new for around $260. So, my approach is look for the obvious like power supply problems, bad grounding etc. If all looks ok go look for a schematic. This is exactly what I did maybe two years ago. When I didn't find a schematic, I threw it in a corner hoping to one day post to see if others had the same problem.

I'll probably end up throwing it back in the corner and try again in another year. In the meantime, I'll work on stuff of value where I can easily find schematics.

It doesn;t matter if someone else had this problem, nor does it matter what brand of amp it is. YOu have a humming preamp. SO we approach any amp the same way.

Isolate the problem. If you turn all the controls to zero, does the hum go away? If so, then which controls have to be up to hear it, and which ones have no effect?

The input jacks are on a small circuit board of their own are they not? Unplug that little board. Hum stay or leave?

The op amps run on +/-15v power supplies, so find them. I assume you have a pair of 15v Vregs on the power amp assembly? Are both producing 15v of CLEAN DC? Or is one down to like 9 or 10v with tons of ripple?

Powr OK? Then look at each op amp. Is there unexpected DC on any output pin? Pins 1 and 7 on the 8-leg dual op amps, and the four corners of the 14 pin ones.

[Jag]

I don't think there are many here who would say anything different, or better, than what Enzo said. I gather that, although you didn't actually say so, you checked everything Enzo suggested?

Many people have had the same symptom, it's VERY common. That doesn't mean the solution is always the same.

I've never had to repair a BX4500H but, I've worked on many other amps that had that exact symptom. The traditional' "divide and conquer" trouble shooting method provides the best results. And no, there is not one global solution to the issue.

[DrGonz78]

Hey not sure about finding that exact schematic but the BVT-4500H might be very similar. Looking over this schematic might help you pinpoint design testing points and troubleshooting... Or I hear they make good door stops too...

behringer_bvt4500h.pdf

[Jazz P Bass]

Ten to one he lost a rail on his opamp power supplies.
But then again, there are how many parts & solder joints in the preamp.
It appears that the OP does not want to spend 'time' on the amp.
He would very much like the answer to why the preamp hums.
Which takes time.
I think we are caught in a loop.

[Enzo]

Hi Progderums, and welcome. When I posted, you had exactly ONE post, so I had no way of knowing your skill level. SO stating basic troubleshooting approaches may or may not have been obvious, I couldn't assume.

You stated you looked for the obvious like power supply problems and grounding. Well, when I stated we look at all amps the same way, that is pretty much what I was suggesting.

Nice try DrGonz, but the BVT4500 is very different. The BX1200 preamp is a lot closer, but even that is different.

Loud hum? My first thought would be DC on an op amp. I can't say that in my experience as a Behringer service center I have found any consistent IC that fails. It could be any of them. IN fact, if I had this in front of me, I'd verify power, then just look at the output pins of each op amp going down the board. Looks at quick glance like there are about five quad op amps - TL074 - so the corner pins of each. I don't see any of them driving LEDs to confuse the issue. If someone had a bad IC4 making a hummy amp, that really doesn't help the guy who has a bad IC7 causing hum. KNowing which volume/gain controls can turn the hum down (if any) at least helps us localize things.

[diydidi]

Hi.
Can anybody repost schematics for BX4500H please?