Hum. How to diagnose (and repair)?

[misterwhizzy]

So my JCA50 has a pretty bad sixty-cycle hum. Other amps in the same socket don't have it nearly as bad. It seems to have to do with the power section, because preamp volume only affects it very, very slightly, and it happens even with V1, V2, or V3 pulled. Is it possible that the bias is too hot? Is it just house wiring or maybe less-than-perfect amp design? Any ideas on how to solve it? I don't mind getting in there if need be, but it's easier to replace preamp tubes, for example, than to remove the whole amp from the chassis and mess with bias.

In the process, I changed V1, V2, and V3 just to see if there was any noticeable difference with different tubes, and there was not.

 

[Blille]

Re: Hum. How to diagnose (and repair)?

Ebtech Hum X.


Sent from my iPhone using Tapatalk Pro

 

[Lazarus1140]

Re: Hum. How to diagnose (and repair)?

No ideas from me, but I'm anxious to see what you learn.

 

[Gtrjunior]

Re: Hum. How to diagnose (and repair)?

First thing to do is plug it in somewhere else. Eliminate the house wiring if possible.

Is this humming something that just started happening or has it been there since you first owned the amp?

 

[ehdwuld]

Re: Hum. How to diagnose (and repair)?

Fluorescent backlighting?

Mine did a howl not so much a hum
It turned out to be a soldier joint on one of the tube sockets
Yours could be similar or a cable routed too close to something inside
Really hard to tell without diving in


EDIT
could be simple as a bad tube

 

[Mincer]

Re: Hum. How to diagnose (and repair)?

My first thought is to bring the amp to another building. Does it still hum with nothing plugged into it?

 

[misterwhizzy]

Re: Hum. How to diagnose (and repair)?

Well, I've been in the house seven years, and I thought I had remembered the inspector saying something about the upstairs wiring. I was hoping it was something there that I could fix at the outlet. No such luck. The outlet was grounded perfectly, based on my multimeter test.

So I have a Hum-X on order. Twenty percent off at Musician's Friend at least makes it worth a try.

 

[Mincer]

Re: Hum. How to diagnose (and repair)?

Let us know if that fixes the issue.

 

[slaveunit]

Re: Hum. How to diagnose (and repair)?

This is a known issue with that amp. The heater traces need to be raised above the pcb. Do a search in the mlp forum and you'll find details. Just know that you'll need to cut some traces by removing the board I believe. Then you'll have to rerun wire that is raised up from the board.

If you can't find the thread pm me and I'll help you find it.

Edit:
Found it.
http://www.mylespaul.com/threads/nad-jet-city.300487/page-2

 

[misterwhizzy]

Re: Hum. How to diagnose (and repair)?

Thanks a lot for looking it up. My sub-par Google-Fu wasn't cutting the muster. I'm afraid I might have to do exactly what he did. I'll give the Hum-X a shot, but I'm not optimistic after seeing this.

 

[slaveunit]

Re: Hum. How to diagnose (and repair)?

Thanks a lot for looking it up. My sub-par Google-Fu wasn't cutting the muster. I'm afraid I might have to do exactly what he did. I'll give the Hum-X a shot, but I'm not optimistic after seeing this.

You're welcome. What made it worse is that the the mlp search function isn't working. So I felt guilty and just figured that I would dig it out for you.

That same fix is what removed the hum from mine. Notice the settings that I had the op of the thread test with. Does yours react the same?

 

[misterwhizzy]

Re: Hum. How to diagnose (and repair)?

That same fix is what removed the hum from mine. Notice the settings that I had the op of the thread test with. Does yours react the same?

Yes. On both channels, the hum is there with the preamp drive all the way down, and it gets louder with increasing master volume. Doesn't sound like a lot of fun to pull the board and all the knobs, etc., but if that's what it takes, I'll go ahead and do it. I've already converted the crunch channel to a bit more Marshally setup, and I could put a sweep pot and/or a replacement bias trimmer in there while it's out. Or just go all in and make the bias points external. And I've been debating since I got it as to whether or not I want a depth pot.

But one thing at a time. And I should probably watch a video on how to cut traces first.

 

[slaveunit]

Re: Hum. How to diagnose (and repair)?

My best guess is that this will be your fix.

The depth control is pretty simple and well worth it imo. I was just reading about the sweep pot. Sounds pretty cool too.

Best of luck with it. Please report back once completed. This thread may help others in the future.

 

[misterwhizzy]

Re: Hum. How to diagnose (and repair)?

Well, the Hum-X arrived today, and unfortunately will be going back probably tomorrow. It made no difference whatsoever, so it looks like I'll have to rerun the heater traces above the PCB. Finding time to do that is the tricky part.

 

[Mincer]

Re: Hum. How to diagnose (and repair)?

Yikes...so that means the noise is from the amp itself, I guess.

 

[misterwhizzy]

Re: Hum. How to diagnose (and repair)?

Yikes...so that means the noise is from the amp itself, I guess.

Yes, but I’m not uncomfortable working inside it. It’s heavily modded already, with a close-to-SLO preamp and EL34 power section. I think I’m going to put a depth pot and a sweep pot in when I redo the heater wiring.

Plus, learning.

 

[misterwhizzy]

Re: Hum. How to diagnose (and repair)?

I started my surgery tonight but didn't have time to finish it. Depth mod is done, sweep will wait until next time. Half the heater wiring is redone, but I haven't worked up the nerve yet to cut the traces. That comes tomorrow. And so do photos.

 

[Blille]

Re: Hum. How to diagnose (and repair)?

Well, the Hum-X arrived today, and unfortunately will be going back probably tomorrow. It made no difference whatsoever, so it looks like I'll have to rerun the heater traces above the PCB. Finding time to do that is the tricky part.

Sorry to hear it didn't work out! Best of luck with the surgery.

 

[misterwhizzy]

Re: Hum. How to diagnose (and repair)?



Wrapping up now. You can see the depth pot in the top left of the photo. I have one more PCB trace to make sure I really got through, then rehousing and off to testing. That board is no joke, though. There is no way I was going to accidentally break anything on it. I still don't particularly like the idea of taking the blade of a screwdriver and utility knife to a PCB, but in this case, it can handle it fine.

 

[GoldenVulture]

Re: Hum. How to diagnose (and repair)?

Are the wires supposed to be reversed like you have at the left hand end ?