LinkBack URL
About LinkBacks
Reply With QuoteCheck the ground connections, starting with the input jack and working toward the tone controls. Then look at the filter caps, working from tones to input.Originally Posted by randomair
Hope this helps!
My acoustic 370 has humming output, it has signal from guitar, but its still hums. The humming gets worse as i adjust the treble mostly. It also has a slight crackle upon shutoff...any ideas? Thanks.
Check the ground connections, starting with the input jack and working toward the tone controls. Then look at the filter caps, working from tones to input.
Hope this helps!
At last! Someone who has the same problem. My 370 amp also hums. Regardless of if I connect a bass or not. I've checked the big caps, all transistors, replaced all small electrolytic caps, ...still hums. Today I'm gonna replace all mylar- and tantalium- caps.
Please inform me if or when you find out what's causing this hum.
//Glenn
Hi jogle, welcome to the forum. You should note that this thread has been dormant for a year now. There are other 370 threads on this board here and there.
How did you "check" the large caps, ESR? Measured value? How much ripple is on the main power supply?
DOn't assume hum is from a bad part, it could easily be from a poor ground connection somewhere. Little mylars and such in the signal path are not likely to introduce hum.
Isolate the problem, is the hum still there when ALL controls are at zero?
Is the hum present in the line out jacks?
Hi Enzo, thanks for trying to help me.
I bought this amp a year ago and I've really tried everything to fix it.
I replaced the big caps, one by one, with a new one. I haven't measured the power supply ripple with an osilloscope or so but the hum is pretty strong.
One thing that I have fixed is a broken choke in the graphic eq section.
The hum is constant, regardless of the volume value. It's only affected by the graphic eq, mostly the 100Hz and 200Hz slider. Its also present in the line out.
Twenty years ago I tried an Acoustic 370 and I was totally sold. Been searching for that sound ever since, and a year ago I found one. It's in mint condition, despite the hum. From 1977, the last year they were produced.
So I really appreciate any help I can get.
//Glenn from Sweden
maybe this is a clue...
I measured the DC voltage on the speaker output with no speaker connected, a tip you gave in another thread. It showed 33,2 V. Not OK I guess. But in the service manual I read that "the amp output is capacitor coupled and will have a DC offset unless it is slightly loaded to discharge the cap." With the speaker connected I only get about 200mV.
Is this high DC offset OK?
//Glenn
Today I managed to isolate the hum. It's coming from somewere in the first EQ section containing bass, mid and treble controls. I think, and hope, that I will be able to fix it.
There is however yet another problem with this amp, but for that I will start a new thread.
Thanks
//Glenn
Long time no see!
I'm back after 1,5 years. This amp has now been checked out by a couple of guys who know what they are doing. They both has come up with the same explanation...
...The Big power transformer is producing such a strong radio interference / magnetic field, that it is picked up by all those chokes in the eq sections. As soon as I switch off the power, the hum disappears, and I can play a couple of seconds until the big cap has unloaded...
One solution could be to install the transformer outside the amp, perhaps in the 301 speaker cabinet, another is bypass both eq sections leaving only the volume control. But all this seems like a bad plan B.
I would really appreciate if someone could come with a better solution.
Hopefully
/Glenn
I would check to see if the hum is present before the EQ section.
Quote:"One thing that I have fixed is a broken choke in the graphic eq section."
Which inductor did you repair?
Do you have the schematic?
Put the EQ section with the inductors in a shielded container.
If their theory is correct, then pull the mounting bolts from the transformer and move it around, rotate it. If that changes the hum in any way, then the magnetic field idea has merit.
Hum comes in 60Hz and 120Hz flavors. (Or 50/100 in other parts of the world) 120Hz is power supply ripple, while 60Hz is ground issues, transformer coupling, etc. Knowing which is then an important clue.
Education is what you're left with after you have forgotten what you have learned.
Hi, and thanks for your reply!
The hum starts after the mid control inductor L101 in the first eq section:
http://acc.homeunix.net/images/schematics/370b.jpg
The hum proceeding this chocke is quiet enough for my needs.
The broken choke I did replace was the 100 Hz (L105), but this did not affect the hum.
One thing that strikes me now when I look att the schematics is the different symbol for L101, L102 and all the other inductors. There's a line added. What does that mean? Has that something to do with a shielded function?
I've been told that they tried to rotate the transformer a quarter, resulting only in minor improvements.
//Glenn
Those two inductors are larger values. The symbol is standard electronic notation. The line represents the iron core of the coil instead of an air core. If they had been wrapped as air cores, they would have been a lot karger.
Education is what you're left with after you have forgotten what you have learned.
Can you try paralleling another cap, same value , over C115?
I've got a 370 with this hum, too. Pushing the 200 Hz slider all the way up generates the loudest hum (volume pot turned all the way down). I've confirmed this PT theory by unbolting the power transformer from the chassis. It bolts to the side of the chassis. The only orientation that I can find that eliminates the hum is to rotate the PT 90 degrees so that the mounting flange faces the EQ board. This, of course, leaves no good way to bolt it in. I tried rotating the PT so that the mounting flange faces down (so I could bolt it to the bottom of the chassis instead of the side), but no matter how I orient it, the hum continues. It's not outrageous hum, and would be un-noticed at any real playing level, but it is noticeable when I turn the amp on.
ST in Phoenix
Here's a pic of the only orientation with which I could achieve no hum (within the limits of the wiring) .
ST in Phoenix
Maybe no help, but I wonder if reversing the primary leads makes a difference. Might change how it couples.
Hmmm, you think the EQ board is picking it up? Can you mount a steel shield between the tranny and the EQ board? Bolted to the floor in there.
Education is what you're left with after you have forgotten what you have learned.
I'm going to find a piece of steel to set in there to see if it makes a difference. I'll be curious to see if the other 370 I have will do the same thing.
I'll try reversing the ac leads, too.
ST in Phoenix
You ought to be able to do the hunk of steel thing pretty simple. Even a sheet of something that wouldn;t be permanent, but just to see if it made the difference.
I remember a SIlvertone amp of some model, and there were two 6L6s right behind the one set of channel controls. There was a stttel plate mounted between the panel and the tubes. I had removed the plate to access the panel controls, then fired up the amp for test without it. WHOA, the controls in the preamp there then picked up what the 6L6s were doing an inch away - instant feedback. Plate restored to position, stable as can be.
Education is what you're left with after you have forgotten what you have learned.
I did some more experimenting with this today. Checking with the scope, the hum is present on the speaker outs (and equivalent places in the power amp) but not on the input to the power amp or the line outs on the back of the amp.
Here are the things I tried that made no difference:
Swapping the PT primary wires.
Pulling the EQ board & moving it away.
Moving the bridge rectifier around.
Moving the output cap (C404) around.
Moving the wiring bundle that goes to the front panel power switch, pilot lamp, ground switch around.
Lifting the ground on the amp.
Lifting the PT out of the chassis & covering the chassis with a grounded steel plate didn't even make much of a difference.
I even turned off the lights in the room to make sure there was no noise coming from them (1 on dimmer, the other a fan remote).
Rotating the PT to 1 of 2 positions is the only thing that DOES make a difference. Neither of those 2 positions could really be permanently mounted.
The level of hum on this amp is not really noticeable unless the 200 Hz & 300 Hz sliders are all the way up and it may be that this is normal for this amp, but I still find it odd.
I guess I'll get the other 370 one up and running and see how it compares.
ST in Phoenix
Here's a pic of the hum. Scope set to 20mV/Div & 5mS/Div.
ST in Phoenix
With all the EQ sliders flat, the waveform drops to about 20mV p-p & is not really noticeable. If I push the 100Hz, 200Hz and 300Hz sliders all the way up, the hum goes up to 120mV p-p on the scope.
ST in Phoenix
Well, the position thing tells us the transformer field is coupling into the amp.
Do you have an old Weller soldering gun? The thing with the two chrome tubes sticking out the front, each with a small chuck the tip is mounted in. Those chrome tubes and the tip are actually a winding around the transformer that is inside the gun. Low voltage and high current. That tip throws off a field. If you trigger on the gun and wave it right over parts on the amp board, you MIGHT find something sensitive to magnetic fields.
Education is what you're left with after you have forgotten what you have learned.
I actually do have one of those around here somewhere. I'll try that.
After thinking about what I posted earlier, I realized that if the EQ sliders are affecting the hum, it HAS to be there at the power amp input. It is, it's just down in the mud at 5mV/Div on the scope. Grounding the input to the PA kills the hum (buzzkill). The bundle of wires from the preamp board to the EQ board do provide some nice antennas (they are twisted, but still).
ST in Phoenix
Using the old Weller as a noise source, it seems like the coils in the preamp are the antennas. With the preamp & the EQ boards free from the chassis, moving them as far as I can within the limits of the wires & rotating them around doesn't provide much relief from the hum. At this point, I'd say it's just part of the design of this amp. I'll see how the other one is after I get it working. I doubt that I'll get to that this weekend, though. Thanks for the help!
ST in Phoenix
Putting the old Weller back in the toolbox, I noticed a wooden box that I didn't remember. Turned out to be a 15 piece router bit set that I bought off ebay a few years back. I forgot all about that set & was about to go buy a bunch of router bits for some other projects. You just saved me a bunch of money tonight, Enzo
ST in Phoenix
Well, when you get to my experience level, you know, these extra abilities kick in, and... uh... my zen approach finds your router bits....
Education is what you're left with after you have forgotten what you have learned.
For the sake of experimentation, I decided to try a star ground topolgy on this amp. I pulled all the grounds & removed the jacks and the preamp board (which is grounded to the chassis when installed) from the chassis. I soldered a short piece of 14 guage solid copper wire to the lug where the power cord ground attaches to the chassis. I then soldered or jumpered all the grounds back to that wire (power cord ground, 2 filter cap grounds, power amp ground, preamp ground, input jack ground, and output jacks ground). I left the bridge rectifier's DC ground attached only to the big filter cap.
What I got was a very unhappy, oscillating amp. After experimenting with tying the various grounds to either the chassis ground or the negative terminal of the main filter cap or the power amp (where most of them were originally), this amp is quietest wired just as it was from Acoustic. Imagine that.
ST in Phoenix
Your hum was coming from the transformer, I thought we had established. That would be 60Hz hum. Star grounding is usually a cure for ground return ripple current hum, which would be 120Hz.
If your EQ inductors were picking up the field, I am surprised a shield didn;t work.
Education is what you're left with after you have forgotten what you have learned.
Why can't you mount the transformer on its side? Looking at your photo of the good orientation, I see mounting tabs at the bottom of the photo, facing front. A couple sturdy L-brackets mounted to the floor would provide mounting points for those.
Education is what you're left with after you have forgotten what you have learned.
Oriented with the mounting flange like it was in that photo, but turned 45 degrees (mounting flange angled towards the corner of the chassis) was one of the 2 quiet positions. I thought about mounting brackets in there for that, but they'd have to be pretty beefy. That PT is heavy. The other quiet position was with the mounting flange down and 1 end tilted up in the air 45 degrees. Not very mountable.
I found a U shaped piece of steel that's just about the size of the PT. It made a difference when around 1 particular side of the PT (the bell on the left in the pic above), but not a huge difference. Rotating the 45 degrees makes the most difference.
ST in Phoenix
Well, after getting the other 370 I have up and running tonight, it seems to have the same hum. I haven't done any in depth testing nor have I moved the transformer around yet, but just listening while working the sliders it seems pretty similar. With all the EQ sliders at midpoint, it's fine. I notice this amp has more shot noise than the other when you turn it up with no input, but I haven't changed any caps in this amp yet nor have I changed the couple of high value resistors to metal films as I did with the 1st amp. I'll be curious to see if those things make a difference with this amp.
ST in Phoenix
There are currently 1 users browsing this thread. (0 members and 1 guests)
Posting Permissions
Bookmarks