acoustic 370 humming output

[randomair]

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.

[Don Moose]

The humming gets worse as i adjust the treble mostly.

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!

[jogle]

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.

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

[Enzo]

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?

[jogle]

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

[jogle]

DC offset

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

[jogle]

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

[jogle]

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

[Jazz P Bass]

Acoustic 370

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?

[Enzo]

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.

[jogle]

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

[Enzo]

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.

[Jazz P Bass]

Acoustic 370

Can you try paralleling another cap, same value , over C115?

[Phostenix]

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.

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.

[Phostenix]

PT Orientation to Reduce Hum

Here's a pic of the only orientation with which I could achieve no hum (within the limits of the wiring) .