Hum problem with SWR Bass 350

[Gerhard Grass]

Hi to all of you,

I have a SWR Bass 350 head (red Version) on my desk and it has a strange problem.
The amp works fine, but powertransformer hums disturbingly loud and the transformer gets a bit warm, even without input signal. No hum from the speaker.

Any suggestions or hints?

thx in advance

Gerhard

[Jazz P Bass]

If you are capable, I would suggest measuring the current that the amp is drawing from the mains at idle.
This measurement may help indicate whether or not there is a problem with the transformer.

[Shawnobi]

Sometimes you can get something called 'lamanation rattle'. It's caused by heat cycling. I think the windings also seperate making it more inneficient and this may also cause the extra heat as the power is being lost that way. If the transformer has bolts across it that you can access, you can sometimes tighten them up and reduce the noise.

[MarkusBass]

If you are capable, I would suggest measuring the current that the amp is drawing from the mains at idle.
This measurement may help indicate whether or not there is a problem with the transformer.

I agree with Jazz P Bass. What happens if you remove the valve from preamp? Have you measure the quiescent current of the power amp? It may be set up incorrectly. Some hum is typical to SWR amps (especially SM900). I would also check the power amp rails.

Mark

[Gerhard Grass]

Hi,

as you suggested, I took some readings:

All values with speaker connected, but no input signal:

AC current: 0,31A@232V (I live in Austria)
Rail voltages: +99V / -98V

AC voltage on the rails:
on neg. rail: about 0V
on pos. rail: 219V AC (!!!)
I checked with several meters and asked a friend to verify my readings.

Could it be the 6800mF/80V cap ? Or should I check the rectifier bridge?

thx Gerhard

[MarkusBass]

All values with speaker connected, but no input signal:
AC current: 0,31A@232V (I live in Austria)
Rail voltages: +99V / -98V
AC voltage on the rails:
on neg. rail: about 0V
on pos. rail: 219V AC (!!!)
I checked with several meters and asked a friend to verify my readings.
Could it be the 6800mF/80V cap ? Or should I check the rectifier bridge?

Gerhard,

I don't have the amp to compare but it seems that the AC current is too high. And also rail voltages are too high. Are you saying that you have 99 Volts on 6800uF/80V capacitors in the power supply? And this does not make you worry?
I don't remember the rail voltages but I'd say that it should be something like +/-70V. Can you check once again this 99V? I also advised you to remove the valve and measue AC current. Did you do it?
Is it possible that the amp is incorrectly set to 230V (was it before 117V)?

Mark

[Gerhard Grass]

Hi Mark,

as you told me, I removed the tube. It didn't change a thing. Checked the rail voltages again and they stay at +/-99V.
As you said, they should be about +/-75V as shown in the schematic and of course that makes me worry. I took a look
with the scope and the rail voltages seem to be too high but ok. On the back of the amp a sign says it is rates for 240V and
there is no switch to change this setting.

So I'm rather confused with this thing.

Gerhard

Gerhard

[MarkusBass]

Checked the rail voltages again and they stay at +/-99V.
As you said, they should be about +/-75V as shown in the schematic and of course that makes me worry. I took a look
with the scope and the rail voltages seem to be too high but ok. On the back of the amp a sign says it is rates for 240V and there is no switch to change this setting.

There no switch but inside the amp (close to the transformer) there are wires that you can swap to change the mains voltage. Take a look at it. Don't turn on the amp unless you want to see a nice explosion. I would remove the transformer from the amp and connect to to external rectifier and capacitors. The transformer is either shorted (primary winding) or set up to incorrect mains voltage.

Mark

[MarkusBass]

I've got an SWR SM400 today. Here is a photo - this is the place you should look at in your amp.



BTW, my amp looks like this (both power amps):



Mark

[Gerhard Grass]

Hi,

the PT on the photo looks very similar to one in my amp. I mailed SWR for a schematic and Mercury Magnetics for technical data and price.
I think, I will have to replace the PT.

I keep you informed.

thx Gerhard

[Jazz P Bass]

SWR 350 Schematic

How did you come to the conclusion that the power transformer is "bad"?

[MarkusBass]

I think, I will have to replace the PT.

Before you that, yau have to be 100% sure that this is the source of the problem. And, as I wrote previously, this can be easily verified.

Mark

[Gerhard Grass]

Hi, the model a have here is a redface. The attached schematics seem to match with my amp.
On the filter caps (6800uF/80V) I have a voltage of +/- 98V. That seems to be too much. The power amp schematic says rail voltage should be +/-77V.
Since the amp is labeled for use with 240V mains everything should be fine.

Gerhard

[slawkowski]

hi, I do not know 350 bass amp but this vintage 1990 PT diagram could help
good luck
slaw

[J M Fahey]

Thanks slawkowski.
This diagram *may* explain the problem.
It shows 2 x 120V windings, each of them tapped at 100V for use in Japan.
I *guess* somebody rewired a 120V American amp into a 220V European one ... and missed, using the 100V windings instead.
240V real wall voltage into a 100+100V winding would produce your exact symptoms.
The hum comes from the beaten to death, soon to explode big capacitors.
The power transistors won't like that too much either.
Hurry.