tracking down hum and grounding

[aortizjr]

Hi...

I just got my hands on a Sunn 200S. And it has buzz going on even when there is no input.

It is also volume independant. Actually it only gets louder when I turn up the EQ. Turning up/down the volume has no effect.

Any ideas?

Also it only has a 2 prong powercord and I would like to convert it to a 3 prong. But where does the ground pin go? I am guessing straight to the chassis. But si there something special I need to know when doing this kind of conversion?

Thanks!

 

[Blue Strat]

Originally posted by aortizjr
Hi...

I just got my hands on a Sunn 200S. And it has buzz going on even when there is no input.

It is also volume independant. Actually it only gets louder when I turn up the EQ. Turning up/down the volume has no effect.

Any ideas?

Also it only has a 2 prong powercord and I would like to convert it to a 3 prong. But where does the ground pin go? I am guessing straight to the chassis. But si there something special I need to know when doing this kind of conversion?

Thanks!

Yes, the ground lead goes to the chassis. A good point is where the power transformer is bolted to the chassis.

Remove the phase inverter tube and check for hum. If it's still there, the source is the power tubes or power supply. If the amp hasn't had a cap job yet, do this first. To trouble shoot amp hum with 30+ year old caps installed is insane.

If the hum goes away when removing the PI tube, it's likely originating just before the EQ circuit since the volume controls have no effect on the hum level and the EQ does.

 

[aortizjr]

That will work perfectly on the ground wire.

I pulled the PI and it has a slight hum.. but not nearly as bad.

I was thinking it was going to need a cap job. Fortunately it is a pretty simple circuit and it is p2p. The can is going to be a real pain...oh well.

I have to order parts.. but hopefully that will do the job.

Thanks!

 

[jonny guitar]

Start with the simplest stuff first. I had a similar situation and did a full cap job, new tubes all around, grid to ground resistor, only to find out that the problem was a simple 40 cent ceramic cap off the tone control.

 

[Richard Guy]

Another thing to look at is the input jack(s). The 'TIP' or 'hot' terminal is grounded until you insert a guitar cord. If it is not, it will sound just like you have a guitar cord inserted without a guitar plugged in.

 

[aortizjr]

Well this thing could really use a cap job anyway. Most of the stuff in there is from '68. If nothing else it will make the amp safer.

Once I know all the caps are good I will go from there. There are definitely a few that have swayed out of spec. Plus they are relatively inexpensive for this amp. It is pretty simple and all the P2P.

In regard to the jacks, they look pretty good. I also use a dummy plug which is just a 1/4" jack that has the tip and sleeve shorted. I use that to simulate having a guitar plugged in.

Also with a guitar plugged in, there is still the hum. Even though the tone is pretty good coming from the guitar.

I will update as things go along.