SWR 550X- Constant hum issue

[marginmonster]

Hi!
My SWR 550X has an inherent hum when no bass is plugged to the input, or if a bass is plugged into the ACTIVE input. The hum goes away when the bass is plugged into PASSIVE/ACTIVE input.

I was a tech in a past life (Navy) and did not see any obvious problems when inspecting the inside of the amp. I looked for burned or broken components, loose wires or crack solder joints. I paid special attention to the input jacks looking for ground issues and checking to make sure I had good continuity from ground on the jack to chassis ground. All good. I swapped the 12AX7 tube out to see if the pre-amp was causing issues. No difference. The input jacks themselves look ok, though it's difficult to see if something internal is broken. Replacing the jacks would be a bare since they are surface mounted to the board.

Does anyone have any ideas what I should look for? I believe the amp has always been like this (I bought it 2nd hand and did not notice the issue when I bought it). Or, do you know of a good shop (in southern CA) that's honest and charges fairly?

Thanks,
Bill Ross

[txstrat]

My first thought were the input jacks as well. The jacks ground the hot terminal, when no cord is plugged in to keep the amp quiet.
Do you have signal when using the ACTIVE input?
I'd check the continuity from the hot terminal of the input jack and the next connection in the signal chain. Maybe the input is open and no signal can go from the jack to the circuit. An open input might lead to the issue you describe.
While the PASSIVE / ACTIVE input is unused = hot tap is grounded, only the first inputs open jack might lead to the hum.
Just a thought.

[txstrat]

The more I think about it I tend to assume the PASSIVE / ACTIVE input might be the culprit.
When the hot terminal to ground connection fails (no cable plugged in) the amp hums because of the open circuit.
Plugging a cord into this input, the ground's beeing made throught the guitar, hence - no more hum.
Using the ACTIVE input still leaves the other inputs grounding open, hence - hum.
I'd re-check the PASSIVE/ACTIVE input thoroughly.

[marginmonster]

Hum Reply

I have signal through the amp when plugged into the Active input, but the hum continues and the signal is distorted (possibly because of the hum and low input). The best I can tell, the input jacks (both active and passive/active) are mechanically working correctly (making and unmaking contacts). I do not have a schematic to check continuity from contacts on the jack into the motherboard. Also, I have not disassembled the mother board from the amp to look more closely at the jack itself and the solder joints. These are most suspect in my mind.

Because the amp seems to work fine using the Passive/Active input and not the Active, I tend to think the jack is defective or there is a cold or cracked solder joint on the mother board. Without a schematic, the best I can do is assume... I am tempted to purchase a new jack and install it. This would renew the solder joints and elliminate mechnical issues. It will also allow me to look more closely at other things on the motherboard. What are you thoughts about this? Do you know where I could get a schematic? I could do real troubleshooting with a decent schmatic.

Thank you

[txstrat]

Because the amp seems to work fine using the Passive/Active input and not the Active, I tend to think the jack is defective or there is a cold or cracked solder joint on the mother board.

Yep, that's what I think too. When the amp works fine while a cord is plugged into the jack (PACSSIVE/ACTIVE), the culprit seems to have to do with the jack or it's related parts of the circuit. That's why I was asking about the continuity between the jack and the board in the first place.
I'd replace the jack and see what happens. So far you don't need a schematic. Or maybe someone's had it posted in this forum. I'd run a search for "SWR 550X".

[Enzo]

I think you DO need a schematic. These inputs are not just a pair of 6db padded like a Fender Twin. There is an extra tube stage after the passive jack. Plugging intot he active bypasses that stage.

[marginmonster]

Thanks!

Thank you for forwarding the schematic. I see what you are talking about the signal path. I am surprised to see that the Active input bypasses the pre amp tube. My thought process to look at where ground should be coming from when nothing is plugged into the amp. By default, it should be there. Both Active/Passive and Active show a ground directly connected to the input jack, but each has it's own dedicated ground pin on the jack. I am going with an assumption that the ground is open to both input jack and finds a path through the bass guitar when plugged into the Passive/Active input. Hopefully this will lead to determining a solution.

FYI: I contacted SWR to see it they would send me a new input jack. With moving parts and contacts (most likely point of failure) this remains my assumed culprit.

Thank you!

[drewl]

Is the input jack(s) a dual ganged one?
I had the same problem with an SWR combo and that jack was flaky, losing it's ground to the front chassis.

[marginmonster]

Thank you! No More Hum

I purchased the original "Cliff" surface mounted 1/4 jacks from a vendor in the mid-west and installed it into my 550X. I think it was a buck for the part and 1.70 to ship it. This completely fixed the problem. It's dead quiet with nothing plugged in as well as when my bass is plugged into either the active or passive input. Apparently it has been messed up since I purchased several years ago. I now love this amp. It is a tone monster and compares well with the best I have ever heard. Also, the DI is completely quiet, which is nice cause I do not have to use my external DI. Thanks for your help and for providing the schematic!