Noisy SWR SM-400 Bass Amp

[smackoj]

I have been fixing up a few SS amps lately. I purchased this SWR SM-400 bass amp in not working condition. I replaced the big filter caps and a few in the preamp section which are associated with the 12AX7 tube that is in the preamp. The problem is that standing alone running or with a guitar plugged in, there is too much hum through the speakers. I tried a few simple things and found that putting my guitar into the Low Gain input jack and putting a male jack into the Hi Gain input, quiets the amp to a respectable noise level. Does anyone know what I might do to lower this noise permanently? The manual for the amp says the Hi Gain input has 5x more gain than the Low input!

Thanks amigos

I am having trouble loading just the schemo so here is a link to the schemo and owner info. You will have to scroll down to the one marked "SM-400" and click on that to open the schemos.      https://support.fender.com/hc/en-us/articles/115002392666-SWR-Amp-Schematics

[shooter]

guitar into the Low Gain input jack and putting a male jack into the Hi Gain input, quiets the amp

appears you are bypassing the tube by doing this, which would lead me to think you're hum is tube related.
was the jack into the hi gain side a shorting jack?

are the jacks plastic, isolated?  look for the usual suspects, things that change when ganging and prodding, coupling caps?

[PRR]

https://support.fender.com/hc/en-us/article_attachments/115007779523/SM-400_Complete.pdf

Yeah, bad ground on hi-gain jack. I don't know how it is built (metal chassis, metal jacks, plastic jacks, sub-boards, jumpers) so I dunno what you are looking for.

[smackoj]

thanks gents; the amp is built in a heavy ga. aluminum chassis. The input jacks are shorting jacks. I will take a few pictures and post them. That may be enough to get to the problem, hopefully. 

[smackoj]

I believe (shooter) that both Hi and Low inputs each use one side of the preamp tube. You can see the red and white input signal wires going to the pcb in front of the 12ax7. Here are a few pics of the inputs. I wonder if replacing the Hi gain input wire with shielded wire would make a difference? The white wire from the Low input is shielded but the red wire from the Hi is not? It could be that someone changed that wire and did not use shielded? It looks as if there may be a small whole about 3/16" away from where the red wire is soldered. I does look like there was something wired there like a shield wire? There is a bit of wire still in the hole and some dried rosin around it.  Here are the pics

[shooter]

both Hi and Low inputs each use one side of the preamp tube.

a quick glance, looks like the 1st half tube is higain , the lowgain "appears" to jump through a bunch of SS hoops before it makes it to the 2nd half of the tube

[PRR]

> I believe ...both Hi and Low inputs each use one side of the preamp tube

I'm with shooter. *According to the schematic*(?), HiGain goes through the tube, LoGain goes to an opamp which mixes the LoGain jack and the HiGain tube.

> putting my guitar into the Low Gain input jack and putting a male jack into the Hi Gain input, quiets

This, to me, is A Clue. Not just using the LoGain side, but also UN-shorting the HiGain input. Very odd. The suspicion is that the "ground" on the un-used HiGain input is hummy, and contrary to expectation, you have to break the shorting connection usually wanted to reduce stray noise at an un-used grid.

This also suggests that shielded wire is not the answer: if hum were coming in otherwise, hum would rise when un-shorted.

I admit that looking at the jacks, it is hard to see how the HiGain jack ground can be any different from the LoGain jack ground. However I can not follow that black wire into the shadows. And I would *think* the black wire should land at the preamp PCB, not wander back toward the power section.

[smackoj]

Sorry for not responding; I will try and find the end of the black wire. I looked at it when I was taking the photos but like you said, "into the shadows" and left the idea sit until you mention it. I will report back on that wire asap.

thanks, jack d

[smackoj]

Hello gentlemen; I hope you are still checking in on this post?  I have some better pictures of that "black wire" that PRR asked about. It runs from the ground pin of the input jack to the ground (?) post of the Balanced Out connector on the back panel. A smaller Brown wire takes off from that same solder point and goes to the ground pin on one of the speaker output jacks on the back panel. Hopefully that is understandable but here are some pics. I may need to post 2x to get all the pics loaded?

[shooter]

runs from the ground pin of the input jack to the ground (?) post of the Balanced Out connector on the back panel

I put circles on what I found for XLR, I drew what I think is the black wire, you can see it goes to the balance pot wiper.

should be able to confirm the schematic with an ohms meter

[smackoj]

hi; I don't follow/understand why a line from the ground pt. of the input would go to the balance pot?

[PRR]

"Ground" goes ALL over.

This balance scheme uses a grounded-wiper pot to cut one side or the other.

Not all grounds are the same!! Particularly around the power supply. But I still can't make out your pictures. Use your finger, trace the black wire from the jack--- where does it physically go?

I would expect it to go pretty near the input tube cathode return, the 1.5K from pin 3 to "ground". Or somewhere near there. If someone has been clever and run it to "a main ground", like the power supply, power supply ripple will contaminate the HI input signal.

[smackoj]

I will take another stab and getting better photos. I  will post them asap.  thanks

[smackoj]

Different hopefully better pics of the black (and brown) wires
Pt A.  Black wire from ground lug of Hi Gain input jack
Pt B.  Black wire and brown wire soldered to back of XLR connector (Direct Out and Line Out) on back of chassis (see earlier picture) This connection is
         not to any of the 3 pins of the XLR jack but to the solder lug above the pin connections.
Pt C.  Brown wire goes to the ground lug of one of the Speaker Out jack.

[shooter]

is the chassis wood?

[smackoj]

Shooter; No, the chassis is thick gauge aluminum

thanks

[shooter]

you have a send/return, got something for each jack?
that "should" get you to looking left, or right

[shooter]

getting anywhere? 
I spent some time with the schematic, and have to admit it took me way longer than it should to get the signal from the inputs to the send jack

[smackoj]

Shooter;  not really getting anywhere. I don't understand why the black wire goes to the "no signal" tab on the XLR Balanced Out jack and then jumps over to the ground lug on one of the 4 spkr output jacks? I think maybe I'll just disconnect the black wire and see what happens?

[shooter]

why the black wire goes

my guess, it's a "case/jack/chassis" ground.
won't take a minute to try, shouldn't have any issue with signal path, or voltages

I would try and find something to jack into the return and play out the PA, that eliminates 1/2 the "box"....mostly

[PRR]

Speaker, XLR... those do not make sense.

I think it goes like this. But I may be wrong.

[smackoj]

Thx PRR. I will give it a try.

[smackoj]

Hi guys; sorry for the huge delay! I tried the black wire to the ground on the pcb but it made the current limiter glow bright. So, I soldered it back from the input jack grd to the ?? lug on the XLR jack where it came off. I removed the brown wire from the XLR over to the output jack grd and Wala, it's working and not buzzing! So, that's the way she's gonna stay.

Thanks for all your help PRR and Shooter.