SWR SM-400 amp issue. Need expert input. FIXED

[40Hz]

This is a question for the people in the TB community who are knowledgeable about electronics on at least a repair tech's level.

Situation: loaned out my SM-400 head and Goliath cab a week ago. (Long story, so please no admonitions. There was a good reason I did so and I’m not sorry I did. )

Normal way of using it is (using the banana jack to 1/4 phono plug cable suppied by SWR) to go bridged 400W mode out using the banana jacks on the back of the amp.

From here:




To here into the 1/4" jack on the Goliath cab.



It came back last night in inoperable condition. No sound. All fuses are fine. Power light comes on. Amp head (not speaker) emitting a faint hum. No sound when going into the front jacks. No sound when plugging an outboard preamp into the effects return. No small thump when first switched on like it usually has. So from my limited knowledge, I'm thinking that the power stage has a major issue.

The person who borrowed it said he may be responsible for it not working because his son may have plugged something in the wrong way when he got it back home and before he returned it since he found it set up in the family room where he had left it stacked and unplugged until he got a chance to return it last night.

The guy is ethical and takes full responsibility for anything damaged while it was in his care. And he's willing to pay to get it repaired.

I said it's 30+ year old amp that has seen regular use throughout it's lifetime. And maybe it just decided to give up the ghos twhen I switched it on as old amps sometimes do. And if that's the case, no fault - no foul AFAIC. So I said I'd run it past some of you guys here who know what you're talking about and get your opinion.

When he found it, instead of it being plugged in like above, he found the 1/4 plug was in the effects send (or maybe return, he couldn't remember which) jack:



And plugged into the banana jack on the back of the cab.



So my question is: would going from the effects send or return into the cab, or possibly plugging the cab directly into the effects return (which I'm guessing is more likely) done anything to flatline the head like that?

The main reason I'm asking is because I don't want him feeling like he should pay to get something fixed that isn't his fault. He's an upright guy, but he isn't exactly rolling in cash. So I don't want him taking responsibility for a problem he didn't cause.

By the same token, considering the amp's age, I really can't see spending my own money having it fixed right now since I'm thinking of just getting a new head anyway.

So for my pal's peace of mind can anybody give me some input on this one? I have some electronics background. But the published schematics for this amp are beyond my level of knowledge. And I don't know anywhere near enough about this type of product to even attempt to determine what may or may not have happened.

So any input would be greatly appreciated.

 

[agedhorse]

No, using the effects send in that way won't hurt the amp, it just won't work.

If the bridge switch is in the stereo (actually dual mono) position but you use the bridge output, you will not get any audio and will see the symptoms you describe. That's the first thing to check.

Otherwise, it's an amp that needs a good tech to service properly. They are not easy to repair and benefits from an experienced service tech.

 

[40Hz]

No, using the effects send in that way won't hurt the amp, it just won't work.

If the bridge switch is in the stereo (actually dual mono) position but you use the bridge output, you will not get any audio and will see the symptoms you describe. That's the first thing to check.

Otherwise, it's an amp that needs a good tech to service properly. They are not easy to repair and benefits from an experienced service tech.

Thanks AgedHorse! Yeah. I've heard from other SWR owners that this particular model is a real hornet to fix due to the mix of tech under the hood. A real "kitchen sink with everything they could cram into it" as one amp guy put it

Ok. Bridge switch is in the bridge mode position, so no joy there. I figured it was something deeper than that. And with the comment above coming from you that's enough to make me think I'll probably retire it for the time being and just get a new head. I've got your D-800 and a few others on my short list. And luckily I don't need a gig amp right now. I also have other smaller amps to work with in the meantime. So I guess I'll just move getting a new gig-sized head up on my list of buying priorities after tax day.

Thx for weighing in.

 

[BassmanPaul]

While I am in absolute agreement with Agedhorse, a thought that crossed my mind was that you should try linking the send and return of the Effects Loop with a signal cable. It costs nothing to check if your problem is related to the loop.

 

[Ant Illington]

This is a question for the people in the TB community who are knowledgeable about electronics on at least a repair tech's level.

Situation: loaned out my SM-400 head and Goliath cab a week ago. (Long story, so please no admonitions. There was a good reason I did so and I’m not sorry I did. )

Normal way of using it is (using the banana jack to 1/4 phono plug cable suppied by SWR) to go bridged 400W mode out using the banana jacks on the back of the amp.

From here:

View attachment 2968806


To here into the 1/4" jack on the Goliath cab.

View attachment 2968807

It came back last night in inoperable condition. No sound. All fuses are fine. Power light comes on. Amp head (not speaker) emitting a faint hum. No sound when going into the front jacks. No sound when plugging an outboard preamp into the effects return. No small thump when first switched on like it usually has. So from my limited knowledge, I'm thinking that the power stage has a major issue.

The person who borrowed it said he may be responsible for it not working because his son may have plugged something in the wrong way when he got it back home and before he returned it since he found it set up in the family room where he had left it stacked and unplugged until he got a chance to return it last night.

The guy is ethical and takes full responsibility for anything damaged while it was in his care. And he's willing to pay to get it repaired.

I said it's 30+ year old amp that has seen regular use throughout it's lifetime. And maybe it just decided to give up the ghos twhen I switched it on as old amps sometimes do. And if that's the case, no fault - no foul AFAIC. So I said I'd run it past some of you guys here who know what you're talking about and get your opinion.

When he found it, instead of it being plugged in like above, he found the 1/4 plug was in the effects send (or maybe return, he couldn't remember which) jack:

View attachment 2968808

And plugged into the banana jack on the back of the cab.

View attachment 2968809

So my question is: would going from the effects send or return into the cab, or possibly plugging the cab directly into the effects return (which I'm guessing is more likely) done anything to flatline the head like that?

The main reason I'm asking is because I don't want him feeling like he should pay to get something fixed that isn't his fault. He's an upright guy, but he isn't exactly rolling in cash. So I don't want him taking responsibility for a problem he didn't cause.

By the same token, considering the amp's age, I really can't see spending my own money having it fixed right now since I'm thinking of just getting a new head anyway.

So for my pal's peace of mind can anybody give me some input on this one? I have some electronics background. But the published schematics for this amp are beyond my level of knowledge. And I don't know anywhere near enough about this type of product to even attempt to determine what may or may not have happened.

So any input would be greatly appreciated.

No help from me but my condolences. I've had that amp for 25 years and everytime I have to connect it, I still get scared and confused, so I see that a mistake could happen.

The good news is that I've seen them used and in good shape for only $200 which is quite the steal if it comes to your needing and wanting another.

 

[Jim C]

Agree with Paul
You could also go from just the left output or right output into the cab
Don't foregt the balance knob I have on of the earlist units and it refuses to die no matter how hard pushed; of course they all can't do this
The lack of an HPF is really dangerous and why I won't use it.
I've watched 2 other bass player blow up a cab with this head; one of the cabs was mine
Seems like a labratory amp and not suited for music IMO

 

[40Hz]

The lack of an HPF is really dangerous and why I won't use it.

I’ve always got a Broughton HPF patched into the effects loop. Picked up that tip off an SWR forum right about the time I was ready to toss the head. Made all the difference. I’ve got everything below about 60 Hz dialed out now. And it’s a whole new beast with one of those.

I've watched 2 other bass player blow up a cab with this head; one of the cabs was mine
Seems like a labratory amp and not suited for music IMO

The problem there was when they were designing that amp 5-strings were just coming out. And if low was good, lower should be better. Or so they thought. So they designed them so their frequency response could go down to some ridiculously low number like 5Hz forgetting there wasn’t a cab on the planet that could reproduce those frequencies. So distortion and overheating was the rule.

Adding a HPF is the fix. With a HPF the amp runs much cooler and the sound gets pristine. Bye-bye fried cabs too.

But yeah. They do kinda feel like somebody crossed a reference amp with a science faire project at times. The aural enhancer does nothing IMO but add more mud. And the compressor is equally useless. Most SM-400 owners soon learn to turn them all the way down and then forget they’re there.

 

[40Hz]

No help from me but my condolences. I've had that amp for 25 years and everytime I have to connect it, I still get scared and confused, so I see that a mistake could happen.

The good news is that I've seen them used and in good shape for only $200 which is quite the steal if it comes to your needing and wanting another.

To be honest, I never really liked the amp all that much. Once I got the HPF fix taken care of however, I decided it was a decent enough amp to keep considering how little I’d get selling it.

These days I mostly just use the power amp section and run a custom preamp I comissioned through it. In that capacity it’s pretty sweet actually.

 

[40Hz]

While I am in absolute agreement with Agedhorse, a thought that crossed my mind was that you should try linking the send and return of the Effects Loop with a signal cable. It costs nothing to check if your problem is related to the loop.

I did as you suggested earlier just for fun. Nada.

I even tried going with an outboard preamp into the effects return like I usually do. Still no joy. Seems it’s the power rather than preamp stage.

@agedhorse gave me an idea however. I want to take a closer look at that bridge/stereo switch. But it’ll have to wait till next week for me to check it.

 

[Al Kraft]

Otherwise, it's an amp that needs a good tech to service properly. They are not easy to repair and benefits from an experienced service tech.

I happen to know one who did an excellent job tuning-up my old SM400S...was worth every penny I spent, including shipping costs to Davis CA!

 

[agedhorse]

Something else to try... place it in dual mono mode and try each amp channel to see if one of the amp channels failed open. That would also make bridge operation not work since the amps are configured in series in bridge mode.

That amp uses the big gull-wing Sanken output transistors. I have 2 pair left, they are kind of hard to find (genuine, not counterfeit). Great transistors, but discontinued (from the original OEM).

These are not easy amps to work on, and there's not as much value in them as I would have expected. You could spend more to repair than to buy another used one (unless yours in in really clean condition).

 

[Al Kraft]

and there's not as much value in them as I would have expected. You could spend more to repair than to buy another used one (unless yours in in really clean condition).

They remind me of the last of the big Jeep Grand Wagoneers from the late 80's/early 90's. They're kind of unique but not really hot collectibles like an old muscle cars, they don't have any real advantage over current vehicles, and good luck finding parts or technicians who can work on them. Still, it's always fun to own a piece of history, even it wasn't earth changing history.

 

[Jim C]

To me the represent one of the first Super Amps.
2 racks space, tons of power, not real heavy, lost of bells and whistles
Mine had to go back to Steve for a weak component (I think a bridge rectifier that I believe had to be used for certification), and replaced it for free and add the transparency control and paid shipping back!
The transparency was in inxesitance when I bought the amp.
He bench tested while he had it in his shop; I think it was 550 watts at 4 Ohms at low distortion
Still have the hand written letter
Reminds me of the service by some guys that used to be in business; GB or something like that...

 

[40Hz]

Something else to try... place it in dual mono mode and try each amp channel to see if one of the amp channels failed open. That would also make bridge operation not work since the amps are configured in series in bridge mode.

That amp uses the big gull-wing Sanken output transistors. I have 2 pair left, they are kind of hard to find (genuine, not counterfeit). Great transistors, but discontinued (from the original OEM).

These are not easy amps to work on, and there's not as much value in them as I would have expected. You could spend more to repair than to buy another used one (unless yours in in really clean condition).

Mine is actually in immaculate condition. It went into a 4U rack road case the day I bought it and it’s been in there ever since. And I take pretty good care of my gear because I don’t like to fix or replace things. But even talking to somebody who wants one, it’s a total buyer’s market for these puppies. I could never get anybody to even go $300 for it. Most times the offers were below $200. And for that price I’d just as soon keep it as a backup. Besides, it’s 2x200W /400w mono power output is considered anemic by many players these days.

But these things also have a bad rep that’s partly deserved due to their excessive lows. And if you don’t know the HPF trick, the sound leaves a lot to be desired. When I was about to toss mine some years back I was with my GF at Jim Mouradian’s shop. He was doing some work on a couple of her guitars when I mentioned how annoyed I was and was just going to scrap mine. He suggested I not do that because (as he put it) there were some really good sounds in the SM-400s. But it could be tricky to dial them in. So I kept it, and once I got it sorted out based on a few tips Jim shared, it really was a rather nice albeit unnecessarily complicated amp. Not really my cuppa. But it worked - and it was paid for - so I kept it.

I do want to pop the hood and take a closer look at the power selector switch. Moving it back and forth earlier, it felt a little funny. And I thought I heard the speaker briefly cut in a few times. Same thing with the dual mono outs. So I’m wondering if the switch (it’s a plastic slider) is the problem. Maybe it took a whack or cracked or something. Fixing a simple switch is something that’s within the scope of things I can do. If it turns out to be more it’ll get a vacation. I’m not about to toss a few hundred into that when for under a grand I can get something better that I actually like with a warranty. So we’ll see.

Fingers crossed.

 

[agedhorse]

Reminds me of the service by some guys that used to be in business; GB or something like that...

... and both were owned by the same parent company at the same time, and shut down an about the same time too... wicked coincidence

Mine is actually in immaculate condition. It went into a 4U rack road case the day I bought it and it’s been in there ever since. And I take pretty good care of my gear because I don’t like to fix or replace things. But even talking to somebody who wants one, it’s a total buyer’s market for these puppies. I could never get anybody to even go $300 for it. Most times the offers were below $200. And for that price I’d just as soon keep it as a backup. Besides, it’s 2x200W /400w mono power output is considered anemic by many players these days.

For all the players who can't understand why the older class AB amps are not being made much anymore, you statement pretty much sums it all up. It's sad (being a linear amp designer from way back) but reality.

 

[40Hz]

For all the players who can't understand why the older class AB amps are not being made much anymore, you statement pretty much sums it all up. It's sad (being a linear amp designer from way back) but reality.

I still think there's a difference in the sound of most Class-D heads compared to AB amps. But maybe it's just my imagining things. Besides, it's now possible to engineer features into amps to make them emulate things and sound better than the old days although even then there were people who were doing solid-state right. Even without today's technology, the old Acoustic Control Corp. did some interesting magic with solid-state back in the 70s. I had their 360 plus some other amps they built back then and those were (and still are) for me some of the best sounding bass amps ever made regardless of underlying technology. (Steve Rabe's involvement in SWR was the main reason I was interested in SWR amps when they first came out after the demise of the old Acoustic.) But whatever. You can't dwell on the past.

I think I'll see what I can do with my SWR but still move ahead on getting another amp. I confident enough in my ability to get (within reason) a good sound out of anything I'm given to use. So me getting a new amp shouldn't be cause for any personal angst. I've still got my old companion, the Ampeg B-15N I bought back in late '69 any time I want to get back to my musical roots. So it's all good.

 

[40Hz]

I happen to know one who did an excellent job tuning-up my old SM400S...was worth every penny I spent, including shipping costs to Davis CA!

I might PM you about that if it gets down to needing real bench time. Audio electronics is not my bailiwick. Turn me loose on a server, PC or a hunk of networking hardware and I'm in my element. (Even if we're now doing so much of it in virtualized environment that the hardware is becoming moot for a lot of things I once needed to do on actual metal. Very cool stuff going on in the IT world. Imagine deciding you needed a new amp or something and in five minutes using just a keyboard you could have one - or several with automatic failover if an instance failed. Gotta love that! That's how servers and now networking hardware is going. I can create and destroy virtual hardware at will as requirements change at no additional cost over what was spent to get the host machine all those virtual machines are running on. Welcome to the Matrix!) But audio is something I never had the time to get deeply into enough to genuinely know what I'm talking about. And these days I'll confess I just want to play music and not ditz with things like I used to.

So much music - so little time.

 

[40Hz]

they don't have any real advantage over current

That in a nutshell is my feeling about a lot of gear. Nice enough for what it is. But nothing so unique or advantageous (most times) that it could ever be called irreplaceable.

 

[NathOBX]

I’ve never seen an expert input on an amp. Does it make your playing sound better?

 

[gitfiddl]

I longed for an SWR SM-400 for a couple decades. I finally bought one in great shape this February for $195. I see them at that price frequently. Today's world is definitely a different one. Whether that's good or bad is a topic for another forum, but I still like old gear.