I'm repairing an SWR Workingman's 12 Combo for a friend. It has an annoying constant 60hz hum which is controlled by the level control - pot on the left side. If the pot is turned full CCW the hum goes away. The hum can be produced if nothing is plugged in, but level is at 12:00 O'clock.
Photo of the amp is attached.
All the other pots are at 12:00 O'clock but they don't affect the hum level much.
To me it looks like Preamp noise, since I can turn it off with the Level control. Power amp and PSU must be OK.
I'll have the amp on my bench by the end of the week and I was hopeful for suggestions on how to diagnose.
I have the schematics of the boards (from this forum).
If all else fails, I was thinking of bypassing the preamp and using a DI straight into the power amp, but hoping for an easy repair instead.
Also, need a suggestion about how to remove the amp from the box - the speaker wire running out the back of the amp down into the box is about 1 inch long. There are no connectors and no easy way to open the speaker box. Do I cut the wire and splice onto this 1 inch stub?
Thanks!
Photo of the amp is attached.
All the other pots are at 12:00 O'clock but they don't affect the hum level much.
To me it looks like Preamp noise, since I can turn it off with the Level control. Power amp and PSU must be OK.
I'll have the amp on my bench by the end of the week and I was hopeful for suggestions on how to diagnose.
I have the schematics of the boards (from this forum).
If all else fails, I was thinking of bypassing the preamp and using a DI straight into the power amp, but hoping for an easy repair instead.
Also, need a suggestion about how to remove the amp from the box - the speaker wire running out the back of the amp down into the box is about 1 inch long. There are no connectors and no easy way to open the speaker box. Do I cut the wire and splice onto this 1 inch stub?
Thanks!
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No speaker jack, huh? Remove the front grill; screws should be in the corners and sides. Then remove the speaker screws and pull it out slowly as you don't know how much slack wire they left you.. Unhook the leads keeping track which one goes to what terminal(polarity). The connectors should be the crimp-on type that slide on. Remove the top screws, these should hold the chassis in place. When pulling the chassis out, feed the speaker leads up through the hole in back as you are pulling it out.
Don't know about the hum issue as I'm not up on my solid state electronics. At least this will let you get it apart and more knowledgeable people can help with the hum issue.
Don't know about the hum issue as I'm not up on my solid state electronics. At least this will let you get it apart and more knowledgeable people can help with the hum issue.
Thanks boobtube - actually no grills either. This must be the economy model. The box is fully carpeted with no access to the speakers. I guess I could cut off the carpet, but even then I'm not sure there will be access without some sawing. Hoping to be less invasive.
Sorry, can't see down by the the speaker in your OP. Will cutting open the cloth leaving enough to wrap around to reach back to the speaker baffle board so you can glue it on later work? Then buy some of the metal grill material I gave you the link to. You'll probably have to cut it to fit. Then follow the original post I gave to take it apart. Other suppliers stock the grill too.18" by 39-1/4" Sheet of Perforated Steel Powder Coated Black for DIY Speaker Grills
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How come no grill?
Is the speaker visible or behind carpet?
Post a picture showing the cabinet front, not just the amp chassis
I'm amazed at your suggesting a:
Is the speaker visible or behind carpet?
Post a picture showing the cabinet front, not just the amp chassis
I'm amazed at your suggesting a:
An externally hosted image should be here but it was not working when we last tested it.
The removal of the chassis, resolving the speaker issue and ordering a grill has been taken care of. Read post #6. Any suggestions about the hum? I'm not up very well on my solid state circuitry.
I'm rereading post # 6 and see no answer, just a suggestion to "cut the cloth so it can be reglued later"
That combined with the OP idea about :
means both on you agree that cutting the carpet is the proper way to access the speaker, which of course is nonsense.
I'm still waiting for the OP to post a picture illustrating how come the speaker is invisible and not unmountable
That combined with the OP idea about :
means both on you agree that cutting the carpet is the proper way to access the speaker, which of course is nonsense.
I'm still waiting for the OP to post a picture illustrating how come the speaker is invisible and not unmountable
Don't cut anything. The chassis is usually VERY tight in its space, its edges push into the carpet. It sides out but often takes a LOT of effort. If the wire to the speaker does not unplug from the rear of the amp, then slide the chassis out enough to see inside, you should be able to disconnect them and then sslide it out the rest of the way. "slide" makes it sound easy, maybe "shove" is a better word.
I'm rereading post # 6 and see no answer, just a suggestion to "cut the cloth so it can be reglued later"
That combined with the OP idea about :
means both on you agree that cutting the carpet is the proper way to access the speaker, which of course is nonsense.
I'm still waiting for the OP to post a picture illustrating how come the speaker is invisible and not unmountable
If you read post #7 he says it will work. At that time, I only was ASKING if it would work for the reason you state, the OP didn't post a pic of the grill of the amp. My son owns a Workingman's 15 which is built the same , so I had a reference as to how the amp is constructed.. If the amp is totally covered with carpet, the only way to access the speaker to remove the connections is to cut the cloth long enough to wrap it around to meet the baffle board and then buy some metal grill and screw it on. The only reason this is needing to be done is that a on a few models, SWR did not provide a speaker jack, it is hardwired to the chassis and the only way to remove the chassis is to disconnect the speaker wires and pull out the chassis with the speaker wire connected to it. Unless you suggest cutting the 1" of exposed wire in the back and splicing it later which of course is ridiculous.
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Here is what he is dealing with. This is why the only way I can see to remove the chassis is to disconnect the speaker leads. And the only way to do that is to access the speaker which is covered with carpet. How else is he to access the speaker? The enclosure is sealed on top so he cannot look down into the enclosure by sliding the chassis partway out without disconnecting the speaker.The chassis is mounted above a sealed enclosure, but there is a 1/4" gap between the chassis and the top of the enclosure, so it will slide out OK. I'm wondering why you are telling him not to do something he has established that will work in post #7. He needs help with the hum.
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Maybe I am stupid, but I don't understand the speaker being "covered with carpet". THat sounds to me like a sheet of carpet in front of the speaker, which can't be.
I believe there is a hardwire speaker lead out the rear of the amp, but how about a photo of the speaker, so we can get to the bottom of this. The cab is a wood box with a large hole in the front. Isn't the speaker placed in the hole with screws around the edge?
In the earlier photos we saw grilles, but with the grille removed we can still see the speaker is front mounted. A rear mounted speaker would mean the back of the cab comes off, and I don't think that is the case. No one builds a cab around a speaker.
I believe there is a hardwire speaker lead out the rear of the amp, but how about a photo of the speaker, so we can get to the bottom of this. The cab is a wood box with a large hole in the front. Isn't the speaker placed in the hole with screws around the edge?
In the earlier photos we saw grilles, but with the grille removed we can still see the speaker is front mounted. A rear mounted speaker would mean the back of the cab comes off, and I don't think that is the case. No one builds a cab around a speaker.
Imagine the attached photo to not have the metal grill but carpet stretched over the speaker. I wanted to know if he could cut the carpet in the same shape of the opening, but to leave extra so it would wrap around the front to extend back on the sides to meet the speaker baffle as it is recessed from the front. Then he would glue it on and attach metal grill to fit the opening to cover the speaker. I sent him a link to a speaker place to see the grill style and told him there are others as well. He'll probably have to paint the wood and figure out spacers for the grill but I see no other way to do it. If you do, by all means it would be great to hear an alternative! Not sure why the OP won't post a pic, but maybe because we solved this part and he's waiting for advice on the hum issue? Maybe it was covered for some reason by a previous owner, who knows?
All of this because there is no jack to unhook the speaker as pictured two posts ago and because the entire speaker cab
is enclosed except for the hole in the top for the speaker leads and of course for the speaker and horn. Easy to see speaker is front mounted.
Sorry for the messy attachment. Forgot to relocate the cursor.
All of this because there is no jack to unhook the speaker as pictured two posts ago and because the entire speaker cab
is enclosed except for the hole in the top for the speaker leads and of course for the speaker and horn. Easy to see speaker is front mounted.Sorry for the messy attachment. Forgot to relocate the cursor.
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I know this is an old thread, but figured I'd add to it just in case anyone else finds it searching for more information.
You guys are making this WAY too difficult. To get at the circuitry, remove the four phillips head screws on the top of cabinet. After doing so, the amp section easily slides out forward. The cable going into the speaker cabinet that you are so worried about has a LOT of slack in it.. just gently pull out what you need as you slide the amp forward. If you need to remove the electronics completely, you can either unsolder the wire from the circuitry or remove the drivers from the front after removing the grill. You won't have to, though.
As far as the hum problem being described, you just have a bad ground strap wire across your input jacks. I'm going to take a wild guess that one or both of your input jacks is/are loose and that, when you attempted to tighten the narrow nut that holds them in place, the jacks spun a bit. When that happened, the bare ground wire that connects to the two input jacks, the tuner jack, and the ground on the board was stressed and either came unsoldered at one or more points or just broke. It's very easy to spot, and an easy repair. Just resolder the bad connection or run a new ground wire if necessary. Takes a couple minutes.
To prevent this from happening again, you could replace the input jacks if the plastic threads have become stripped, or, if they're still in good shape (doubtful), carefully snug them down again and just make sure you don't rotate those jacks in the future.
Good luck. Cutting carpeting and cables is not the answer.
You guys are making this WAY too difficult. To get at the circuitry, remove the four phillips head screws on the top of cabinet. After doing so, the amp section easily slides out forward. The cable going into the speaker cabinet that you are so worried about has a LOT of slack in it.. just gently pull out what you need as you slide the amp forward. If you need to remove the electronics completely, you can either unsolder the wire from the circuitry or remove the drivers from the front after removing the grill. You won't have to, though.
As far as the hum problem being described, you just have a bad ground strap wire across your input jacks. I'm going to take a wild guess that one or both of your input jacks is/are loose and that, when you attempted to tighten the narrow nut that holds them in place, the jacks spun a bit. When that happened, the bare ground wire that connects to the two input jacks, the tuner jack, and the ground on the board was stressed and either came unsoldered at one or more points or just broke. It's very easy to spot, and an easy repair. Just resolder the bad connection or run a new ground wire if necessary. Takes a couple minutes.
To prevent this from happening again, you could replace the input jacks if the plastic threads have become stripped, or, if they're still in good shape (doubtful), carefully snug them down again and just make sure you don't rotate those jacks in the future.
Good luck. Cutting carpeting and cables is not the answer.
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