Trace Elliot Smc 715 150 Watts 1x15" Back View Images
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Not sexy, I know - Trace Elliot 715 SMC bass amp opened up - is it safe?
- Thread starter big jilm
- Start date
- #1
I have this old solid state boat anchor, but it's my only bass amp - and it's on the fritz. Trace Elliot 715 SMC GP7 bass combo. 150 watts, 1x15".
When I play through it, all of a sudden the volume will drop dramatically and it will sound distorted. I can sometimes hit the top of the amp with my fist and make it work again for a little while, then it happens again.
I tried cleaning the effects loop jacks, and tried just bridging it with a cable. No help. Checked the speaker connections - they look good (to me). So, I pulled the head out:
Is it like a tube amp where voltage is still in there? I don't want to get shocked! What I would like to do is plug it up and play while poking the wires with a wooden stick - is that safe? I figure since I can knock it with my fist and have it work sometimes, it might just be a loose connection or something. Anyway, I want to be safe, so I'm not doing anything with it for the moment (if ever).
Anyway, I was hoping you guys might have some ideas about what might be wrong, and if it's ok to poke with a wooden chopstick/skewer in there while the amp is on.
Thanks!
- #2
THAT amp is most likely low voltage...50 Volts and under I am assuming. Not making light of the Voltage numbers, you still need to take care.....just use your head.
Go ahead and chop-stick it.
What you mainly want to avoid, with any device that is open, is grabbing the amp with both hands, or touching something inside the amp, while the other hand rests on, or steadies.
the chassis.....NOT touching the amp with both hands at the same time is the idea.
COULD be all sorts of stuff.
ONE thing MIGHT be.....all those spade connectors, the slide on connectors, they can be oxidized and give Bad/Intermittent contact. Often, just the act of disconnecting and reconnecting will clean them enough to renew contact. You can stick those and any other connection you see.
best
- #3
This is important, yes. But to say this is all that matters for personal safety is incorrect, just as it is incorrect to say that B+ is not a "real" danger.What you mainly want to avoid, with any device that is open, is grabbing the amp with both hands, or touching something inside the amp, while the other hand rests on, or steadies the chassis.....NOT touching the amp with both hands at the same time is the idea.
For example, you also want - very much, trust me - to avoid having even parts of just one hand contact two separate circuit points at the same time (for example, finger on one point in the circuit, palm on the other). This can easily happen if you get absorbed in what you are poking at, and without thinking try to push some wire or other object aside with the palm of your hand while your finger on the chopstick is deep in the amp. Your finger on the chopstick slips slightly, your palm is in contact with ground - and oops, shock and nasty burn right through the skin. Will it happen every time? No. Can it happen, does it happen? Yes.
So yes, use a chopstick, and also stay alert and aware. Take breaks. Consider your environment too. And obviously for things like checking out spade connectors, the amp should be off and unplugged and you should have a pretty fair idea of what those connectors connect to.
OP, if you're not used to working on a live amp, it will take you maybe 5 or 10 minutes to read a reliable and very clearly written account of how to stay safe, whether with a tube amp or solid state: the section titled "Safety Precautions," to be found on p. 189 onwards of Teemu Kyttälä's book "Solid-State Guitar Amplifiers," available as a free PDF. Worth taking the few minutes required.
- #4
Where did I say that.?This is important, yes. But to say this is all that matters for personal safety is incorrect, just as it is incorrect to say that B+ is not a "real" danger.
Oh.....I forgot what forum I was on.
Good Luck OP
- #5
Would you rather I have paraphrased the part where you say it's a myth?
I don't think it's a good idea to write vaguely or dismissively on this topic.
Small shocks are better than large shocks. No shocks are better than small shocks. No burns are better than some burns. Think about helping people out here.
- #6
The Mains are Just As Dangerous, and is discussed much less frequently. Nothing vague about it.
It is ALL important, and in a few sentences, I did as much to impress that as forum length
Anyway.....good luck OP. Shortly you will hear from some good techs.
best
- #7
All I want to do right now is play and poke the wires - from 1' away with a BBQ skewer to see if they are loose.
- #8
Teen electrocuted while taking apart unplugged computer
Plus, there was a long thread awhile back on The Gear Page in which someone raised the question, how dangerous can the high DC voltage inside a guitar amp really be, if no one has ever heard of someone being killed by it? Here is the link:
Ever hear of a death from tube amp's lethal voltage?
The thread brought forth several pages worth of anecdotes in which techs and DIY'ers recalled bad incidents in which they got shocked & perhaps almost killed - mostly by mains AC, but in some cases, from high DC voltage from filter caps, amp circuits, etc. I've talked to a retired electronics engineer, & have also read, that what they especially respected when dealing with DC were old TV sets, microwave ovens, etc.
Most likely if we searched long enough on the web, we could find quite a few obits over the years of electricians, appliance repair techs, etc., who were killed by high voltage DC just as the teenager was. When these deaths happen they're not going to show up on The Gear Page. That doesn't make the danger of high voltage DC from a big filter cap or a B+ supply a "myth." There are other threads too on other forums where people talk about burned hands, numbed arms, and very nearly being killed when dealing with filter caps in amps & making a mistake.
Perhaps the reason there are more deaths from mains AC is because it is more accessible - it can be right on the chassis of an appliance, for example. That's why earth grounding matters. I've never heard anyone say otherwise, on TGP or elsewhere.
- #9
That amp does not have "about 50V or less inside"
but, being 150W, requires some +/- 50/60V rail s (notice plural) , which being in series actually means 100/120V DC end to end.THAT amp is most likely low voltage...50 Volts and under I am assuming.
Both rails are present at the bridge rectifier and big filter caps, which might intuitively be perceived as screaming "don't touch me" (wouldn't base safety just on that though ) but also everywhere in the power amp PCB, often in side by side tracks separated by 1 or 2 mm
Now you are measuring voltages at the PCB, feeling relatively safe because you are a foot away from Mains hardware , your fingers slip less than an inch and you are toast.
Which is the definition of a guy who touches + 50/60V with his right hand, and -50/60V with the left one.
In case you don't notice, it's right across the chest and anything inside it (hint hint) .
- #10
One more time, just so I'm clear:
So, if I'm careful, I should be able to play the amp while poking wires with a wooden stick and be safe, correct?
- #11
- #12
Yes - if by "play the amp" you mean "have it turned on" while poking with the stick. The stick will keep you safe so long as your fingers are not near the tip and you've got your other hand in your pocket or behind your back, as the cliché goes.One more time, just so I'm clear: So, if I'm careful, I should be able to play the amp while poking wires with a wooden stick and be safe, correct?
Here's a link to a decent video on safety from Premier Guitar - as I remember, about halfway through the tech brandishes a chopstick and talks about it being a useful tool:
Amp Safety 101
On the other hand if at some point you want to turn the amp off and unplug it and work inside it USING YOUR HANDS to find bad connections (e.g. check out things like spade connectors by pulling them off & putting them back on), then you should to learn how to identify & safely drain the big filter caps. It's not "a screwdriver across the cap" as some old salts still believe; and some kind of inexpensive multimeter is a big help. A couple of TGP threads can help you:
Always Drain Your Filter Caps
Resistor for draining filter cap
- #13
Well, using a wooden BBQ skewer, I think I found the connection that is giving trouble. Hitting a note on my bass (farty and low volume) I pushed on this connection:
It's like a plug with three small wires (green, black, and red) and one larger wire (gray with a green sleeve) going into it, then plugged into a board.
Looks like somebody siliconed this one - I don't know if it was like that from the factory, or an attempted fix, but there's clear silicon on it.
BAM! Volume was back, no fartiness. I blew it out with some compressed air, sprayed some contact cleaner at it, and played for a bit. Fine. Tried to make it mess up by pushing at the connection with the skewer, and I couldn't get it to malfunction - I finally got it to go bad again, and got it working the same way.
Is that part replaceable?
Can I touch that part of the amp if the amp is off and unplugged?
- #14
JM Fahey knows much more than I do and hopefully he will give you his thoughts. I will just mention some of what I can see from looking at the photos.
The part in question (the black plug) is simply to connect the wires it carries to the PCB. The plug was most likely glopped in place at the time the amp was built simply to keep it from coming loose - amps vibrate, and glue or glop is pretty routine for making sure things stay on PCBs. The top part of the plug inserts into a socket & the socket is soldered to the PCB. Possibly the plug was loose in its socket, although that would be odd - usually they have little plastic prongs that snap in place on the socket and keep things tight.
The big cap (well, "big" in relative terms) in the background of the photo looks like a filter cap. Those are the kinds of caps that can carry voltage even if the amp is turned off. You'd have to know this particular amp to know if the caps bleed off after the amp is turned off, or still hold voltage.
I'm not sure about the wires - the lettering for the plug on the PCB that I can read seems to say "+V 0V -V" which may correspond to the red wire, the green wire, and the black wire. This, plus the two diodes in the photos at left, makes it seem possible (I couldn't say for sure) that these wires connect secondaries from a power transformer to the PCB, then to a rectifier, & from there to the filter caps. So you could look & see if the wires seem to connect to a transformer. But I don't know what the gray wire is or where it connects, so I could be clueless here.
I don't think the plug itself is the most dangerous part of an amp circuit in the world - transformer secondaries are not active if an amp is turned off. HOWEVER, you don't know your way around this PCB, and in case those nearby filter caps are holding voltage, I wouldn't suggest getting your hands on any of it. Not even the plug because to wrestle with the plug you would likely start using two hands.
Also, yes - plugs like this can be replaced, it's just a cheap part - but it's a bit of a minor PIA. Something was clearly loose but it seems possible your poking with the stick fixed it enough to last. I think you either take it to a tech or just see how it does. Me, I'd see how it does. I might be tempted to add a bit more glop if it seems like the glop might have loosened up - you could poke at it with the chopstick to see. For glop I would just use any silicone glue.
But again, hopefully JMF will comment, because he really knows this stuff.
(EDITED) P.S. Just in terms of safety stuff - if you had one hand on the bass strings and the other on the chopstick, that would have made it imperative that you not slip up. I wish we had discussed this part beforehand. It would be semi-safe (and maybe you did this) to put the instrument on a stand so that you could pluck an open string, set it vibrating as loud as you wanted, and then with your hands completely off the instrument turn to the amp & prod it with the chopstick. Safer than that would be to have a friend play the note on the bass & not you. And if no one was available, you could put a tone into the amp - very easy to do - and let the tone make the farty noise while you are free to concentrate completely on chopsticking. That would have been best.
- #15
Thanks for the info here! I'll be very careful - likely I will just try to re-goop, I guess. I certainly don't want to touch anything that I shouldn't.All right! Good chopsticking!!JM Fahey knows much more than I do and hopefully he will give you his thoughts. I will just mention some of what I can see from looking at the photos.
The part in question (the black plug) is simply to connect the wires it carries to the PCB. The plug was most likely glopped in place at the time the amp was built simply to keep it from coming loose - amps vibrate, and glue or glop is pretty routine for making sure things stay on PCBs. The top part of the plug inserts into a socket & the socket is soldered to the PCB. Possibly the plug was loose in its socket, although that would be odd - usually they have little plastic prongs that snap in place on the socket and keep things tight.
The big cap (well, "big" in relative terms) in the background of the photo looks like a filter cap. Those are the kinds of caps that can carry voltage even if the amp is turned off. You'd have to know this particular amp to know if the caps bleed off after the amp is turned off, or still hold voltage.
I'm not sure about the wires - the lettering for the plug on the PCB that I can read seems to say "+V 0V -V" which may correspond to the red wire, the green wire, and the black wire. This, plus the two diodes in the photos at left, suggest that these wires may connect secondaries from a power transformer to the PCB. So you could look & see if they seem to go to a transformer.
I don't think this is the most dangerous part of an amp circuit in the world - transformer secondaries are not active if an amp is turned off. HOWEVER, you don't know your way around this PCB, and in case those nearby filter caps are holding voltage, I wouldn't suggest getting your hands on any of it. Not even the plug because to wrestle with the plug you would likely start using two hands.
Also, yes - plugs like this can be replaced, it's just a cheap part - but it's a bit of a minor PIA. Something was clearly loose but it seems possible your poking with the stick fixed it enough to last. I think you either take it to a tech or just see how it does. Me, I'd see how it does. I might be tempted to add a bit more glop if it seems like the glop might have loosened up - you could poke at it with the chopstick to see. For glop I would just use any silicone glue.
But again, hopefully JMF will comment, because he really knows this stuff.
P.S. Just in terms of safety stuff, I'm sure you already know - but having your hands on the bass strings made it imperative that you not slip up on the chopstick. Obviously you didn't so all is well. Myself, I would probably do it differently: for example I would put the instrument on a stand so that I could pluck a string, set it vibrating as loud as I want, and then with my hands off the instrument I would prod with my chopstick. I don't think I would do both things at once.
- #16
- #17
I had the bass around my neck - I played a note, then used the stick. OK - next time, I'll put the bass on a stand, hit the note, hands off the bass, use the stick. Thanks very much for all the help so far!!!But please do see my P.S. on my comment above - I have edited & expanded it a bit. I am unhappy with the thought you might have had the bass in one hand and the chopstick in the other at the same time! If you ever need to do this sort of thing again, it's easy to put a tone into the amp rather than try and play and prod at the same time. The tone can come from something simple like a computer or iPhone app signal generator - you hook a male-to-male cord out of the headphone jack into a phone plug so you can plug it into the amp - you can get a nice low frequency note or whatever you want. You might want to keep the volume down on the headphone out - you could ask in this forum how to set all this up with a voltmeter so you don't blast the preamp in the bass amp.
- #18
That plug sends power and ground from the main Power Supply to the front mounted Preamplifier.
Red/Black are DC rails, 95% of the time they send +/-15V or thereabouts, because either you have a small supply just for that, or, most common, they reduce main rails (estimated around +/-50V) to +/-15V using regulators or a couple 1W/2W/5W resistors and Zener diodes, BUT some designers send the full voltage and the preamp has its internal regulators/Zeners.
Please repeat the last closeup but at 2X the distance and from a vertical point (now some parts are obscured) , so the above picture now is the bottom right quarter of the new one.
Repeat it if needed, parts values should be sharp and readable.
That said, I see what looks like a Zener diode just below the right large filter cap, I read part of the label, it's BZX55C** , "**" should read 15V or 16V and there should also be another Zener diode nearby (now hidden).
If so, it's confirmed that you are sending +/-15V through Red/Black , and green should be Power Supply Ground.
The grey cable is an audio shielded one, shield is audio ground and center wire is hot, and poor contact at that connector is very compatible with losing signal with vibration (or chopsticking).
2 main possibilities, both solvable.
1) there you have a 5 pin "comb" . a male connector soldered to the PCB, needle thin pins separated by 0.1" (2.54 mm) , the most common standard for connectors, specially in PCs, DVD players, etc.
It's somewhat flimsy for permanently moving, vibrating and mistreated MI amps , so a prime suspect.
I would not be surprised *at all* (and that's an understatement) to find cracked solder at the bottom of that PCB (you should dismount the Power Amp module to do so, probably 4 screws and you lift it away from chassis and sit it upside down over a magazine just in case, to avoid it touching chassis.
Then you reinforce soldering by touching each of the 5 solder pads with a fine tip solder iron (around 25W is fine) and a little, very little bit of solder.
Just melt the old solder (should take, say, 2 to 5 seconds, if 10 seconds or more your soldering pencil is too weak) and then you slightly touch that molten solder with the tip of a piece of good quality thin solder wire (0.7mm to 1.0 mm, not more) .
You just want a bit of fresh solder to melt and mix with the old one, and its flux/rosin to clean the surface.
Should take 2-4 seconds and then you rise both away and let solder cool undisturbed.
It should cool on its own into a smooth bright surface.
Possible problem: you apply too much and the resultant solder blob touches and joins to a nearby one (they are real close).
Just in case, have a solder sucker nearby, remelt and suck the blob away, then reapply a little solder to each pad involved.
Quite confident that this should solve your problem.
2) the pull out female connector on the cable, which is plugged into the male PCB connector has weakened contacts , or some individual wire to female connector has weakened (they are not soldered but crimped, as in "mechanically crushed together" or the wire itself has some problem (corrosion, or cracked after much flexing) .
In any case, you should replace the full connector but it's not easy, specially without the proper crimping tool , whict to boot is too expensive to be used just once, so IF that's the case, I'd normally suggest to pull the full connector, even the male PCB mounted one, melt/suck the holes clean, chop the female connector off and straight solder those wires in the holes, the old way.
IF needed, I'll explain it in its own post, to avoid making this one even longer
PS: and there's a simplified "Argentine Way" to do it , not that Pro loooking but which pampers the PCB which is something we must care about all the time.
In my book, "the PCB IS the amplifier", anything else is "parts" and can be bought somewhere.
Ok, go resolder those suspect connector pads, reassemble carefully, and test, you might be done with it.
If not, we continue with Plan B
- #19
Wow - AMAZING post, Mr. Fahey - this is exactly why I say that TGP is the best site on the net for good knowledgeable people who are willing to take time out of their life to help.Thanks for the trust shown ,it's a great honour.That plug sends power and ground from the main Power Supply to the front mounted Preamplifier.
Red/Black are DC rails, 95% of the time they send +/-15V or thereabouts, because either you have a small supply just for that, or, most common, they reduce main rails (estimated around +/-50V) to +/-15V using regulators or a couple 1W/2W/5W resistors and Zener diodes, BUT some designers send the full voltage and the preamp has its internal regulators/Zeners.
Please repeat the last closeup but at 2X the distance and from a vertical point (now some parts are obscured) , so the above picture now is the bottom right quarter of the new one.
Repeat it if needed, parts values should be sharp and readable.
That said, I see what looks like a Zener diode just below the right large filter cap, I read part of the label, it's BZX55C** , "**" should read 15V or 16V and there should also be another Zener diode nearby (now hidden).If so, it's confirmed that you are sending +/-15V through Red/Black , and green should be Power Supply Ground.
The grey cable is an audio shielded one, shield is audio ground and center wire is hot, and poor contact at that connector is very compatible with losing signal with vibration (or chopsticking).
2 main possibilities, both solvable.
1) there you have a 5 pin "comb" . a male connector soldered to the PCB, needle thin pins separated by 0.1" (2.54 mm) , the most common standard for connectors, specially in PCs, DVD players, etc.
It's somewhat flimsy for permanently moving, vibrating and mistreated MI amps , so a prime suspect.
I would not be surprised *at all* (and that's an understatement) to find cracked solder at the bottom of that PCB (you should dismount the Power Amp module to do so, probably 4 screws and you lift it away from chassis and sit it upside down over a magazine just in case, to avoid it touching chassis.Then you reinforce soldering by touching each of the 5 solder pads with a fine tip solder iron (around 25W is fine) and a little, very little bit of solder.
Just melt the old solder (should take, say, 2 to 5 seconds, if 10 seconds or more your soldering pencil is too weak) and then you slightly touch that molten solder with the tip of a piece of good quality thin solder wire (0.7mm to 1.0 mm, not more) .
You just want a bit of fresh solder to melt and mix with the old one, and its flux/rosin to clean the surface.
Should take 2-4 seconds and then you rise both away and let solder cool undisturbed.
It should cool on its own into a smooth bright surface.
Possible problem: you apply too much and the resultant solder blob touches and joins to a nearby one (they are real close).
Just in case, have a solder sucker nearby, remelt and suck the blob away, then reapply a little solder to each pad involved.Quite confident that this should solve your problem.
2) the pull out female connector on the cable, which is plugged into the male PCB connector has weakened contacts , or some individual wire to female connector has weakened (they are not soldered but crimped, as in "mechanically crushed together" or the wire itself has some problem (corrosion, or cracked after much flexing) .
In any case, you should replace the full connector but it's not easy, specially without the proper crimping tool , whict to boot is too expensive to be used just once, so IF that's the case, I'd normally suggest to pull the full connector, even the male PCB mounted one, melt/suck the holes clean, chop the female connector off and straight solder those wires in the holes, the old way.
IF needed, I'll explain it in its own post, to avoid making this one even longer
PS: and there's a simplified "Argentine Way" to do it
In my book, "the PCB IS the amplifier", anything else is "parts" and can be bought somewhere.
Ok, go resolder those suspect connector pads, reassemble carefully, and test, you might be done with it.
If not, we continue with Plan B
I will snap the pics you request in the morning - I REALLY appreciate the wisdom and help you are providing for me. I really like this old boat anchor of an amp - it would be great to get it up and running full-time!
I will post again in the morning.
My thanks -
big jilm
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Trace Elliot Smc 715 150 Watts 1x15" Back View Images
Source: https://www.thegearpage.net/board/index.php?threads/not-sexy-i-know-trace-elliot-715-smc-bass-amp-opened-up-is-it-safe.1643845/
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