sc20h power

[fer1991]

Hi everyone, I've got an oldy thing to discuss today. In the past I've taken my sc20h to the service to fix the fx loop (I'm leaving here a post where I talk about it) with very very bad experience with the service.

the link: https://www.marshallforum.com/threads/anyone-get-the-sc20-loop-fix-implemented.126766/

In all the things they did wrong, they messed up with the cable's order from the power supply going to the board. As I previously recorded a video for references, the second I saw that I asked them and they said that they were missorderer from the factory.
Since the whole job was poorly done, I reverted that, and even reinstalled properly the original resistors values they changed. But, is there any way to check if the transformer is properly connected?? Besides looking at my photos before the service or any other photo from google. Is there any way to measure with a multimeter the cables from the transformer and to know where they are supposed to be connected?? Thanks a lot everyone

 

[PelliX]

Hi everyone, I've got an oldy thing to discuss today. In the past I've taken my sc20h to the service to fix the fx loop (I'm leaving here a post where I talk about it) with very very bad experience with the service.

the link: https://www.marshallforum.com/threads/anyone-get-the-sc20-loop-fix-implemented.126766/

You mean a Marshall Authorized Dealer (or whatever they're referred to as) or sending it in to Marshall themselves? Just curious, it's after the fact, now...

In all the things they did wrong, they messed up with the cable's order from the power supply going to the board. As I previously recorded a video for references, the second I saw that I asked them and they said that they were missorderer from the factory.

So you have "documentation" of the wires and where they connect? I can gladly provide a photo of the "correct" (or assumed correct - certainly working) layout in mine. But are you really saying that they swapped things around? They had to remove the boards to perform the changes, but one would expect them to get that right - whoever they are.

Since the whole job was poorly done, I reverted that, and even reinstalled properly the original resistors values they changed.

Was the FX loop "worse" than before, or were you experiencing different problems...?

But, is there any way to check if the transformer is properly connected?? Besides looking at my photos before the service or any other photo from google. Is there any way to measure with a multimeter the cables from the transformer and to know where they are supposed to be connected??

Well - and I mean this in a sincere but partially tongue in cheek way - if the amp is working, chances are the wiring is correct. You could indeed work out which tap is which and measure the voltages, but if something were amiss, you would have released the magic smoke...

 

[fer1991]

You mean a Marshall Authorized Dealer (or whatever they're referred to as) or sending it in to Marshall themselves? Just curious, it's after the fact, now...



So you have "documentation" of the wires and where they connect? I can gladly provide a photo of the "correct" (or assumed correct - certainly working) layout in mine. But are you really saying that they swapped things around? They had to remove the boards to perform the changes, but one would expect them to get that right - whoever they are.



Was the FX loop "worse" than before, or were you experiencing different problems...?



Well - and I mean this in a sincere but partially tongue in cheek way - if the amp is working, chances are the wiring is correct. You could indeed work out which tap is which and measure the voltages, but if something were amiss, you would have released the magic smoke...

When I read somewhere about a "fix" or "upgrade" I wrote Marshall uk and got answered by the official service in my country. They were far away from where I live but took the amplifier to them. When I got it back, I saw the poorly work done, and since I could do a better job I put everything as it were. It took me a few headaches to find the original values since not even Marshall would tell me what they were. Eventually the bad service did the right thing and told me what were the original values.

I've got photos of what they did, I just have to look for them at home to share them here. But the amplifier turned on with the wrong conections, and with the original conections.

The fx just fixed the volume drop issue, not the poping thing, and the treble control became a lot more harsh.. the notes had lost their weight. At the same time I reverted the resistors I corrected this power supply cable issue.

The amplifier "worked" when they gave it to me, but I couldn't stand the poorly work, so I did not test the amplifier for too long, I didn't want it to blow up, or something. I assumed that the factory would have done it better, and google photos showed everyone wired the same as the original way. But, since the technician only followed schematics, and the amplifier didn't blow up in the first 5 seconds, could he be that wrong about that?

 

[PelliX]

It took me a few headaches to find the original values since not even Marshall would tell me what they were.

I believe the service bulletin shows the resistors including their values - else somebody here (myself included) could have provided you with those. Ah well, got it sorted out, right.

But the amplifier turned on with the wrong conections, and with the original conections.

I'm going to assume this likely reversed AC going to the rectifier(s) or heater circuit - which effectively makes zero difference unless you're one of those people who actually ensures you put the plug in the socket the right way, the socket is wired correctly, etc. I don't know where you live, but reversed wiring on sockets is pretty common many places. Considering it turned on (without sparks and all that), they didn't mix up the B+ with the heater wiring which would be detrimental. Presuming you tested High and Low power modes and they operate correctly, they didn't mix that up, either.

and the amplifier didn't blow up in the first 5 seconds, could he be that wrong about that?

...that sums it up, pretty much yes. Once you flick that power switch without other precautions (removing valves, variac, current limiter), you basically cross the proverbial Rubicon. Glad it worked for you!

As for the technician, I don't know what schematics he was following. It may well be that the exact wiring is omitted. Uncommon, but what do I know...

 

[fer1991]

I'm going to assume this likely reversed AC going to the rectifier(s) or heater circuit

Since I'm no expert in schematics or Photoshop, I'll try this way. They changed the order from the 4 wires coming from the power supply, this way. (I wrote the original wire on the sides)

 

[PelliX]

Uhhm, that's a bit confusing. Do your High and Low power modes work normally?

It's probably what I suggested earlier - just swapped AC polarity, in which case there's nothing real to worry?

 

[Tatzmann]

View attachment 114561

 

[fer1991]

Uhhm, that's a bit confusing. Do your High and Low power modes work normally?

It's probably what I suggested earlier - just swapped AC polarity, in which case there's nothing real to worry?

To be honest, I really don't know how transformers work and I don't know what every cable it's supposed to do, so I'm gonna leave it as it came from the factory. The only things I could try to guess with this is that the blue and brown cables look like they were positive and negative, they leave the blue in front of a fuse and put a red one in front of the other fuse. But then again, I'm just guessing.

 

[PelliX]

Well, @Tatzmann provided a link to the schematic for the SV20 (which has practically the same power section as the SC20). So Red and Brown are a pair, so are Blue and White. Mixing the wires in a pair is not really a big deal, honestly. As I stated earlier, if they had mixed up something "important", you would have had smoke.

But the only part left to check for you would be the High and Low power modes. If it's indeed louder on the High position of the standby switch and quieter on the Low position - it's working correctly.

This is the correct wiring (my SC20). ^

 

[fer1991]

This is the correct wiring (my SC20). ^

Thanks for the photo, that's the way I put the wires. Since brown and red are a pair, and so white and blue, wouldn't it make more sense to put the leads in front of the fuses?? So I guess he took the red as a lead cable instead of the brown??

(Hi and Low works fine, and it worked with the service wiring as well)

 

[PelliX]

Thanks for the photo, that's the way I put the wires. Since brown and red are a pair, and so white and blue, wouldn't it make more sense to put the leads in front of the fuses?? So I guess he took the red as a lead cable instead of the brown??

Well, in GENERAL, you want the fuse on the live wire, not the neutral. So that's an argument against swapping them around. The problem is, however, that in most parts of the world, where plugs fit in a socket in two ways (rotated 180 degrees), there's no telling which would be the live and neutral wires without keeping track of it. Then, as I mentioned earlier, there's the chance of the socket having its polarity reversed. So unless you test your socket, ensure the power cord is connected the right way around and so on - there's no benefit. There's a ~50/50 chance that he did you a favor, too. See what I mean?

(Hi and Low works fine, and it worked with the service wiring as well)

Good, then you're fine!

 

[fer1991]

There's a ~50/50 chance that he did you a favor, too. See what I mean?

Yes, you're right, on top of all of that, I live in Argentina where we had type C connectors, and now type I. So every socket is wired differently. Yes.. every socket should be installed by a professional.. but that's not reality. So you are right, I just can not know witch's live and witch's neutral, and even if I did, I won't be opening the amp every time I switch the power-sockets so like you said, there's 50% chance that he did me a favor, and 50% chance of that favor disappearing as soon as I plugged the amp at home.
Thanks a lot for your time and help. I really appreciate it . I thought something like this could be the answer but I don't have the knowledge to point it out firmly.

So, If I understood it well, everytime the socket is wired differently it could be similar to switching the wires internally, so I can not know if there's a final way to wire the amp, so the factory way should be the right one, isn't it??

 

[PelliX]

Yes, you're right, on top of all of that, I live in Argentina where we had type C connectors, and now type I. So every socket is wired differently. Yes.. every socket should be installed by a professional.. but that's not reality. So you are right, I just can not know witch's live and witch's neutral, and even if I did, I won't be opening the amp every time I switch the power-sockets so like you said, there's 50% chance that he did me a favor, and 50% chance of that favor disappearing as soon as I plugged the amp at home.
Thanks a lot for your time and help. I really appreciate it . I thought something like this could be the answer but I don't have the knowledge to point it out firmly.

No prob!

So, If I understood it well, everytime the socket is wired differently it could be similar to switching the wires internally, so I can not know if there's a final way to wire the amp, so the factory way should be the right one, isn't it??

Yes, effectively, every new socket you encounter *might* be wired the other way around. I don't know much about Argentinian electrics, but mixups are common in most parts of the world.

Of course, a simple way to check is to plug the power cable in the socket and check which wire is live with a tester screwdriver. If the polarity is reversed, turn the plug in the socket, and it will be "correct".

L = Live (which should cause the lamp in the screwdriver to light up), N = Neutral, E = Earth (Ground).

 

[fer1991]

L = Live (which should cause the lamp in the screwdriver to light up), N = Neutral, E = Earth (Ground).

Well.. I checked at home, and we use the Live wire at the left side of the female plug. I even detected one extension cable that was wired opossited (as Australia does), and that's way I always had some ground noise when using the guitar and using the phone while connected to the wall in that extension.

What I'm thinking now it's that my amplifier comes from Spain, and my voodoo lab comes from Germany, and they are 220v as we are here, that's what I thought I should look after, but now I have my doubts about if somehow I'm bypassing fuses or making the ground path work unpropperly

 

[PelliX]

Well.. I checked at home, and we use the Live wire at the left side of the female plug. I even detected one extension cable that was wired opossited (as Australia does),

Yeah, doesn't surprise me

and that's way I always had some ground noise when using the guitar and using the phone while connected to the wall in that extension.

Uhm... ground noise would typically not depend on the polarity applied to your amplifier. If you're talking about a mobile phone, then the polarity will also play no significant role in the shielding against that... maybe I'm misunderstanding you here?

What I'm thinking now it's that my amplifier comes from Spain, and my voodoo lab comes from Germany, and they are 220v as we are here, that's what I thought I should look after, but now I have my doubts about if somehow I'm bypassing fuses or making the ground path work unpropperly

Well, a fuse on the neutral will not protect in the way it would on the live wire, so depending on the expected fault condition it *could* be a major problem. To ensure this works correctly, you must consider polarity at all points in the circuit. For you that's the transformer wiring in the amp, the polarity of the socket and of the plug. Ground is ground, though. If you have a multimeter, measure the resistance between the ground on the plug of the amp (unplugged) and its chassis - you should have continuity (or a very very low resistance). Check the earth wiring in your house, too. Presuming both are fine, you should be OK.

If you want to spend some money to make these problems go away, there are (expensive) power conditioners and so on that will take care of this, or (quite cheap) socket testers that will simply show you whether the ground is OK, which is live and which is neutral. I've never used one myself (I just use a multimeter), but I've heard plenty of people being happy with them.

 

[fer1991]

Yeah, doesn't surprise me

Looking again at your photo and my videos, Marshall uses blue and brown for Live and Neutral, that's how they reach the fuses.

Wouldn't a polarity change be swapping only the white/blue - red/brown above/below order?

- white/fuse/blue
- red/fuse/brown

What I mean is don't know why he swapped brown and red and if what I just said would have melted the amp down in seconds

 

[PelliX]

Yes, but the primary fuse is located in the receptacle of the power cable - so correct polarity on the plug/cable must be ensured. On the inside of the chassis, the blue (neutral) and brown (live) lead to the power switch and then to the transformer (primary side). From there on you get the fuses for other sections inside the device. If you feel comfortable doing so, check the polarity with your meter - before the rectifier it's AC and after that it's DC of course - but even with the wrong setting your meter should show positive or negative readings.

 

[fer1991]

If you feel comfortable doing so, check the polarity with your meter - before the rectifier it's AC and after that it's DC of course - but even with the wrong setting your meter should show positive or negative readings.

I feel comfortable doing this kind of tests and I have a multimeter, but I'm not certain about how it should be done. I don't know exactly where I have to take the measures and what should they read

 

[fer1991]

So, we've got the polarity corrected at our cables, the female does have the lead in the right side. So, polarity has never been an issue?

 

[PelliX]

I feel comfortable doing this kind of tests and I have a multimeter, but I'm not certain about how it should be done. I don't know exactly where I have to take the measures and what should they read

Well, assuming that you have the 'socket and cable' but sorted out, as per this diagram:



Then you can proceed to proceed to measure the voltages on the secondary pairs of the transformer. Ideally, you should be able to put the positive lead on the fuse and get a positive reading. If you get a netgative reading (of the correct voltage, or close to it) it's been reversed and would need to be corrected.

So, we've got the polarity corrected at our cables, the female does have the lead in the right side. So, polarity has never been an issue?

Eh, wait! You guys have polarized plugs!

Well, whether the polarity entering your amplifier is correct at the socket is another matter, but you indeed checked that - so that's cool. Then it's just the secondaries of the transformer you want to check.

This "Schuko" is what you find in many parts of Europe and variations of similar designs. As you can see, it can be rotated by the user... so polarity is a bit of a gamble unless you pay due care and attention.