Selmer TnB 50 MKIII noise

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Selmer TnB 50 MKIII noise

Postby loz7 on Fri May 12, 2017 12:39 pm

I have a mkiii selmer 50 that has a nasty noise i.e like a lead is plugged in with no guitar attached or a loose ground. I have checked all the grounds which seem ok and there doesnt seem to be any dry joints. The only thing that makes it stop is when I put a lead into the FX loop IN and put the other end of the lead halfway into the OUT socket,the noise stops but then there is no signal when a guitar is plugged in..

Is the FX loop circuit a gain stage for both the bass and trebble channels?
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Re: Selmer TnB 50 MKIII noise

Postby Vortexion on Fri May 12, 2017 7:38 pm

Most likely possibilities are as follows:

1) Does the hum/buzz go away when the input volume controls are turned all the way down? If so, it's likely that one of the instrument input sockets is not properly shorting its "hot" connection to ground when there's no jack plug inserted. This situation leaves the input wide open, picking up room hum like an antenna. This could be because the shorting contacts need cleaning, or it could be that the leaf springs themselves have gone slack/lost their springiness from years of use. Slackness can sometimes be fixed by carefully bending them back together with pliers, but equally sometimes that can make the situation worse, 'cos it's very easy to bend the leaf springs into the wrong shape to grip a jackplug properly - and then you'd have to replace the relevant jack socket(s) altogether.

2) If the noise persists even though the input volume controls are down, then I'd suspect a similar spring contact problem in one of the FX loop IN/OUT jacks. These aren't wired to short to ground when you leave a plug out (because that would silence the signal completely - exactly what's happening when you do your "one jack in / other one half in" thing). Instead, they're wired so that when there are no jacks inserted, the signal is routed straight through from the "hot" of one socket to the "hot" of the other. Fixing a lack of contact issue here would require the same "clean it/bend it or replace it" approach described above.

The thing with dirty/oxidised jack spring contacts is that the actual contact point is quite small, and while the thing may look as if it's closing nicely, it only needs a thin film of dirt/corrosion to get in the way of good conductivity.

Of course, the grounding issue may be nothing to do with any of the sockets, but if you're sure that the grounds in the rest of the amp are OK, then they are the most likely culprits
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Re: Selmer TnB 50 MKIII noise

Postby loz7 on Sat May 13, 2017 7:53 am

Vortexion wrote:Most likely possibilities are as follows:

1) Does the hum/buzz go away when the input volume controls are turned all the way down? If so, it's likely that one of the instrument input sockets is not properly shorting its "hot" connection to ground when there's no jack plug inserted. This situation leaves the input wide open, picking up room hum like an antenna. This could be because the shorting contacts need cleaning, or it could be that the leaf springs themselves have gone slack/lost their springiness from years of use. Slackness can sometimes be fixed by carefully bending them back together with pliers, but equally sometimes that can make the situation worse, 'cos it's very easy to bend the leaf springs into the wrong shape to grip a jackplug properly - and then you'd have to replace the relevant jack socket(s) altogether.

2) If the noise persists even though the input volume controls are down, then I'd suspect a similar spring contact problem in one of the FX loop IN/OUT jacks. These aren't wired to short to ground when you leave a plug out (because that would silence the signal completely - exactly what's happening when you do your "one jack in / other one half in" thing). Instead, they're wired so that when there are no jacks inserted, the signal is routed straight through from the "hot" of one socket to the "hot" of the other. Fixing a lack of contact issue here would require the same "clean it/bend it or replace it" approach described above.

The thing with dirty/oxidised jack spring contacts is that the actual contact point is quite small, and while the thing may look as if it's closing nicely, it only needs a thin film of dirt/corrosion to get in the way of good conductivity.

Of course, the grounding issue may be nothing to do with any of the sockets, but if you're sure that the grounds in the rest of the amp are OK, then they are the most likely culprits


Thanks for the reply, all the inputs short to ground and the noise is present with the volume controls up or down. Also the FX loop IN and out are connected properly but I will check them again. The only other thing I can think of is that the mains transformer has been changed, and the noise is like a loud buzz/static so it could be that I guess?
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Re: Selmer TnB 50 MKIII noise

Postby Vortexion on Sat May 13, 2017 9:22 am

loz7 wrote:The only other thing I can think of is that the mains transformer has been changed, and the noise is like a loud buzz/static so it could be that I guess?

Seems doubtful to me. If the hum/buzz were as a result of a faulty mains tranny installation, you wouldn't be able to silence the noise by doing your trick with the FX loop jack sockets and patch cord. The fact that you are able to silence the noise (and, as you've said, cut off the signal entirely) with that trick means that the source of the problem must lie either in the FX loop connections themselves or at some point earlier in the circuitry (but not before the instrument volume controls, 'cos you've checked for that).

Suggestion:
You may already have tried this, but FWIW, here's a diagnostic worth doing. Obtain (or make) a very short length of unscreened wire (5 inches or so) with a croc clip at each end, and use this to short the two "hot" connection solder tags of the FX loop IN/OUT sockets. Don't insert any jack plugs. Switch the amp on. If you find that this has killed the noise and you are able to play normally through the amp, then you can be sure that the problem lies in a dirty/slack connection between those two jack sockets. If it doesn't kill the noise, then the source of the grounding issue must lie somewhere earlier in the signal path.

Let us know how you get on! :fingersx:
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Re: Selmer TnB 50 MKIII noise

Postby stratele52 on Sat May 13, 2017 10:33 am

Look center tap to tubes heaters from Power Transformer is missing or defective
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Re: Selmer TnB 50 MKIII noise

Postby loz7 on Sat May 13, 2017 11:00 am

Vortexion wrote:
loz7 wrote:The only other thing I can think of is that the mains transformer has been changed, and the noise is like a loud buzz/static so it could be that I guess?

Seems doubtful to me. If the hum/buzz were as a result of a faulty mains tranny installation, you wouldn't be able to silence the noise by doing your trick with the FX loop jack sockets and patch cord. The fact that you are able to silence the noise (and, as you've said, cut off the signal entirely) with that trick means that the source of the problem must lie either in the FX loop connections themselves or at some point earlier in the circuitry (but not before the instrument volume controls, 'cos you've checked for that).

I thought you might say that and of course you were right,I jumped another transformer to the amp but still the same static noise just a little different.


I tried a jumper lead between the FX in and out but it made the noise worse slightly..I notice that the heater on the mains tranny has a single 6.3v tap and the other side of the heaters goes to the centre tap for the HT and to ground..

Here's some voltages I took from the grids of the preamp valves:

V1 Pin 2:
0.017V + loud hiss when I touch the probe
V1 Pin 7:
0.014V + hum when I touch the probe

V2 Pin 2
0.28V + Very loud hum and hiss when I touch the probe

V3 Pin 2
0.014V +high pitch osilation noise when I touch the probe
V3 Pin 7
0.016V + Noise dissappears almost completely when I touch the probe but when I touch the chassis with my hand at the same time a really weird insecty noise and hum!

PI grids are about 10v and 11v(which is normal I think?)

I tried grounding the grids of the first 3 preamp grids but it just hummed and or crackled and didnt make any difference to the noise :cry:
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Re: Selmer TnB 50 MKIII noise

Postby stratele52 on Sat May 13, 2017 11:06 am

.I notice that the heater on the mains tranny has a single 6.3v tap and the other side of the heaters goes to the centre tap for the HT and to ground..

This is not good if I understand what you write.
No heater wires at the tubes should be at ground

Pictures please.

FX loop look ok to me
Any link for schematic ?
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Re: Selmer TnB 50 MKIII noise

Postby stratele52 on Sat May 13, 2017 11:11 am

Shematic here;

http://schems.com/bmampscom/selmer/selm ... smkiii.pdf

I'm pretty sure your heater wiring are wrong and the issue is a noise
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Re: Selmer TnB 50 MKIII noise

Postby Vortexion on Sat May 13, 2017 11:28 am

stratele52 wrote:Shematic here;

http://schems.com/bmampscom/selmer/selm ... smkiii.pdf

I'm pretty sure your heater wiring are wrong and the issue is a noise

Interestingly, that schematic shows no centre-tap for the heater supply. This is either a mistake on the drawing or a design change from previous versions of the Selmer TnB.
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Re: Selmer TnB 50 MKIII noise

Postby stratele52 on Sat May 13, 2017 11:33 am

This is a common desing for low budget amps and this desing make noisy amp. A artificial c.t .with two 100 ohms resistors remove all the noise.

I must see how is the wiring of your amp first, look wrong.
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Re: Selmer TnB 50 MKIII noise

Postby loz7 on Sat May 13, 2017 11:44 am

stratele52 wrote:.I notice that the heater on the mains tranny has a single 6.3v tap and the other side of the heaters goes to the centre tap for the HT and to ground..

This is not good if I understand what you write.
No heater wires at the tubes should be at ground

Pictures please.

FX loop look ok to me
Any link for schematic ?


Thats what I thought, but heaters normally hum if they are wired wrong, this is a static type noise..

That is the same schematic I am looking at- on my transformer there is a 6.3v tap and a 5v tap but nothing else. photo attached..

Image
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Re: Selmer TnB 50 MKIII noise

Postby loz7 on Sat May 13, 2017 11:48 am

Earlier when I connected the other mains transformer to the selmer it had 2x 3v taps and a centre tap to ground but the amp was still noisy-
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Re: Selmer TnB 50 MKIII noise

Postby stratele52 on Sat May 13, 2017 11:57 am

loz7 wrote:[
Thats what I thought, but heaters normally hum if they are wired wrong, this is a static type noise..

T ]



You write ;
" have a mkiii selmer 50 that has a nasty noise i.e like a lead is plugged in with no guitar attached or a loose ground. "
__________________________
Look at this schematic, is your heater wiring are like this ; one wire to ground and other to heater wire from Power Transformer ?
If yes, it is wrong.
I need to see heaters wiring picture,

http://el34world.com/charts/Schematics/ ... _schem.pdf
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Re: Selmer TnB 50 MKIII noise

Postby loz7 on Sat May 13, 2017 12:15 pm

stratele52 wrote:
loz7 wrote:[
Thats what I thought, but heaters normally hum if they are wired wrong, this is a static type noise..

T ]



You write ;
" have a mkiii selmer 50 that has a nasty noise i.e like a lead is plugged in with no guitar attached or a loose ground. "
__________________________
Look at this schematic, is your heater wiring are like this ; one wire to ground and other to heater wire from Power Transformer ?
If yes, it is wrong.
I need to see heaters wiring picture,

http://el34world.com/charts/Schematics/ ... _schem.pdf



Thanks for the reply, yes that is the same heater wiring as the fender. here are the photos of the wiring fromt he valves to the transformer:

http://jpegbay.com/gallery/006216621-.html#1
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Re: Selmer TnB 50 MKIII noise

Postby stratele52 on Sat May 13, 2017 12:27 pm

Heater wiring is wrong ; the brown wire go to P.T center tap wich go to ground ; this is a noise issue
It is a very bad wiring job :(

Did PT have a another center tap for B+ ( DC voltage ) ? It must have one more


Thank's for picture,

Some more may help , not too close from the amp wiring , I must see more of the circuits

I'must live home in 30 minutes for the day
Last edited by stratele52 on Sat May 13, 2017 12:32 pm, edited 1 time in total.
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