Switch with internal arcs - Help!

lwi · 22nd November 2001, 09:42 PM

I have a DIY guitar amp (a mesa/boogie 2:90 copy), and it has a problem.

One of the switches produces a rather large popping noise when turned off.
I believe it's because of internal arcs inside the switch.
I have read here and a few other places that a contact suppressor and a vdr might help.

Can anyone provide me with more detailed info on this?

thanks in advance,

Lwi

rtirion · 22nd November 2001, 10:00 PM

If the switch is the main power switch, I would suggest replacing it. After 10 years or more of service the contacts inside the switch a probably damaged (burned to a more or lesser degree).

You can place a ceramic disc capacitor over the contacts of the switch 10nF/630VAC to supress arcing and this will increase the life of your switch and amp.

Be extremly carefull when you replace the switch. 120Vac or 240 Vac is extremly lethal. If you are not sure how to replace the switch or put in the capacitor, please do not attempt this your self, but have a qualified technician carry out this small repair. Maybe you can take your amp over to a local repair shop. I am sure they will be happy to take care of this for a reasonable price.

lwi · 22nd November 2001, 10:37 PM

Thank you very very much

the switch is brand new so it's age is not an issue.
I built the amp myself, I just finished it recently, adding a cap shouldn't be a problem

cheers

lwi

walker · 22nd November 2001, 11:01 PM

Iwi, I have done many such repairs and agree with Rtirion. Just thought I would mention RC snubbers, they include a series resistor with the capacitor for even longer switch life, (less stress). I use Evox-Rifa units, 0.1uF + 47ohm 1000V, (these seem to suit our 240v system and equipment upto 500VA). If you want the best supression you can calculate the correct values of R and C with the following rule of thumb;
R=E/Iload
C(uF)=Iload(Amps)

I also agree that though the job appears simple, it should be done by some one with the apropriate training.

Regards WALKER

lwi · 23rd November 2001, 03:45 AM

sorry guys, it did not work. It is still popping.

here is the part of the schematics concerning my problem:

("output" = to the power tubes)

the switch (this switch!) is used for decreasing (when turned off) the cathode voltage. The end result is a drop in volume, but the tubes still saturate as if playing at loud volumes.
Putting a 10nF cap on the contacts of the switch resolved nothing.

any thoughts?

walker · 23rd November 2001, 04:39 AM

Iwi does the pop sound come from the switch or through the speaker?

Regards WALKER

PS As usual I was off on a tangent thinking about the mains power switch.

lwi · 23rd November 2001, 04:49 AM

through the speaker.... its quite loud when I turn it off... but less so when turned on.

walker · 23rd November 2001, 05:03 AM

I suspect that 10nF was too small but seeing the circuit I'm now concerned that a cap shouldn't be used at all. Could you scan a larger section of the circuit please.

Regards WALKER

lwi · 23rd November 2001, 06:36 AM

here you go:

Just to clarify: the feedback is fed into a small eq section (two switches and a presence pot) wich is the added to the normal signal.

if anything else seems nebulous, just say so.

thanx

walker · 23rd November 2001, 07:55 AM

Iwi, a cap across the switch will effect the sound I believe, though by how much I’ve no idea, (depending on the value of course). You could try 1uF but listen out for loss of top end, before and after it is in the circuit, (switch open of course). Snubbers would not be any better. The solution maybe to replace the switch with a pot, (power dissipation permitting). I don't really like that solution though either.
The Messa circuit must be different or maybe the manual suggested the selection be made before turning on.
I have repaired a Messa before, (love the wicker speaker cloth) I don’t believe I have any schematics. If I remember correctly, they had Groove Tubes, don’t you love marketing?

Try posting this in the valve forum, some of these guys would, I suspect, be quicker than I on thermionics questions.
Regards WALKER

PS I know this is redundant, but, there is lethal voltages on this circuit, be werry careful!

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22nd November 2001, 09:42 PM	#1
lwi diyAudio Member Join Date: Nov 2001	I have a DIY guitar amp (a mesa/boogie 2:90 copy), and it has a problem. One of the switches produces a rather large popping noise when turned off. I believe it's because of internal arcs inside the switch. I have read here and a few other places that a contact suppressor and a vdr might help. Can anyone provide me with more detailed info on this? thanks in advance, Lwi

22nd November 2001, 10:00 PM	#2
rtirion diyAudio Member Join Date: Nov 2000	If the switch is the main power switch, I would suggest replacing it. After 10 years or more of service the contacts inside the switch a probably damaged (burned to a more or lesser degree). You can place a ceramic disc capacitor over the contacts of the switch 10nF/630VAC to supress arcing and this will increase the life of your switch and amp. Be extremly carefull when you replace the switch. 120Vac or 240 Vac is extremly lethal. If you are not sure how to replace the switch or put in the capacitor, please do not attempt this your self, but have a qualified technician carry out this small repair. Maybe you can take your amp over to a local repair shop. I am sure they will be happy to take care of this for a reasonable price.

22nd November 2001, 10:37 PM	#3
lwi diyAudio Member Join Date: Nov 2001	Thank you very very much the switch is brand new so it's age is not an issue. I built the amp myself, I just finished it recently, adding a cap shouldn't be a problem cheers lwi

22nd November 2001, 11:01 PM	#4
walker diyAudio Member Join Date: Nov 2001 Location: Perth West Australia	Iwi, I have done many such repairs and agree with Rtirion. Just thought I would mention RC snubbers, they include a series resistor with the capacitor for even longer switch life, (less stress). I use Evox-Rifa units, 0.1uF + 47ohm 1000V, (these seem to suit our 240v system and equipment upto 500VA). If you want the best supression you can calculate the correct values of R and C with the following rule of thumb; R=E/Iload C(uF)=Iload(Amps) I also agree that though the job appears simple, it should be done by some one with the apropriate training. Regards WALKER

23rd November 2001, 03:45 AM	#5
lwi diyAudio Member Join Date: Nov 2001	sorry guys, it did not work. It is still popping. here is the part of the schematics concerning my problem: ("output" = to the power tubes) the switch (this switch!) is used for decreasing (when turned off) the cathode voltage. The end result is a drop in volume, but the tubes still saturate as if playing at loud volumes. Putting a 10nF cap on the contacts of the switch resolved nothing. any thoughts?

23rd November 2001, 04:39 AM	#6
walker diyAudio Member Join Date: Nov 2001 Location: Perth West Australia	Iwi does the pop sound come from the switch or through the speaker? Regards WALKER PS As usual I was off on a tangent thinking about the mains power switch.

23rd November 2001, 04:49 AM	#7
lwi diyAudio Member Join Date: Nov 2001	through the speaker.... its quite loud when I turn it off... but less so when turned on.

23rd November 2001, 05:03 AM	#8
walker diyAudio Member Join Date: Nov 2001 Location: Perth West Australia	I suspect that 10nF was too small but seeing the circuit I'm now concerned that a cap shouldn't be used at all. Could you scan a larger section of the circuit please. Regards WALKER

23rd November 2001, 06:36 AM	#9
lwi diyAudio Member Join Date: Nov 2001	here you go: Just to clarify: the feedback is fed into a small eq section (two switches and a presence pot) wich is the added to the normal signal. if anything else seems nebulous, just say so. thanx

23rd November 2001, 07:55 AM	#10
walker diyAudio Member Join Date: Nov 2001 Location: Perth West Australia	Iwi, a cap across the switch will effect the sound I believe, though by how much I’ve no idea, (depending on the value of course). You could try 1uF but listen out for loss of top end, before and after it is in the circuit, (switch open of course). Snubbers would not be any better. The solution maybe to replace the switch with a pot, (power dissipation permitting). I don't really like that solution though either. The Messa circuit must be different or maybe the manual suggested the selection be made before turning on. I have repaired a Messa before, (love the wicker speaker cloth) I don’t believe I have any schematics. If I remember correctly, they had Groove Tubes, don’t you love marketing? Try posting this in the valve forum, some of these guys would, I suspect, be quicker than I on thermionics questions. Regards WALKER PS I know this is redundant, but, there is lethal voltages on this circuit, be werry careful!