Switch with internal arcs - Help!

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.

lwi

Member
2001-11-22 10:34 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

Member
2000-11-24 12:02 am
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.
 
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

Member
2001-11-22 10:34 pm
sorry guys, it did not work. It is still popping.

here is the part of the schematics concerning my problem:

An externally hosted image should be here but it was not working when we last tested it.

("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?
 
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!
 

lwi

Member
2001-11-22 10:34 pm
Yea, I knew it would affect the sound somewhat.
I also toyed with the pot idea, but its not a very elegant solution.

you are absolutely right avout the fact that the original Mesa schematics are a little different: a switch turns on a LED acting on a photo-sensitive resistor (which I replaced with a switch). The effect is most likely more gradual than the simple flick of a switch.

Mesa don't really like to see their schematics showing up all around the net... the site i got them from was forced to take the off shortly after.

thanks for your help anyways.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.