Correct Zap Procedure?

[mwd]

In researching zapping dendrites I have seen 1x, 2x, 3x 9volt batteries, car chargers, removing tubes and using the high voltage from a pin. I also read, when doing this for percussion on an M3, that the blue wire on terminal K should be zapped.

But should the wires be desoldered from the terminal or left on? If they are unsoldered do you zap the wire or the terminal or both then resolder? I have read enough to be thoroughly confused.

[David Anderson]

I'll use 3 x 9V batteries, and if that doesn't work, I'll thoroughly clean out the percussion switch box.

I don't like the idea of people who don't have a thorough understanding of electronics trying the high voltage method. There have been a number of stories on here over the years of people who caused collateral damage that way.

[Sweet Pete]

Unsolder terminal K,then zap.Then resolder K back on.I have basic electronic skills.I use pin 1 off the 12AU7 perc tube.I keep one hand behind my back.
I'm here to tell you about it so the big mistake hasn't yet happened on my watch!
People don't read or study the safety procedures.They won't read a schemo or a map either.
The first thing a courier reaches for is a map.Digital or otherwise.Use batteries if you are not prepared to 'follow procedure'!
You should see a voltage drop at K with your meter when a note is 'keyed' on perc.No drop? Zap!
Main thing is,unsolder then resolder K! And be safe!

[David Anderson]

The inherent problem with zapping metal whiskers is the chance that there is another whisker nearby to take the place of the one you zapped. If you thoroughly flush out the percussion switch box, you get rid of current and future problems.

I've seen cases where a quick zap with batteries cleared the problem. I've also seen cases where it cleared it for two minutes, but then it came back, over and over, and was not a reasonably permanent solution. It depends on how densely metal whiskers have formed on the inside of the switch assembly.

[Sweet Pete]

Almost as easy to clean the switchbox.The extra hour spent doing it right saves frustration from re-appearance of issue.
When time allows....

[mwd]

OK so I see many mention cleaning percussion box which I had done. I also have a drop to zero when a key is pressed. Still don't have percussion so I was wondering if I had previously zapped wrong or needed to try it again and it sounds as if I don't.

[David Anderson]

The voltage at Terminal K should be between 36V and 29V with no key depressed (higher voltage on Fast, lower on Slow) and 0V with an upper manual key depressed. If that is the case and you still don't have percussion, it's not a triggering problem.

I have seen a couple of no-percussion cases when one of the wirewound voltage-dropping resistors inside the preamp opened up. Someone here recently had a dead 6C4. Zapping isn't a magic cure-all.

Metal whiskers are often the cause of percussion not working, but they are not the only cause. Zapping only fixes metal whisker problems.

[mwd]

Thank you. I believe mine was 22.5 v and I can't remember the switch config. The steps I have taken are clean the percussion box, zap, and replace the 3 tubes. I may remove the screws and tilt the amp just to see if there is something unhooked, melted, etc? I'm kinda' stuck with my limited trouble shooting knowledge.

[David Anderson]

You might see something, but you might not. An open resistor often looks like any other resistor. At a certain point, you either have to learn to do the work or hire someone who does.

I have an old car I want to get running again. I'm having the carburetor professionally rebuilt because I want it done by someone who knows what he's doing. Other things that I know how to do, I'll do myself, but not that part.

[mwd]

I know what you are saying unfortunately there are no Hammond repair options or even electronic repair facilities here in rural New Mexico. So it will be up to what I can glean from this forum :-)