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Crate GX-212+ Mysteriously Switches Channels

Started by dafremen, August 31, 2013, 01:17:13 PM

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dafremen

Hello everyone,

First, I'd like to thank all of you who have contributed to this forum and especially to the Crate Amplifier discussions. With your help, I've been able to nurse this amp along. The schematic has been invaluable. Thanks again.

I've finally reached a stumper: I'll have the amp in Clean Channel and at random intervals while playing, it will switch to O/D and stay there. This is where it gets weird (especially since I thought I was pretty thorough in looking for cold solders): If I smack the amp (like Fred Sanford opening his secret cash drawer), it will go back into CLEAN and stay there. Smacking it while it's in Clean Channel, will cause it to momentarily switch to O/D (it blinks red is all.)

I took the switch apart, cleaned the contacts, applied VERY LIGHT dielectric grease, checked again for cold solders..saw nothing. I'm beginning to wonder if there's a cold solder in the foot switch circuit that just isn't obvious? I don't know, I've spent 3 days on it and I'm throwing my hands up.

Our drummer had a sense of humor about the amp being this flaky..until we started auditioning leads. Then it wasn't so funny anymore. Unfortunately, my budget means no new gear til tax time..so it's fix it or go without.

Anybody familiar with this problem? Have any new approaches I might try?

Thanks again and rock on,

daf

Enzo

Grease?  On the contacts?  It sure sounds like a footswitch jack.  I want those cutout contacts to be completely clean, no nothing on them.  OH wait, you greases a switch?  I still want that clean too.  I doubt it is the switch, I think it is the FS jack.

Flip the board over, nothing plugged into the jack.  Carefully measure the resistance across each cutout contact.  Ideally it is zero.  IN practice it has a small resistance.  End remember your meter leads have a small resistance.

If I measure more than half an ohm, I consider it a problem.  A resistance of mayve 2 ohms will work fine, 2 ohms won;t affect it.  But the fact it has 2 whole ohms resistance tells me teh contacts are not clean, and at a later moment they may measuer open.

While you are doing that, do the FX loop jacks too, and any involving power amp in.

dafremen

The dielectric grease was a touch I added because they have a contact slider that "pinches" the switch "rails"(poles) as it slides. There was visible wear so I thought dielectric grease would not only reduce friction, but also provide for continuous contact between the slide and the "rails."

There's nothing plugged into the foot switch jack when it glitches on me, but I'll check those resistances anyhow.

What makes it seem like an intermittent physical open/short is the fact that hitting the cab corrects the problem temporarily.

daf

Roly

I'm sorry but I don't understand what you mean by "dielectric grease".

According to Wikipedia "Dielectric grease is electrically insulating and does not break down when high voltage is applied", and this is consistent with the normal use of "dielectric" which is as an insulator in capacitors.  As such this is the very last thing I would apply to any contacts that were supposed to mate and conduct, connector or switch.  If this "dielectric grease" is silicon-based you may be in a bit of trouble because silicon compounds are almost impossible to remove once applied.

I found the circuit for the GX-120/GX-212 here;

http://elektrotanya.com/crate_gx_120,_gx_212_sch.pdf/download.html

Quote from: dafremenSmacking it while it's in Clean Channel, will cause it to momentarily switch to O/D (it blinks red is all.)

It's physically sensitive, and the blinking LED's show it's happening before the control circuit for the signal switching FET's.

Looking at the circuit lower right, channel A is selected when FET Q19 is off, when point (A) is negative, when Q17 is off, when its Base is pulled positive via D15 and D16. Which brings us to the footswitch jack and contacts.

If this amp has its sockets soldered directly into the PCB then have a very close examine of the solder joints.  This style of construction is very prone to damaged solder joints due to leads being tripped over and wrenching the socket in its solder joints.
If you say theory and practice don't agree you haven't applied enough theory.

dafremen

Perhaps the product is mislabeled then, because this is the product used on spark plug boots between the plug and the boot. Bad spot for an insulating lubricant/corrosion preventative, so I'm going to have to assume the package is mislabeled.

Found the problem, though. It was a cold solder in the foot switch circuit, near the foot switch input jack. Thanks for your help.

daf

Roly

No, that is exactly what these insulating compounds are used for.  With 30-40,000 volts the connection into the spark plug is not effected, but the object there is to stop flashover from the high voltage wiring to grounded parts of the engine or tracking through dirt on the insulating body of the plug or rubber cap.  In your amp situation the voltage on the switch is only 16 volts and this compound will not help its operation at all.

http://en.wikipedia.org/wiki/Silicone_grease#Dielectric_grease

Glad you found your broken solder joint.   :tu:

HTH
If you say theory and practice don't agree you haven't applied enough theory.

dafremen

#6
Good information. I'm now disposing of what I thought was an electrically conductive lubricant. THe switch works great, like I said I applied it in "cloth applied" amounts, ie.. super thin. Guess I lucked out there. Yea, for reference, it was R77 that had developed a loose solder joint. I soldered the jack again too just to be sure and all of the others as well.

No red flash when i bang on it. No intermittent switches between channels. Hurray..I can avoid making our band look stupid next audition! :)

daf

P.S. It did seem strange when I read it, since I remembered the term "dielectric" being used to describe the insulating medium between conductors in a capacitor.


J M Fahey

Oil/Grease is always insulating .

yet it does not prevent contact between 2 metal parts, because it simply slides away under light pressure

The problem with oil/grease in power carrying contacts is that sparks will carbonize it, even little by little, and sooner or later that may be a problem.

Enzo

daf, if you are still reading here, the channel switching circuit runs THROUGH the FS jack, and those contacts complete the circuit when the jack is empty.  SO if they get dirty, or their solder cracks as you found, as far as the amp is concerned it is the same as plugging in a foot switch an clicking the switch to open.

Roly

Quote from: J M Faheyit simply slides away under light pressure

This certainly true of petrolium products, but silicon compounds are a special case because they can have an absolutely phenominal surface tension, thousands of times greater.  We all know how one drop of oil can stretch out to cover a large patch of driveway, but silicon compounds were banned in Australian telephone exchanges because it was discovered that uncured residue of sealants could migrate along metres of cable into relay contact sets and cause failures due to the formation of insulating hard silicon oxide crystals even with micro-arcing, holding "closed" contacts apart. {and it's amazing what you can discover when you have a research division dealing with literally millions of contacts in-house.}

The message is - use silicon compounds around electronics with caution; in some situations the very properties that make them so attractive can turn out to be a problem.  As a sidebar, it is also wise to use neutral cure rather than acetic acid (vinegar smell) cure sealants around electronics.
If you say theory and practice don't agree you haven't applied enough theory.

tonyharker

That should be of course SILICONE, not silicon. 

dafremen

Thanks again everyone. I guess my "assumption" (boy there's the "make an A-- out of ME" bit) was that the silicone was mixed with conductive material of some sort.

daf