Gallien-Kruger 400RB-IV - Little Sound w/Occasional Screeching and Overload?

[louloomis]

Hey Everyone,

I have the Gallien-Krueger 400RB-IV (relatively new amp). The symptom has changed slightly but it's like the pots were scratchy and I could get sound, then the sound became almost nothing but if I worked the gain or master, there would be insane shrieking screeching coming through the speaker cab as though it was about to blow.

Is this a problem with the preamp? Could it be dirty controls? How much should this be to fix the amp? It's a one-owner amp (me) and I've almost never used it since I owned it. Any ideas?

Thanks.
LL

[52 Bill]

It could be dirty or broken pots or a loose or broken connection or any number of things. It could take a good tech 15-30 minutes to diagnose the problem, so figure maybe an hour to fix it. Depending on where you live, $40-$80.

If you have basic skills, you could try and spray clean the pots and jacks to see if that clears up the problem.

[louloomis]

It could be dirty or broken pots or a loose or broken connection or any number of things. It could take a good tech 15-30 minutes to diagnose the problem, so figure maybe an hour to fix it. Depending on where you live, $40-$80.

If you have basic skills, you could try and spray clean the pots and jacks to see if that clears up the problem.

Thanks for the response. I was quoted 175-200 bucks to fix it. It seemed really high to me.

LL

[Chuck H]

I "fixed" one with similar symptoms by cleaning jacks and pots. The loop jacks are switching jacks and, since they're not used much, become dirty or oxidized with no male jacks scraping them clean on a regular basis. If you do it yourself get potentiometer "cleaner" and "lubricant". Some products claim to do do both and I'm told they work fine. Blast a puff of keyboard cleaner into the pots to blow out loose debris and then spray a small amount of cleaner in. Rotate the pot back and forth. If using a dual purpose product, your done. If not, wait a few minutes and then spray the lube and rotate. To clean jack switches you can spray the cleaner onto a piece of business card, wedge it into the contact point and rub it back and forth a few times. I'll admit that on badly corroded jacks I've used 1000 grit wet/dry sandpaper this way, but your not suppose to do that. It can rub plating off the contacts. Follow by applying the cleaner to the actual jack contact points and plugging a male jack in and out a few times. Nothing to it. Oh... Be sure not to get any spray cleaner on instrument or furnature finishes.

[km6xz]

I am sure Chuck meant "deoxidizer" not Cleaner. Contact cleaner is the worst thing for contacts when the problem is not dirt. It is oxide layer of metal where atmospheric oxygen chemically bonds with the top few molecules of metal of the contact surface. It is a natural results if oxygen interacting with metals. The oxide of a metal is an insulator, a good one, that prevents current flow until some elevated voltage potential difference between the two contacts of a switch or the wiper and resist of a pot. The method of returning current flow is to remove the oxygen bonded to the top layers of metal. That requires a "de-oxidizing" chemical that has a stronger attraction to the oxygen than the metal. There are a number of chemicals which easily achieve that goal; Caig Labs "De-Oxit", Cramolin "R-5", etc.
Cleaners including contact cleaner is not the same think and is a strong solvent that can make the oxide grow faster and thicker soon after seeming to work initially for a few days. For example Radio Shack sells a degreaser that they label as "Contact Cleaner" which will ruin pots and decrease reliability of switch contacts and relays.
De-Oxit leaves a slight film on the surface of the metal which protects for months. Very little of the chemical is needed, since its job is to change local chemical bonds, not flood or wash away dirt. Use as little as you can because excess, if it migrates into the shaft bushing will wash away the permanent lube that makes the pots turn smoothly.
It a contact is too corroded, where the metal surface is pitted and burned, this might help but in most cases it is better to replace the part. Before resorting to burnishing, make sure the contact material is suitable or you can ruin the contact using abrasives.
Oxide is on all exposed surfaces of metal and if the layer of insulation is shallow, normal operation with wipe or self-burnish the contact or pot.. Using the pot regularly or using jacks, with insertions and removals of plugs, will keep it working for a long time. The contacts that do not get exercised are the ones where current flow is most likely to be impacted.
Good luck

[louloomis]

Thanks a lot for this guidance. Very helpful and will give it a go.

LL

[louloomis]

Before I begin anything, here's an update:

The same problem persists if I:

1. Plug the bass directly into the Effects Loop Return.
2. Plug the bass into the Input of the amp while having a cable run from the Effects Loop Send to the Effects Loop Return.

Also, while doing #2, the amp turned off by itself.

Any further thoughts before I begin doing the cleaning as instructed above?

Thanks.
LL

[Chuck H]

That new symptom doesn't jive with the schem. I think it may have been incidental and not related to the original problem. Be sure to clean any board plug pins as well. Nothing to lose by cleaning (ahem... deoxidizing) any mobile, switchable and plug-able contacts. If you still have troubles it could be a bad solder joint. With the amp in operation you can often find them by poking components and connections with a wooden stick. The amp will likely make some kind of audible report if you move a bad joint or connection.

[louloomis]

Everyone,

Just wanted to give you an update and I hope it's not premature but...........IT'S ALIVE!!! IT'S ALIVE!!!!!

I spray cleaned (using De-Oxit D5) these:

1. input jack
2. effects loop send
3. effects loop return
4. speaker output jacks (both)
5. the 3 wire connector pin that goes from the transformer to the board on which is mounted the fan, etc. (since I could easily access it)
6. the 2 wire connector pin that goes from the preamp board to the board on which is mounted the fan, etc. (since I could easily access it)

I'll continue to test but right now it sounds great!

Can't tell you how truly appreciative I am - please accept my thanks to all the contributors. If anything changes, I'll let you know!

It's ALIVE!

Regards,
LL

[Chuck H]

Very good sounding amps. Glad it's working! Always nice when things go like you HOPE they will.

[louloomis]

One last question, please.

Now, of the original symptoms, only the following is occurring and it happens like this:

1. I set the Volume is at about "9 o'clock"
2. I turn the Master knob up and get close to let's say "11 o'clock"
3. As I get close to "11 o'clock" or slightly beyond, I get this insane screeching sound and a white ring temporarily (temporarily because I then turn the Master down again) appears around the interior perimeter of each speaker in my cabinet as if the speakers are about to burst into flames or something. This was mentioned in my first post but now it exists without the other issues mentioned.

I know this sounds crazy, but when I've gotten feedback before with other amps, it was nothing like this insane screeching. Is this feedback and I just never have experienced this via the bass before or is something else going on here? What's up with the white ring around the perimeter of my speakers (like where the foam surrounds are - I think they're called foam surrounds).

Any thoughts? Am I about to blow my speakers or?

Thanks.
LL

[Chuck H]

The white ring thing definitely confuses me. The screeching is probably instability due to a bad ground. It could be a bad pot or a cold solder joint. It will take some bench gear and experience to fix it. The diagnosis will require voltage reading (and knowing what they mean) and signal tracing.

[J M Fahey]

You didn't mention the white ring in your first post or any other.
You didn't say a word about your cabinet. Brand ? Model ?
You won't blow your speakers but you will definitely kill your tweeter (or horn).

[Chuck H]

Ah... Foam surround... Is there a horn or tweeter in the cabinet? And if yes, is there a working crossover in the cabinet? I cant remember if the 400RB has a crossover or not. If yes, is it being used? Correctly? Is there anything else in the signal chain? Compressor, EQ, etc? What about your guitar? Does the problem happen with or without an instrument plugged in? If only with an instrument plugged in, is it possible it could be the instrument pickups are microphonic?

Remove everything from the signal chain except your instrument. If the problem stops it's one of the removed components. If the problem persists try the amp with nothing plugged into the input. If the problem stops, try the amp with another instrument. If the problem stops then it may be your pickups are microphonic. It's the instrument. If the problem persists make sure the cabinet is connected correctly if crossovers need to be employed. If it is, try the amp with another cabinet. If the problem stops, it's the cabinet. If the problem persists then you've isolated the problem to the amplifier.

[Steve Conner]

The white ring is the light from the tweeter protection bulb shining out through the speaker surrounds. It indicates the cabinet is being fed with excessive high frequency signal, but we knew that already from the problem description of "insane screeching" Hopefully the bulb will blow before the tweeter does, but it can be fiddly to replace.

I'm not sure what causes the screeching. My best guess would be a loose connection, dry solder joint or bad ground as was mentioned earlier. Try the "Enzo Whack". Bash the amp with your fist while it's screeching, see if that affects the symptoms.

But don't let it screech for too long or you'll burn out your tweeter bulb.